CBIP Publication No 295

Pub. No. 295

MANUAL ON TRANSFORMERS

MANUAL ON TRANSFORMERS Publication No. 295

Editors

Editors G.N. Mathur R.S. Chadha

G.N. Mathur R.S. Chadha CENTRAL BOARD OF IRRIGATION AND POWER

2006

MARCH

2006

2006 ISBN 81-7336-302-1

"Reproduction of articles in publication in any form is permissible subject to proper acknowledgement and intimation to the publishers. The publishers have taken utmost care to avoid errors in the publication. However, the publishers are in no way responsible for the authenticity of data or information given by the contributors."

Central Board of Irrigation and Power Malcha Marg, Chanakyapuri, New Delhi 110 021 Phone : 2687 5017/2687 6567 Fax : 2611 6347 E-mail : [email protected]/[email protected] Web : www.cbip.org

(ii)

WORKING GROUP ON TRANSFORMERS Chairman Shri R.C. Agrawal Chief (T&D), TBG Bharat Heavy Electricals Ltd. Bhopal Members Shri M.M. Goswami AGM (Engg.) Power Grid Corp. of India Ltd., Gurgaon

Shri M.Z.M.A. Sayed Divisional Engineer BEST, Mumbai

Shri Anil Chanana DGM Power Grid Corporation of India Ltd.

Shri Sanjay Kar Chowdhury Assistant Manager (Sub-Station) CESC Ltd., Kolkata

Shri Kamal Sarkar DGM (OS) Power Grid Corp. of India Ltd., Gurgaon Shri B.N. De Bhowmick DGM (OS) Power Grid Corp. of India Ltd., Gurgaon Shri V.K. Bhaskar Chief Manager (OS) Power Grid Corp. of India Ltd., Gurgaon

Shri Ranjit Singh Nain Executive Engineer/PTRW Haryana Vidyut Prasaran Nigam Ltd. Ballabhgarh, Haryana Shri K.K. Bhatia Addl. Chief Engineer Gujarat Energy Transmission Corp. Ltd. Vadodara

Shri Hirdesh Gupta Dy. Chief Design Engineer National Thermal Power Corp. Ltd., Noida

Shri K.S. Kattigehallimatt Chief Engineer KTPCL, Bangalore

Shri S.K. Malik Suptd. Engineer (O&M Circle) Bhakra Beas Management Board, Panipat

Shri D. Majumdar Dy. Chief Engineer (CTC) Damodar Valley Corporation, Jharkhand

Shri R.K. Garg Bhakra Beas Management Board, Chandigarh

Shri P.K. Kognolkar Addl. Director Central Power Research Institute, Bhopal

Shri J.S. Batra Bhakra Beas Management Board, Chandigarh Shri R.M. Malhotra Delhi Transco, New Delhi

Shri S.C. Bhageria AGM, TRE Bharat Heavy Electricals Ltd., Bhopal

Shri S.N. Katakwar Superintending Engineer (Dist.) Maharashtra State Electricity Board Mumbai

Shri R.K. Tiwari AGM Bharat Heavy Electricals Ltd. Bhopal

(iii)

Shri S.K. Srivastava AGM, TRE Bharat Heavy Electricals Ltd. Jhansi Shri C.P. Deshmukh Senior DGM Bharat Heavy Electricals Ltd. Bhopal Shri R.K. Mohapatra Deputy General Manager, TRE Bharat Heavy Electricals Ltd. Jhansi Shri B. Naik Manager (T) Bharat Heavy Electricals Ltd. Jhansi Shri A.K. lkka Bharat Heavy Electricals Ltd. Bhopal Shri R.K. Agrawal Bharat Heavy Electricals Ltd. Bhopal Shri M. Vijaykumaran Technology Expert ALSTOM, Allahabad Shri K. Bheema Prakash General Manager ALSTOM, Allahabad Shri K. Raghuraman General Manager ALSTOM, Allahabad Shri Mohan Manager ALSTOM, AITPL Bangalore Shri V.K. Lakhiani GM (Q&A) Crompton Greaves Ltd. Mumbai

Shri A.B. Bhatia DGM Crompton Greaves Ltd., Gwalior Shri Dharam Vir DGM, Transformer Division Crompton Greaves Ltd., Mumbai Shri Pramod Rao Crompton Greaves Ltd., Nasik Shri P. Ramchandran Asstt. Vice President ABB Ltd., Vadodara Shri M.L. Jain Vice-President, Technology EMCO Ltd., Thane Shri Gautam Mazumdar Manager EMCO Ltd., Thane Shri K. Samba Murthy General Manager (Q&A) Vijay Electricals Ltd., Hyderabad Shri S.K. Mahajan Consultant Voltamp Transformer Pvt. Ltd., Vadodara Shri S.R. Karkhanis Deputy General Manager The Tata Power Company Ltd., Mumbai Ms. C.R. Bhonslay Manager (Engineering Department) The Tata power Company Ltd., Mumbai Shri M.L. Mittal GM (Retd.) BHEL, Habibganj, Bhopal Late Shri A.K. Kapur ED (Retd.) Power Grid Corp. of India Ltd. A-55, East of Kailash New Delhi

(iv)

lR;eso t;rs

10.3.2006

FOREWORD India had a meager installed power generating capacity of approximately 1,300 MW at the time of independence in 1947. Since then we have made rapid progress and on date this figure stands at 1,23,668 MW. Matching with the generating capacity, transmission and distribution networks have also grown. However, despite this spectacular growth of power sector, still we have about 8% shortage in energy and about 12% shortage in peak power. Due to these shortages, despite best efforts the voltage and frequency in different grids sometimes varies beyond acceptable limits and lead to power cuts. Transformer is a vital link for taking power supply from generating station to consumer premises. Matching with the growth in power sector, about 56,000 MVA power transformers and 26,000 MVA transformers for distribution network in the country are being manufactured annually. The manufacturers have also upgraded the manufacturing technology matching with the increase in the complexity of the grid besides their impressive R&D setup to support the technology and meeting export requirement. In transmission and distribution sector, cost of the power transformer represents the largest portion of the capital investment and financial consequences of failure of transformer lead to considerable loss of revenue besides break down of power supply. Central Board of Irrigation & Power (CBIP) has been playing a key role to disseminate the latest technological advancement information covering almost all aspects of power sector. In early 70s, it was felt that industry should have detailed reference specifications for transformers. (v)

Accordingly, using the CBIP as a platform, a "Working Group on Transformer" was formed with representatives from users and manufacturers to prepare detailed specifications for transformers. In 1976, CBIP issued the specifications as Technical Report 1, "Manual on Transformers" under Research Scheme on Power for the first time. This manual was subsequently revised in 1987 and in 1999. The transformer manual issued by CBIP is being widely used by power engineers as a reference book covering all aspects of transformers for almost all applications. Almost all the utilities in the country are referring the CBIP Manual on Transformers while formulating their own purchase specifications on the basis of technical information contained there in. I am happy to note that this manual has been revised and updated now with the help of Working Group experts from all eminent organizations, and contains the latest technological information including installation, testing, commissioning and repairs of transformers besides additional chapters on 800 kV transformers, Condition Monitoring and Diagnostic Techniques and chapters on various components of the transformers of all sizes and voltages including dry type transformers. I congratulate CBIP and all experts of the Working Group for bringing out this manual covering latest state-of-art technology and I am sure that this document will be of great use to engineering fraternity as a reference book.

(R.V. SHAHI)

(vi)

PREFACE Transformer is a vital component of all electric power systems and every pulse generated, passes through several transformers before it is finally consumed in any appliance or equipment or industry. In our country the transformers are subjected to severe stress condition due to wide variation in voltage and frequency in system due to huge gap in demand and supply causing higher failure rate of transformers as compared to western countries. With the escalations in the cost of material and other inputs into its manufacture, the requirement of un-interrupted power and above all the steep rise in the cost of power has forced the manufacturers to compete in innovations and improvise designs as well as manufacturing practices to deliver a low-loss, competitive costing transformers with built-in monitoring features to ensure its long troublefree service life. On the other hand power utilities are also under immense pressure to maintain and upgrade their system to meet the rising demand in power with no tolerance for any breakdown in supply. Moreover with the introduction of 'Availability Based Tariff, setting-up of the 'Electricity Regulatory Commissions' and the 'Consumer Grievance re-dressal Forums' all over the country, the essentiality of uninterrupted power now has a large commercial value. In line with our quality policy, to promote and coordinate professional excellence in water and power sector CBIP has prepared more than 300 Manuals on all important subjects. To disseminate the latest developments among the engineer's fraternity, CBIP has updated these manuals from time to time. First edition of the Transformer manual was published in 1976 and it was updated in 1987 and again in 1999. To incorporate the latest developments and facilitate the professional engineers associated in planning, designs, procurement, testing, erection & commissioning of the entire range of Distribution and Power transformers upto 800 kV and also to assist in maintenance / monitoring of the key parameters of the installed transformers, this Manual has been (vii)

updated. In this edition a chapter on 'Repair of transformer at site' has also been added. This trend of repair of transformer at site is becoming popular since it saves in to and fro transportation time as well as HV test cost since some of the tests including HV Test are not to be repeated. For this purpose CBIP had constituted a working group, under the Chairmanship of Shri R.C. Aggarwal, General Manager (T), BHEL, of highly experienced engineers from large power utilities, designs organisation, manufacturers and testing stations from NHPC, PGCIL, NTPC, BHEL, ABB, ALSTOM, CGL, EMCO, VOLT AMP, CEA, WBSEB, KPCL etc., who have put in their knowledge and experience in bringing out this updated transformer manual. I thank all experts of the working group for their valuable contribution. I am sure that this Manual shall be of immense value and provide good reference document to the practising engineers especially those looking after planning, procurement, design, maintenance, testing and commissioning of power systems including transformers.

G.N. Mathur Secretary Central Board of Irrigation and Power New Delhi 110021

(viii)

Contents Foreword

(v)

Preface

(vii)

SECTION A General

1

SECTION B Specifications for Three Phase 11 kV/433 - 250V Class Distribution Transformers (upto and including 100 kVA)

65

SECTION B1 Specifications for Single Phase 11 kV / 250V and 11 kV/ √3/ 250V Distribution Transformers (10, 15 & 25 kVA Ratings)

77

SECTION C Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class)

87

SECTION D Specifications for Power Transformers of Voltage Class below 145 kV

99

SECTION E

Specifications for 145 kV Class Power Transformers

107

SECTION F


115

SECTION G Specifications for 420 kV Class Power Transformers

123

SECTION G1 Specifications for 800 kV Class Power Transformers

135

SECTION H Specification for Earthing Transformers

147

SECTION I

Specifications for Valves for Transformers

157

SECTION J

Test Requirements for Transformers

165

SECTION K Erection, Commissioning and Maintenance

223

SECTION K1 Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors

295

SECTION L

319

Capitalisation Formula for Transformer Losses

SECTION M Specifications for Protective Schemes for Power and Distribution Transformers

323

SECTION N Specifications for Voltage Control of Power Transformers

337

SECTION O Specifications for Fire Protection of Power Transformers

353

(ix)

SECTION P

Specifications for Transformer Bushings upto 800 kV Voltage Class

363

SECTION Q Specifications for Dry Type Transformers

375

SECTION R Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above

383

SECTION S

397

Guidelines for Repair of Power Transformers at Site

About the Authors

419

(x)

SECTION A

General

SECTION A General 1.0

GENERAL DESIGN OF APPARATUS

1.1

Compliance with Specifications

1.1.1 Except where otherwise specified or implied herein, the transformers shall comply with the latest edition of Indian Standard 2026 (hereinafter referred to as "IS"). 1.2

Design and Standardisation

1.2.1 The transformer and accessories shall be designed to facilitate operation, inspection, maintenance and repairs. All apparatus shall also be designed to ensure satisfactory operation under such sudden variations of load and voltage as may be met with under working conditions on the system, including those due to short circuits. 1.2.2 The design shall incorporate every reasonable precaution and provision for the safety of all those concerned in the operation and maintenance of the equipment keeping in view the requirements of Indian Electricity Rules. 1.2.3 All material used shall be of the best quality and of the class most suitable for working under the conditions specified and shall withstand the variations of temperatures and atmospheric conditions arising under working conditions without undue distortion or deterioration or the setting up of undue stresses in any part, and also without affecting the strength and suitability of the various parts for the work which they have to perform. 1.2.4 Corresponding parts liable to be replaced shall be interchangeable. 1.2.5 Cast iron shall not be used for chambers of oil filled apparatus or for any part of the equipment which is in tension or subject to impact stresses. This clause is not intended to prohibit the use of suitable grades of cast iron for parts where service experience has shown it to be satisfactory, e.g., large valve bodies. 1.2.6 All outdoor apparatus, including bushing insulators with their mountings, shall be designed so as to avoid pocket in which water can collect. 1.2.7 Means shall be provided for the easy lubrication of all bearings and where necessary of any mechanism or moving part, that is not oil immersed. 1.2.8 All mechanism shall, where necessary, be constructed of stainless steel, brass or gunmetal to prevent sticking due to rust or corrosion. 1.2.9 All taper pins used in any mechanism shall be of the split type complying with IS: 2393 for these items.

Manual on Transformers

2

1.2.10 All connections and contacts shall be of amp le section and surface for carrying continuously the specified currents without undue heating and fixed connections shall be secured by bolts or set screws of ample size, adequately locked. Lock nuts shall be used on stud connections carrying current. All leads from the winding to the terminal board and bushings shall be adequately supported to prevent injury from vibration including a systematical pull under short circuit conditions. Guide pulls shall be used where practicable. 1.2.11 All apparatus shall be designed to minimise the risk or accidental short-circuit caused by animals, birds or vermin. 1.2.12 Provision shall be made to fix safety fence around top cover of transformers of rating 100 MVA and above, for safe working during installation and servicing for large capacity transformers. 1.2.13 In tank on load tap changers shall be located such that the space above the diverter switch chamber will be free of inter connecting pipes etc. for lifting the diverter switch unit for inspection and maintenance purposes. 1.2.14 Dryness of the insulation may be ensured by measuring the water extraction during vacuum drying. The water extraction per tonne of insulation per hour may be limited to 50 grams maximum. Alternatively dryness can be judged by dew point measurement. 1.3

Galvanising

1.3.1 Galvanising where specified shall be applied by the hot-dipped process or by electrogalvanising process and for all parts other than steel wires shall consist of a thickness of zinc coating equivalent to not less than 610 gm of zinc per square meter of surface. The zinc coating shall be smooth, clean and of uniform thickness and free from defects. The preparation of galvanising and the galvanising itself shall not adversely affect the mechanical properties of the coated material. The quality will be established by tests as per IS: 2633. Alternative to galvanising, zinc spraying or aluminising can also be considered. 1.3.2 All drilling, punching, cutting, bending and welding of parts shall be completed, and all burrs shall be removed before the galvanising process is applied. 1.3.3 Galvanising of wires shall be applied by the hot-dipped process and shall meet the requirements of the relevant Indian Standard. The zinc coating shall be smooth, clean and of uniform thickness and free from defects. The preparation for galvanising itself shall not adversely affect the mechanical properties of the wire. 1.3.4 Surfaces which are in contact with oil shall not be electrogalvanised/cadmium plated. 1.4

Labels

1.4.1 Labels shall be provided for all apparatus such as relays, switches, fuses, contained in any cubicle or marshalling kiosks as shown in Fig. 1. 1.4.2 Descriptive labels for mounting indoors or inside cubicles and kiosks shall be of material that will ensure permanence of the lettering. A matt or satin finish shall be provided to avoid dazzle from reflected light. Labels mounted on dark surfaces shall have white

General

3

lettering on a black background. Danger notices shall have red lettering on a white background. 1.4.3 All plates shall be of incorrodible material. 1.4.4 Labels shall be attached to panels with brass screws or with stainless steel screws or these can be stuck with suitable adhesive also. 1.5

Bolts and Nuts

1.5.1 Steel bolts and nuts exposed to atmosphere shall be of following material : l Size 12 mm or below – stainless steel l Above 12 mm – steel with suitable finish like electrogalvanised with passivation. 1.5.2 All nuts, bolts and pins shall be locked in position with the exception of those external to the transformer, under gasket pressure. 1.5.3 All bolts, nuts and washers exposed to atmosphere and in contact with non-ferrous parts which carry current shall be of phosphor bronze. 1.5.4 If bolts and nuts are placed so that they are inaccessible by means of ordinary spanners, suitable special spanners shall be provided by the supplier. 1.5.5 Bolts and nuts shall not be less than 8 mm in diameter except when used for small wiring terminals. 1.6

Cleaning and Painting

1.6.1 Before painting or filling with oil or compound, all ungalvanised parts shall be completely clean and free from rust, scale and grease, and all external surface cavities on castings shall be filled by metal deposition. 1.6.2 All blast cleaned surfaces (except machined faces that have to be protected) must be cleaned in accordance with ISO specification no. ISO 8501 Part 1(This standard specification is based on and now supersedes Swedish Standard SIS 05 59 00) to a minimum standard of ‘ASa2½’ or ‘BSa2½’ prior to paint application. 1.6.3 External and internal surfaces of all transformer tanks and chambers and other fabricated steel items shall be cleaned of scale, rust and surface dirt by blast cleaning or other suitable approved method. After cleaning, these surfaces should be immediately covered with paint. Hot oil resistant varnish on white synthetic enamel/epoxy paint is to be used for painting the inside of all oil filled chambers, including transformer tanks. Only one thin layer ( ≅ 25 microns) of this is to be applied. 1.6.4 Except for hardware, which may have to be removed at site, all external surfaces shall receive at least four coats of paint. The type and thickness of paint shall be chosen to suit pollution level at site. 1.6.5 Selection of paint system for different environmental conditions shall be in line with ISO : 12944. 1.6.6 For rural or mild atmosphere, alkyd enamel primer and finish system may be used in four coats to give a total dry film thickness of at least 80 microns.


4

1.6.7 For urban or industrial situation two coats of epoxy zinc phosphate or zinc chromate primer topped with two coats of aliphatic polyurethane glossy finish paint is recommended. The total dry film thickness should preferably be between 100 and 130 microns. 1.6.8 In case of highly polluted area, chemical atmosphere or at a place very near the sea coast, paint as above with one intermediate coat of high build MIO (Micaceous iron oxide) as an intermediate coat may be used to give a total dry film thickness of 150 to 180 microns. 1.6.9 All interior surfaces of chambers or kiosks that are in contact with air shall receive at least three coats of paint, of which the topcoat shall be of a light shade. If heaters are not provided in the chamber, then the top coat should be of anti condensation type. 1.6.10 Any scratch, bruise or paint damage incurred during transportation and unloading at site should be made good by the purchaser as soon as the damage is detected. This is to be done by thoroughly cleaning the damaged area and applying the full number of coats as was applied originally. Manufacturer should supply the necessary paint for this touch up painting at site. 1.6.11 One coat of additional paint shall be given at site over all external surfaces, including hardware, after erection by the purchaser. Supplier shall furnish necessary information on the make and grade of the top-coat paint. In general, it is possible to apply enamel paint over epoxy polyurethane coating and the vice versa is not recommended. As far as possible the make and grade of the recoat shall be same as the original coat. 1.7

Oil

1.7.1 The transformers and all associated oil-filled equipment shall normally be supplied alongwith the first filling of oil and 10 percent excess quantity of oil shall also be supplied in non-returnable drums. The oil shall conform to IS : 335. Alternatively, if the purchaser so desires, oil may be supplied in tankers directly from the refinery for transformers which are despatched from factory to site in gas filled condition. 1.8

Prevention of Acidity

1.8.1 The design and all materials and processes used in the manufacture of the transformer, shall be such as to reduce to a minimum the risk of the development of acidity in the oil. Special measures, such as nitrogen sealing or the use of inhibited oil shall not be resorted to, unless otherwise specified by the purchaser. 2.0

ELECTRICAL CHARACTERISTICS AND PERFORMANCE

2.1

Type of Transformers and Operating Conditions

2.1.1 All transformers, unless otherwise specified shall be oil immersed and may be either core or shell type and shall be suitable for outdoor installation. Normally oil immersed transformer shall be provided with conservator vessels. Where sealed transformers are specified, there shall be no conservator but adequate space shall be provided for expansion of oil without developing undue pressure. The types of cooling shall be as stated in the relevant specifications.

Fig. 1 Typical lables and dial plates

General

5


6

2.1.2 Transformers designed for mixed cooling shall be capable of operating under the natural cooled condition upto the specified load. The forced cooling equipment shall come into operation by pre-set contacts in WTI and the transformer will operate as a forced cooled unit. 2.1.3 Transformer shall be capable of remaining in operation at full load for 10 minutes after failure of the oil and/or water circulating pumps or blowers without the calculated winding hot-spot temperature exceeding l50o C. Transformer fitted with two coolers each capable of dissipating 50 percent of the losses at Continuous Maximum Rating (CMR) shall be capable of remaining in operation for 20 minutes in the event of failure of the oil and/or water circulating pumps or blowers associated with one cooler without the estimated winding hot-spot temperature exceeding l50o C. 2.2

Continuous Maximum Rating and Overloads

2.2.1 Transformers provided with mixed cooling shall comply, as regards its rating, temperature rise and overloads, with the appropriate requirements of IS : 2026 when operating with natural cooling and with mixed cooling. 2.2.2 All transformers, except where stated shall be capable of operation continuously, in accordance with IS loading guide at their CMR and at any ratio. In case bi-directional flow of power is required, that shall be specifically stated by the purchaser. 2.2.3 Temperature rise test shall be performed at the tapping as desired by the purchaser. If nothing has been stated by the purchaser, the test shall be carried out at the tapping with the highest load losses. 2.2.4 The transformer may be operated without danger on any particular tapping at the rated kVA provided that the voltage does not vary by more than + 10 percent of the voltage corresponding to the tapping. 2.2.5 The transformer shall be suitable for continuous operation with a frequency variation of + 3% from normal 50 Hz. Combined voltage and frequency variation should not exceed the rated V/f ratio by 10%. Note : Operation of a transformer at rated kVA at reduced voltage may give rise to excessive losses and temperature rise

2.3

Voltage Ratio

2.3.1 The voltage between phases on the higher and lower voltage windings of each transformer measured at no-load and corresponding to the normal ratio of transformation shall be those stated in the ordering schedule. 2.3.2 Means shall be provided in accordance with clauses 8 and 9 for varying the normal ratio of transformation.

General

2.4

7

Electrical Connections

2.4.1 Transformers shall be connected in accordance with the IS vector symbol specified in ordering schedule of the requirements. 2.4.2 Auto connected and star/star connected transformers shall have delta connected stabilising windings if specified in the order. Two leads from one open corner of the delta connection shall be brought out to separate bushings. Links shall be provided for joining together the two terminals so as to complete the delta connection and earthing it external to the tank. 2.5

Duty under Fault Conditions

2.5.1 Except where modified below, it is to be assumed that the capacity of generating plants simultaneously connected is such that normal voltage will be maintained on one side of any transformer when there is a short-circuit between phases or to earth on the other side. Any transformer may be directly connected to an underground or overhead transmission line and switched into and out of service together with its associated transmission line. 2.5.2 All transformers shall be capable of withstanding any external short-circuit according to IS : 2026 without damage. 2.5.3 Transformers with tertiary windings shall be capable of withstanding the mechanical and thermal effects of any external short-circuit to earth with the short-circuit MVA available at the terminals not exceeding the values given in the ordering schedule with the neutral points on both HV and LV windings directly connected to earth as per the requirements of IS : 2026. 2.5.4 Transformers directly connected to generator (generator step-up transformers) shall be designed for exceptional circumstances arising due to sudden disconnection of the load and shall be capable of operating at approximately 25 percent above normal rated voltage for a period not exceeding one minute and 40 percent above normal rated voltage for a period of 5 seconds. However, the purchaser will install the over fluxing protection device in case of generator step-up transformers. Note : All inter-connected tra nsformers of 50MVA and above shall also be provided with over fluxing protection device by the purchaser.

2.6

Stabilising Windings

2.6.1 If specified in the order, the stabilising winding shall be capable of carrying continuously the load specified therein. 2.6.2 The design of stabilising winding shall be such as to take care of the effect of transferred surges and the tenderer shall offer suitable surge protection wherever necessary.


8 2.7

Losses

2.7.1 The accepted losses of each transformer shall be stated in the order. The tolerance on the losses of each transformer shall be in accordance with IS : 2026. 2.8

Regulation and Impedance

2.8.1 The impedance voltage at principal tap and rated kVA shall be stated in the order and tolerance shall be in accordance with IS : 2026. 2.8.2 For all transformers, the value of impedance on any other tapping shall be generally subject to the approval of the purchaser at the time of order. Any specific requirement may be mentioned at the time of enquiry as a prequalification instead of at the time of order. 2.9

Flux Density

2.9.1 The maximum flux density in any part of the core and yokes, of each transformer at normal voltage and frequency shall be such that the flux density in over-voltage condition as per clause 2.2.5 shall not exceed 1.9 Tesla (19,000 lines per cm2 ) However, in case of transformers with variable flux the voltage variation which would affect flux density at every tap shall be kept in view while designing transformers. 2.10 Vibration and Noise 2.10.1 Every care shall be taken to ensure that the design and manufacture of all transformers and auxiliary plant shall be such as to have minimum noise and vibration levels following good modern manufacturing practices. 2.10.2 The manufacturers will ensure that the noise level shall not exceed the figures as per Table 0 -1 of NEMA Pub. No. TR - 1. 2.11 Suppression of Harmonics 2.11.1 All the transformers shall be designed with particular attention to the suppression of harmonic voltage, especially the third and fifth, so as to eliminate wave-form distortion and from any possibility of high frequency disturbances, inductive effects or of circulating currents between the neutral points at different transforming stations reaching such a magnitude as to cause interference with communication circuits. 3.0

CORES

The cores shall be constructed from high grade cold rolled non-ageing grain oriented silicon steel laminations. 3.1

Magnetic Circuit

3.1.1 The design of the magnetic circuit shall be such as to avoid static discharges, development of short-circuit paths within itself or to the earthed clamping structure and the production of flux components at right angles to the plane of the laminations which may cause local heating.

General

9

3.1.2 Every care shall be excercised in the selection, treatment and handling of core steel to ensure that as far as is practicable, the laminations are flat and the finally assembled core is free from distortion. 3.1.3 Adequate oxide/silicate coating is to be given on the core steel. However, laminations can be insulated by the manufactures if considered necessary. 3.1.4 Oil ducts shall be provided where necessary to ensure adequate cooling. The winding structure and major insulation shall not obstruct the free flow of oil through such ducts. Where the magnetic circuit is divided into pockets by cooling ducts parallel to the planes of the laminations or by insulating material above 0.25 mm thick, tinned copper strip bridging pieces shall be inserted to maintain electrical continuity between pockets. 3.1.5 The framework and clamping arrangements shall be earthed in accordance with clause 5.2. 3.1.6 When insulation is provided for the core to core bolts and core to clamp plates, the same shall withstand a voltage of 2000 V AC for one minute. 3.1.7 For consideration of overfluxing the transformer shall be suitable for continuous operation for values of overfluxing factor upto 1.1, this factor being v/vm X fn /f. The manufacturer shall state the overfluxing capability and corresponding withstand durations for the transformers for factors 1.1, 1.25 and 1.4. 3.2

Mechanical Construction of Cores

3.2.1 All parts of the cores shall be of robust design capable of withstanding any shocks to which they may be subjected during lifting, transport, installation and service. 3.2.2 All steel sections used for supporting the core shall be thoroughly sand blasted or shot blasted after cutting, drilling and welding. Any non-magnetic or high resistance alloy shall be of established quality. 3.2.3 Adequate lifting lugs shall be provided to enable the core and windings to be lifted. 3.2.4 Adequate provision shall be made to prevent movement of the core and winding relative to the tank during transport and installation or while in service. 3.2.5 The supporting framework of the cores shall be so designed as to avoid the presence of pockets which would prevent complete emptying of the tank through the drain valve, or cause trapping of air during filling. 4.0

WINDINGS

4.1

General

4.1.1 All star connected windings for system of 66 kV and above shall have graded insulation as defined in IS : 2026. All windings for system voltages lower than 66 kV shall be fully insulated. All neutral points shall be insulated for the voltages specified in IS : 2026.


10

4.1.2 Power transformers shall be designed to withstand the impulse and power frequency test voltages as specified in IS : 2026. 4.1.3 The windings shall be designed to reduce to a minimum the out-of-balance forces in the transformer at all voltage ratios. 4.1.4 The insulation of transformer windings and connection shall be free from insulating composition liable to soften, ooze out, shrink or collapse and be non-catalytic and chemically inactive in transformer oil during service. 4.1.5 The stacks of windings shall receive adequate shrinkage treatment before final assembly. Adjustable devices shall be provided for taking up any possible shrinkage of coils in service. 4.1.6 The coil clamping arrangement and the finished dimensions of any oil ducts shall be such as will not impede the free circulation of oil through the ducts. 4.1.7 No strip conductor wound on edge shall have a width exceeding generally six times its thickness. 4.1.8 The conductors shall be transposed at sufficient intervals in order to minimise eddy currents and equalise the distribution of currents and temperatures along the windings. 4.2

Bracing of Windings

4.2.1 The windings and connections of all transformers shall be braced to withstand shocks which may occur during transport, or due to switching short-circuit and other transient conditions during service. 4.2.2 Coil clamping rings, if provided, shall be of steel or of suitable insulating material. 5.0

INTERNAL EARTHING ARRANGEMENTS

5.1

General

5.1.1 All metal parts of the transformer with the exception of the individual core laminations, core bolts and associated individual clamping plates shall be maintained at same fixed potential. 5.2

Earthing of Core Clamping Structure

5.2.1 The top main core clamping structure shall be connected to the tank body by a copper strap. The bottom clamping structure shall be earthed by one or more of the following methods: (a)

By connection through vertical tie-rods to the top structure

(b)

By a connection to the top structure on the same side of the core as the main earth connection to the tank

General

5.3

11

Earthing of Magnetic Circuit

5.3.1 The magnetic circuit shall be earthed to the clamping structure at one point only through a link placed in an accessible position beneath an inspection opening in the tank cover. The connection to the link shall be on the same side of the core as the main earth connection. The link should be brought out using bushing/terminal board on all transformers above 31.5 MVA. 5.3.2 When magnetic circuits are subdivided into separate isolated sections by ducts perpendicular to the plane of laminations all such sections should be earthed. 5.4

Earthing of Coil Clamping Rings

5.4.1 Where coil clamping rings are of metal at earth potential, each ring shall be connected to the adjacent core clamping structure on the same side of transformer as the main earth connections. 5.5

Size of Earthing Connections

5.5.1 All earthing connections with the exception of those from the individual coil clamping rings shall have a cross-sectional area of not less than 0.8 cm2 . Connections inserted between laminations of different sections of core as per clause 5.3.2 shall have a cross-sectional area of not less than 0.2 cm2 . 6.0

TANKS

6.1

Tank Construction

(a)

All transformer reactor tanks should generally be of conventional type i.e., tank body with top cover, Bell shaped construction can be specified for 100 MVA and higher rating transformer unless otherwise specified.

(b)

Top cover of conventional type transformer and Bell type construction may be bolted or welded to the tank body rim. Inspection covers shall always be bolted type.

6.1.1 The transformer tank and cover shall be fabricated from low carbon steel suitable for welding and of adequate thickness. The tanks of all transformers shall be complete with all accessories and shall be designed so as to allow the complete transformer in the tank and filled with oil, to be lifted by crane or jacks, transportation by road, rail or ship/boat without over straining any joints and without causing leakage of oil. 6.1.2 The transformer conservator tank, if equipped with an air cell, need not be designed for full vacuum but a vacuum-tight valve should be provided in the Buchholz relay pipe connection. Alternatively an equalising connection may be provided between the inside of air cell and conservator for evacuating the conservator along with air cell, which may be removed after evacuation and oil filling.


12

6.1.3 The main tank body excluding tap-changing compartments, radiators and coolers shall be capable of withstanding vacuum given in the following table : Highest system voltage kV

MVA rating

Up to 72 kV

Up to 1.6 above 1.6 and up to 20 above 20 for all MVA ratings

Above 72 kV

Vacuum gauge pressure kN/m 2

(mm of Hg)

34.7 68.0 100.64

250 500 760

100.64

760

6.1.4 The base of each tank shall be so designed that it shall be possible to move the complete transformer unit by skidding in any direction without any damage when using plates or rails. 6.1.5 Normally a detachable underbase will be used, but in case transport facilities permit, a fixed underbase can be used. 6.1.6 Where the base is of a channel construction, it shall be designed to prevent retention of water. 6.1.7 Tank stiffners shall be designed to prevent retention of water. 6.1.8 Wherever possible the transformer tank and its accessories shall be designed without pockets where gas many collect. Where pockets cannot be avoided, pipes shall be provided to vent the gas into the main expansion pipe. The vent pipes shall have a minimum inside diameter of 15 mm except for short branch pipes which may be 6 mm minimum inside diameter. 6.1.9 All joints other than those which may have to be broken shall be welded when required they shall be double welded. All bolted joints to the tank shall be fitted with suitable oil-tight gaskets which shall give a satisfactory service under the operating conditions and guaranteed temperature rise conditions. Special attention shall be given to the methods of making hot oil tight joints between the tank and the cover as also between the cover and the bushing and all other outlets to ensure that the joints can be remade satisfactorily at site and with ease with the help of semi-skilled labour. 6.2

Lifting and Haulage Facilities

6.2.1 Each tank shall be provided with : (a)

Lifting lugs suitable for lifting the transformer complete with oil

(b)

A minimum of four jacking lugs, in accessible positions to enable the transformer complete with oil, to be raised or lowered using hydraulic or screw jacks. The minimum height of the lugs above the base shall be: -

Transformers upto and including 10 tonnes weight - 300 mm (approx.) so as to accommodate suitable jacks beneath the jacking parts

General

(c)

13

Transformers above 10 tonnes weight - 500 mm (approx.) so as to accommodate suitable jacks beneath the jacking lugs

Suitable haulage holes shall be provided

6.2.2 To facilitate safe handling at site, the longitudinal and transverse axes and the centre of gravity of main transformer tank should be marked permanently on all four sides. 6.3

Tank Cover

6.3.1 Each tank cover shall be of adequate strength, and shall not distort when lifted. Inspection openings shall be provided as necessary to give easy access to bushings or changing ratio or testing the earth connection. Each inspection opening shall be of ample size for the purpose for which it is provided and at least two openings one at each end of the tank, shall be provided. 6.3.2 The tank cover and inspection covers shall be provided with suitable lifting arrangements. Unless otherwise approved inspection covers shall not weigh more than 25 kg each. 6.3.3 The tank cover shall be fitted with pockets for a thermometer and for the bulbs of oil and winding temperature indicators. Protection shall be provided, where necessary, for each capillary tube. 6.3.4 The thermometer pocket shall be fitted with a captive screwed top to prevent the ingress of water. 6.3.5 The pockets shall be located in the position of maximum oil temperature at CMR and it shall be possible to remove the instrument bulbs without lowering the oil in the tank. 6.4

Axles and Wheels

6.4.1 Requirement of the roller will be specified for plinth mounted transformers. If required only one set of roller of each size to be asked for. 6.4.2 If specified, transformers are to be provided with wheels and axles. They shall be of such dimensions and so supported that under any service conditions they shall not deflect sufficiently to interfere with the movement of the transformer. Suitable locking arrangements will be provided to prevent the accidential movement of the transformer. 6.4.3 All wheels should be detachable and shall be made of cast iron or steel as required. 6.4.4 Wherever specified, flanged wheels shall be provided suitable for use on gauge track as specified in the detailed specification and shall be so placed that pinchbar can be used to move the transformer. 6.4.5 The direction of motion shall be specified in case of undirectional movement.

14


6.4.6 If wheels are required to swivel, they shall be arranged so that they can be turned through an angle of 90° when the tank is jacked up clear of the rails or floor. Means shall be provided for locking the swivel movements in positions parallel to and at right angles to the longitudinal axis of the tank. 6.5

Conservator Vessels, Oil Gauges and Breathers

6.5.1 A conservator complete with sump and drain valve shall be provided in such a position as not to obstruct the electrical connections to the transformer having a capacity between highest and lowest visible levels of 7.5% of the total cold oil volume in the transformer and cooling equipment. The minimum indicated oil level shall be with the feed pipe from the main tank covered with not less than 15 mm depth of oil and the indicated range of oil level shall be from minimum to maximum. 6.5.2 If the sump is formed by extending the feed pipe inside the conservator vessel, this extension shall be for at least 25 mm. The conservator shall be designed so that it can be completely drained by means of the drain valve provided, when mounted as in service. 6.5.3 One end of the conservator shall be bolted into position so that it can be removed for cleaning purposes. 6.5.4 Normally one oil gauge, magnetic/prismatic/plain type as specified shall be provided. 6.5.5 The oil level at 30°C shall be marked on the gauge. 6.5.6 Taps or valves shall not be fitted to oil gauge. 6.5.7 The oil connection from the transformer tank to the conservator vessel shall be arranged at a rising angle of 3 to 9 degrees to the horizontal up to the Buchholz Relay and shall consist of : (a)

For transformers up to and including 1000 kVA 25 mm inside diameter pipes as per IS : 3639

(b)

For transformers from 1001 to 10,000 kVA 50 mm inside diameter pipes as per IS: 3639

(c)

For transformers of over 10,000 kVA 80 mm inside diameter pipes as per IS : 3639

6.5.8 A valve shall be provided at the conservator to cut-off the oil supply to the transformer, after providing a straight run of pipe for at least a length of five times the internal diameter of the pipe on the tank side of the gas and oil actuated relay and at least three times the internal diameter of the pipe on the conservator side of the gas and oil actuated relay. 6.5.9 Each conservator vessel shall be fitted with a breather in which silica gel is the dehydrating agent and designed so that : (a)

The passage of air is through the silica gel.

General

15

(b)

The external atmosphere is not continuously in contact with the silica gel.

(c)

The moisture absorption indicated by a change in colour of the tinted crystals, can be easily observed from distance.

(d)

All breathers shall be mounted at approximately 1,400 mm above ground level.

(e)

Self indicating (blue) silica gel contains the dye cobalt chloride which has potential health hazards. An alternative to the blue self indicating silica gel is SILICA GEL ORANGE with an organic indicator. The colour changes from orange to light yellow as it absorbs moisture.

6.5.10 One non-return valve, which may automatically cut off the flow of oil from conservator towards the main tank may be provided in the pipe connection between the Buchholz relay and conservator for transformers of 100 MVA and above. 6.6

Filter and Drain Valves sampling Devices and Air Release plugs

6.6.1 Each transformer shall be fitted with the following : (a)

The filter and drain valves as specified.

(b)

A drain valve as specified below shall be fitted to each conservator. For diameter up to 650 mm: Size of the valve 15 mm. For diameter above 650 mm : Size of the valve 25 mm.

(c)

A robust oil sampling device shall be provided at the top and bottom of the main tank. The sampling device shall not be fitted on the filter valves specified under (a) above.

(d)

One 15 mm air release plug.

(e)

For transformers above 100 MVA rating, one 100 mm bore valve shall be provided for attaching vacuum connection and with provisions for attaching a vacuum gauge, a pressure gauge or an oil level indicator.

6.6.2 All other valves opening to atmosphere shall be fitted with blank flanges. 6.7

Cooler and Radiator Connections

Valves and valve mountings shall be provided as specified under "Cooling Plant" Clause 7. 6.7.1 All valves shall be of gun-metal or cast steel or may have cast iron bodies with gunmetal fittings. They shall be of full way type with internal screw and shall be opened by turning counter clock-wise when facing the handwheel.


16

6.7.2 Means shall be provided for padlocking the bottom valves in the open and closed positions. This is required for the valves where opening device like hand-wheel, keys, etc., are the integral part. 6.7.3 Every valve shall be provided with an indicator to show clearly the position of the valve. 6.7.4 All valves shall be provided with flanges having machined faces. 6.7.5 The drilling of valve flanges shall comply with the requirements of IS : 3639. 6.8

Pressure Relief Device

6.8.1 The pressure relief device shall be provided of sufficient sizes for rapid release of any pressure that may be generated within the tank, and which might result in damage to the equipment. The device shall operate at a static pressure of less than the hydraulic test pressure for transformer tank. Means shall be provided to prevent the ingress of rain water. 6.8.2 Unless otherwise approved the relief device shall be mounted on the main tank, and, if on the cover, shall be fitted with skirt projecting 25 mm inside the tank and of such a design to prevent gas accumulation. 6.8.3 If a diphragm is used it shall be of suitable design and material and situated above maximum oil level. 6.8.4 If a diaphragm is put at the base of pipe, an oil gauge is required on the stand pipe for indicating fracture of diaphragm. 6.8.5 One of the following methods shall be used for relieving or equalising the pressure in the pressure relief device: (a)

An equaliser pipe connecting the pressure relief device to the conservator, or

(b)

The fitting of a silica gel breather to the pressure relief device. The breather being mounted in a suitable position for access at ground level.

6.8.6 If specified, the pressure relief valve (spring operated type) capable of releasing the pressure in the tank when it rises above a predetermined safe limit, shall be provided. It shall be provided with a microswitch for actuating trip contact when it operates. It shall also give a visual indication of valve operation by raising a flag. The flag and the switch shall remain operated until they are reset manually. The operating pressure of the pressure relief valve shall always be less than the tank test pressure. The microswitch shall have IP 55 protection and the fasteners shall be of rust proof material. 6.8.7 An oil splashguard shall be provided to the pressure relief device to restrict spillage of hot oil in the event of operation of the pressure relief device.

General

6.9

17

Accommodation for Auxiliary Apparatus

6.9.1 If specified, facilities shall be provided for the mounting of internal/external neutral current transformer(s) adjacent to the neutral terminal(s) and tank. 6.10 Earthing Terminal 6.10.1 Two earthing terminals capable of carrying for 4 seconds the full lower voltage, shortcircuit current of the transformer. Provision shall be made at positions close to each of the bottom two corners of the tank for bolting the earthing terminals to the tank structure to suit local conditions. The design of earthing terminals shall be as per IS 3639 - Part 3 (Fittings and accessories for Power Transformers Part 3 : Earth Terminals. 6.11 Rating, Diagram and Property Plates 6.11.1 The following plates shall be fixed to the transformer tank at an average height of about 1750 mm above ground level as shown in Figs. 2 and 3. (a)

A rating plate bearing the data specified in the appropriate clauses of IS : 2026

(b)

A diagram plate showing the internal connections and also the voltage vector relationship of the several windings in accordance with IS: 2026 and in addition a plan view of the transformer, giving the correct physical relationship of the terminals. When links are provided in accordance with clause 2.3 for changing the transformer ratio, then approved means shall be provided for clearly indicating ratio for which the transformer is connected. No load voltage shall be indicated for each tap.

(c)

Where specified a plate showing the location and function of all valves and air release cocks or plugs is to be provided. This plate shall also warn operators to refer to the maintenance instructions before applying the vacuum treatment for drying (Fig. 4).

6.11.2 The above plates shall be of material capable to withstanding continuous outdoor service. 6.12 Joints and Gaskets 6.12.1 All gaskets used for making oil tight joints shall be of proven material such as granulated cork bonded with synthetic rubber or synthetic rubber gaskets conforming to IS : 4253, unless otherwise specified.

18


Fig. 2 Typical rating and diagram plate for OFAF cooled power transformer with on-load tapchanger

General

Fig. 3 Typical rating and diagram plate of off circuit OFWF cooled power transformer

19


20

Fig. 4 Typical valve schedule for power transformer

7.0

COOLING PLANT

7.1

General

7.1.1 Radiators and coolers shall be so designed as to avoid pockets in which moisture may collect and shall withstand the pressure tests.

General

21

7.1.2 Unless the pipe work is shielded by adequate earthed metal the clearance between all pipe work and live parts shall be more than the clearance for live parts to earth. 7.2

Radiators Mounted Directly to the Tank/Banked

7.2.1 Detachable radiators as per section I of this manual. 7.2.2 Valves shall be provided on the tank at each point of connection to the tank. 7.2.3 Where separate radiator banks are provided, the conservator vessels specified in clause 6.5 can be mounted thereon. 7.2.4 All coolers shall be suitable for mounting on a flat concrete base. 7.2.5 The oil circuit of all coolers shall be provided with the following: (a)

A valve at each point of connection to the transformer tank

(b)

Removable blanking plates to permit the blanking off the main oil connection of each cooler.

(c)

A drain valve of 25 mm at the lowest point of each bank of cooler

(d)

A thermometer pocket fitted with a captive screwed cap on the inlet and outlet oil branches of each separately mounted cooler bank.

(e)

A filter valve as specified in clause 6.6 at the top and bottom of each cooler bank of cooler.

(f)

Air release plugs of 15 mm.

7.2.6 In addition the following are to be provided only with water cooled oil coolers which shall be as per IS : 6088 : (a)

A suitable differential pressure gauge or equivalent suitable device fitted with electrical contacts to give an alarm when differential pressure between cooler oil outlet and water inlet pressure drops below a preset value.

(b)

Oil and water flow switches, fitted with electrical contacts, in the pipework adjacent to the coolers.

7.2.7 The disposition of flow indicators is to be as shown in Fig. 5. 7.2.8 Water cooled oil coolers shall be double tube type in which water shall circulate through the inner tube and oil in between the outer tube and shell. The design of shell and tube assembly shall be such as to facilitate cleaning without any risk of water mixing with the oil. The material of the tube plates and tube shall be such that corrosion shall not take place due to galvanic action. A water analysis report shall be furnished, in time, to enable supplier to ensure a suitable material for tube and tube plates.


22

7.2.9 Any leakage which may take place in the oil cooler shall be of the oil into the water and not the reverse, and means shall be provided to ensure that the pressure of the oil in the cooler is always greater than the pressure of the water. The water pressure in the cooler will be kept as low as possible. Further, the cooling water discharge should be free to the atmosphere to reduce the pressure in the cooler. 7.3

Oil Piping and Flanges

7.3.1 The necessary oil piping shall be provided for connecting each transformer to the coolers and oil pumps. The oil piping shall be with flanged gasketed joints. Cast iron shall not be used. 7.3.2 The drilling of all water and oil pipe flanges shall comply with IS: 3639 and IS: 1536 (Section I -specification for valves for transformers.)

Fig. 5 Flow indicators and alarms

General

23

7.3.3 A suitable expansion piece shall be provided in each oil pipe connection between the transformer and the separately mounted oil coolers. 7.3.4 Drain valves/plugs shall be provided in order that each section of pipework can be drained independently. 7.4

Oil Pumps

7.4.1 Each forced oil cooler shall be provided with a motor driven oil pump of the submerged motor type and of adequate capacity. It shall be possible to remove the pump and motor from the oil circuit without having to lower the level of oil in the transformer or coolers and without having to disturb the pump foundation fixing. Oil pump shall be capable of dealing with the maximum output of transformer and total head which may occur in service and with the varying head due to changes in the viscosity of the oil. 7.4.2 Each pump assembly shall be furnished with oil flow indicator with alarm contacts to indicate normal pump operation and oil flow. 7.4.3 For mixed type cooling, the pump should be of axial flow type to permit oil circulation when pump is idle. 7.4.5 Under no circumstances, the degree of forced circulation create a static electrification hazard in any part of a transformer under any operating condition. 7.5

Air Blowers and Ducts

7.5.1 Air blowers for use with oil coolers or for air blast cooling shall be motor driven. They shall be suitable for continuous operation outdoors and capable of dealing with the maximum output and total head required in service. The bearings shall be of sealed type, which does not require frequent lubrication. 7.5.2 Air blowers shall be capable of withstanding the stresses imposed when brought up to full speed by the direct application of full line voltage to the motor. 7.5.3 Air blowers shall be complete with all necessary air ducting and coolers shall be designed so that they operate with a minimum of noise or drumming. In order to reduce the transmission of noise and vibration the blowers shall be either mounted independently from the coolers or, alternatively, an approved form of antivibration mounting shall be adopted. It shall be possible to remove the blower complete with motor without disturbing or dismantling the cooler structure framework. 7.5.4 Blades or runners fabricated to form hollow sections shall not be used. 7.5.5 Blades shall be suitably painted for outdoor use. 7.5.6 If fans are mounted at a height less than 2.5 m suitably painted wire-mesh guards with a mesh not greater than 25 mm shall be provided to prevent accidental contact with the blades. Fans mounted at more than 2.5 m height shall be provided with outside guards against birdage. Guards shall be provided over all moving shaft and couplings.


24 7.6

Motors

7.6.1 Motors shall be of the squirrel cage totally enclosed weather-proof type and shall comply with Indian Standards as applicable for continuously rated machine. The motors shall be capable of operating at all loads without undue vibration and with a minimum of noise. They shall be suitable for direct starting and for continuous running from 415-240 volts three-phase, 4 wire 50 Hz supply. 7.6.2 All motors shall be capable of continuous operation at any frequency between 48 and 51 Hz, together with any voltage within 5 percent of the nominal value. Motors upon which the primary equipment depends for its continued operation at full load shall also be capable of continuous operation at 85 percent of the nominal voltage at normal frequency without injurious over-heating. 7.6.3 All motors shall have ball or roller bearings and grease lubricators shall be fitted with hexagon nipples to relevant Indian Standard. 7.6.4 Vertical spindle motors shall have bearings capable of withstanding the thrust due to the weight of the moving parts and the action of impeller. 7.6.5 The stator windings shall be adequately braced and suitably impregnated to render them non-hygroscopic and oil resistant. Weather-proof motors shall be provided with suitable means of breathing and drainage to prevent accumulation of water. 7.6.6 Each terminal box shall be fitted with means of terminating the external wiring for outdoor use. 7.6.7 Varnished cambric or glass insulation shall be used for connections from the winding to the terminals. All motor terminals shall be of the stud type and totally enclosed. 7.6.8 Each pump, or blower and its motor shall be mounted on a common base plate and the drive shall be direct. 7.7.

Cooler Control

7.7.1 Each motor or group of motors shall be provided with a three pole electrically operated contactor and with control gear of suitable design both for starting and stopping the motor manually and also automatically from the contacts on the winding temperature indicating device specified in clause 13. Additional terminals for remote manual electrical control of motors shall be provided. Overload and single phasing protection shall be provided but no-volt release shall not be fitted. HRC fuses shall be provided for the main supply. This equipment shall be accommodated in the marshalling box specified in clause 15. 7.7.2 Where small motors are connected in groups, the group protection shall be arranged so that it operates satisfactorily in the event of a fault occurring on a single motor. 7.7.3 Where blowers and oil pumps are provided, the connections shall be arranged as to allow the motors or groups of motors to be started up and shutdown either collectively or individually. 7.7.4 All motor contactors and their associated apparatus shall be capable of holding in and operating satisfactorily and without over heating for a period of ten minutes if the supply

General

25

voltage falls for the period, to 75 per cent of normal at normal frequency. The motor contactors and associated apparatus shall be capable of normal operation with a supply voltage of 85 per cent of the normal value and at normal frequency. 7.7.5 All contacts and other parts which may require renewal, adjustment or inspection shall be readilly accessible. 7.7.6 The control arrangements are to be so designed as to prevent the simultaneous starting of motors of a total rating of more than 20 HP. 7.7.7 Alarm indication for failure of group of fans and oil pump shall be provided. 7.7.8 Alarm indication shall be provided to indicate failure of power supply. 7.7.9 The start up or shut down of any pump or combination of pumps must not cause maloperation of any gas and oil actuated relay. 7.7.10 For transformers with OFWF cooling required to meet peak load requirements and are thus switched on or off during the day, the oil pump shall be kept running when the transformer is off for a short period but water circuit is switched off. In case the transformer is switched off for a longer time, the oil pump can also be switched off but it shall be run at least one hour earlier before the transformer is energised again. 8.0

VOLTAGE CONTROL (OFF-CIRCUIT TYPE)

Voltage Control (off-circuit type) should conform to section N of the specification. 9.0

VOLTAGE CONTROL (ON-LOAD TYPE)

Voltage control (on-load type) should conform to section N of the specification. 10.0 PARALLEL OPERATION TAPCHANGER

OF

TRANSFORMERS

WITH

ON-LOAD

10.1 Besides the local and remote electrical control specified in clause 9, on-load tapchangers, when specified, should be suitable for remote electrical parallel control as in clause 10.2. 10.2 Remote Electrical Parallel Control 10.2.1 In addition to the methods of control as in clause 9, the following additional provision shall be made. 10.2.2 Suitable selector switch be provided, so that any one transformer of the group can at a time be selected as "Master", "Follower" or "Independent". 10.2.3 Necessary interlock blocking independent control when the units are in parallel, shall be provided. 10.2.4 The scheme will be such that only one transformer of a group can be selected as "Master". 10.2.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tapchanging when transformers in a group operating in "Parallel control" are one tap out-of-step.


26

11.0 BUSHING INSULATORS AND TERMINALS The bushing should comply with IS 2099, IS 12676 and section P of this specification. The over voltage power frequency test level or the BIL of bushings should be one step higher than that of the windings. 11.1 Transformers shall be fitted either with bushing insulators or with cable boxes, as stated in order. Where accommodation for current transformers is required on 72.5 kV bushings and above, the requisite details will be notified to the supplier at the time of tendering. 11.2 Special precautions shall be taken to avoid ingress of moisture into paper insulation during manufacture, assembly, transport and erection. 11.3 Each porcelain bushing or insulator, and paper bushing shall have marked upon it the manufacturer's identification mark, and such other mark as may be required to assist in the representative selection of batches for the purposes of the sample tests. 11.4 Clamps and fittings made of steel or malleable iron shall be hot dip galvanised. All fasteners of size 12 mm and above shall be hot dip galvanised and fasteners of size less than 12 mm shall be of stainless steel. 11.5 The bushing flanges shall not be of re-entrant shape which may trap air. 11.6 Bushing turrets shall be provided with vent pipes which shall be connected to route any gas collection through the Buchholz relay. The take off point of the vent pipes shall be the top most point on the bushing turret so that there will not be any air trapped in the bushing turret. 11.7 The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows: Rated System Voltage kV 11 22 33 47 66 110/132 132 220 220 400 800

Basic Insulation level kV peak 75 125 170 250 352 550 650 950 1050 1425 1950

Minimum clearances Phase to Phase to phase (mm) earth (mm) 280 330 350 530 700 1220 1430 2000 2350 4000 5800

140 230 320 480 660 1050 1270 1800 2150 3500 5000

Note : 1. These clearances are applicable for transformers to be installed up to an altitude of 1000 m above mean sea level. 2. For altitude exceeding 1000 m the clearance should be increased by 3 percent for every additional 300 m. 3. Air clearance of 3500 mm between phase to earth for 400 kV system can be relaxed by maximum 200 mm as fas as air release pipe emanating from bushing turret is concerned.

General

27

12.0 CABLE BOXES AND DISCONNECTING CHAMBERS 12.1 Cable boxes shall be suitable for terminating the cables directly or alternatively shall be in the form of sealing end-chambers for accommodation sealing ends into which the cable will be terminated, as specified in the order. 12.2 Cable boxes shall be designed to accommodate all the cable joint fittings or sealing ends required by the manufacturers of the cables, including stress/cones or other approved means for grading the voltage stress on the terminal insulation of cables operating at voltages of 22 kV and above, between phases. They shall also be provided with expansion chambers for the filling medium and means of preventing the formation of air spaces when filling. Drain plugs of ample size shall be provided to enable the filling medium to be removed quickly. 12.3 The cable boxes shall be fitted with suitable non-ferrous wiping glands with combined armour and earthing clamps. The ends of all wiping glands shall be tinned before despatch to site. Wiping glands for single core cables shall be insultated from the box. Wiping glands insulation cables shall be capable of withstanding a dry high voltage test of 2,000 volts AC for one minute. Air insulated cable boxes for PVC cables may be provided with compression glands. Sufficient wiping glands shall be provided for the termination of required number of cables. 12.4 Where cable boxes are provided for three core cables, the seating sockets on the two outer phases shall preferably be inclined towards the centre to minimise bending of the cable cores. Where there is more than one core per phase, the socket block shall be so designed as to minimise bending of the cable cores. 12.5 Where cables for 1 kV and above are terminated in the cable box, if specified an oilfilled disconnecting chamber with removable links shall be provided for testing purposes. A barrier shall be provided on both sides of the disconnecting chamber to prevent ingress of the oil used for filling the chamber into the cable box or the transformer. It shall only be necessary to remove part of the oil in the chamber itself when making the necessary testing connections. 12.6 Where sealing end chambers are provided, the disconnecting chamber may be omitted and the facilities for testing shall be provided in the sealing end chamber itself. A barrier shall then be provided between the sealing end chamber and the main tank subject to the provision of the next paragraph. 12.7 The barrier between the main tank and the disconnecting or cable sealing end chamber may be omitted, where the design is such that the cover of the disconnecting or cable sealing end chamber can be removed without lowering any oil level other than in the chamber itself, in order to make the necessary testing connections. 12.8 The disconnecting or sealing end chamber shall have a removable cover and the design of the chamber shall be such that ample clearances are provided to enable either the transformer or each cable to be subjected separately to high voltage tests when filled with transformer oil.


28

12.9 An earthing terminal shall be provided in each disconnecting or sealing end chamber to which the connectioins from the transformer winding can be earthed during cable testing. 12.10 The cable boxes and disconnecting or sealing end chambers shall also be capable of withstanding for 15 minutes, both at the time of the first tests on the cables and at any subsequent time as may be required, between phases and to earth a DC test equal to 2E kV or an AC test equal to 4E/3 kV. 12.11 During these tests the links in the disconnecting or sealing end chamber or cable box will be withdrawn and the transformer winding with connections thereto will be earthed. 12.12 Unless otherwise approved the creepage distances and clearance to earth and between phases shall not be less than those specified in Table 1. In case of compound filled cable box with shrinkable tape, the allowable minimum clearances shall be subject to the agreement between manufacturer and the user. 12.13 Cable boxes suitable for semi-fluid compound filling shall be tested with transformer oil at room temperature and at a pressure of 0.7 kg/cm2 for 12 hours during which no leakage shall occur. 12.14 Terminals shall be marked in a clear and permanent manner. 12.15 Unless otherwise specified main cabling jointing and filling of cable boxes will be carried out by the customer. However, filling medium will be supplied as a part of the cable box by the manufacturer. Table 1 Voltage class kV

1.1 3.6 6.6 12 24 36

Insulating medium

Clearance between phases (mm)

Clearance to earth direct (mm)

Creepage over porcelain to similar material (mm)

Creepage over cable surface (mm)

Air Air Air Air Compound Air Semi-fluid compound or oil Air Semi-fluid compound or oil

25 50 90 130 50 241 100 351 125

20 50 70 80 50 140 75 222 100

25 90 192 75 384 125 576 150

192 125 384 190 576 250

13.0 TEMPERATURE INDICATING DEVICES AND ALARM 13.1 Oil temperature indicator shall be provided as required in detail specification, i.e., Section D to G. 13.2 All transformers above 10 MVA shall be provided with a device for indicating hottest spot winding temperature. The device shall have a dial type indicator and in addition a

General

29

pointer to register the highest temperature reached. The number of contacts as specified will be provided. 13.3 Except where outdoor type of indicators are supplied, the temperature indicators shall be housed in the marshalling box. If specified, for transformers above 10 MVA a remote repeater indicator electrically operated from winding temperature indicator is to be provided for mounting on the control panels. Unless otherwise specified the remote repeater indicator shall be of flush mounting type. 13.4 The tripping contacts of winding temperature indicators shall be adjustable to close between 60o C and 120°C and alarm contacts to close between 50°C and 100°C and both shall re-open when the temperature has fallen by about 10°C. 13.5 The contacts used to control the cooling plant motors on the above devices shall be adjustable to close between 50o C and 100o C and re-open when the temperature has fallen by any desired amount between 15o C and 30o C. 13.6 All contacts shall be adjustable on a scale and shall be accessible on removal of the cover. Micro switches shall be preferred to mercury switches. 13.7 The temperature indicators shall be so designed that it shall be possible to check the operation of the contacts and associated equipment. 13.8 Connections shall be brought from the device to terminals placed inside the marshalling box. 13.9 Cooler failure or oil and water flow alarm shall be provided as specified in clause 7.2.6. 14.0 GAS AND OIL ACTUATED RELAYS 14.1 Each transformer shall be fitted with gas and oil actuated relay equipment to IS : 3637 having contacts which close following oil surge or low oil level conditions. Micro switches shall be preferred to mercury switches. 14.2 Each gas and oil actuated relay shall be provided with a test cock to take a flexible pipe connection for checking the operation of the relay. 14.3 Where specified to allow gas to be collected at ground level, a pipe approximately 5 mm inside diameter shall be connected to the gas release cock of the gas and oil-actuated relay and brought down to a point approximately 1.25 m above ground level, where it shall be terminated by a cock. 14.4 A machined surface shall be provided on the top of each relay to facilitate the setting of the relays and to check the mounting angle in the pipe and the cross level of the relay.


30

14.5 The design of the relay mounting arrangements, the associated pipework and the cooling plant shall be such that maloperation of the relays shall not take place under normal service conditions. 14.6 The pipework shall be so arranged that all gas arising from the transformer shall pass into the gas and oil-actuated relay. The oil circuit through the relay shall not form a delivery path in parallel with any circulating oil pipe, nor shall it be tied into or connected through the pressure relief vent. Sharp bends in the pipework shall be avoided. 14.7 When a transformer is provided with two conservators, the gas and oil actuated relays shall be arranged as follows: If the two conservators are connected to the transformer by a common oil pipe, one relay shall be installed in the common pipe. If the two conservators are piped separately to the transformer, two relays shall be installed, one in each pipe connection. Adequate clearance between oil pipework and live metal shall be provided. 15.0 MARSHALLING BOX 15.1 A steel weather and vermin proof enclosure having degree of protection IP 55 shall be provided for the transformer ancillary apparatus. The box shall have domed or sloping roofs and the interior and exterior painting shall be in accordance with clause 1.6. 15.2 The marshalling box, wherever provided shall accommodate the following equipments, alternatively weather proof instruments can be mounted outdoor. (a)

Temperature indicators

(b)

Control and protection equipment for the local electrical control of tap changer, if the same cannot be accommodated in the motor driving gear housing.

(c)

Control and protection equipment for the cooling plant; and

(d)

Terminal boards and gland plates for incoming and outgoing cables

15.3 All the above equipments except (d) shall be mounted on panels and back of panel wiring shall be used for interconnection. 15.4 The temperature indicators shall be so mounted that the dials are not more than 1600 mm from ground level and the door(s) are of adequate size. 15.5 To prevent internal condensation an approved type of metal clad heater shall be provided, controlled by a suitable thermostat.

General

31

15.6 All incoming cables shall enter the kiosks from the bottom and the gland plate shall be not less than 450 mm from the base of box. The gland plate and associated compartment shall be sealed in suitable manner to prevent the ingress of moisture. 15.7 Undrilled gland plate shall be provided for accomodating glands for incoming and outgoing cables. 16.0 CONTROL CONNECTIONS BOARD AND FUSES

AND

INSTRUMENT

WIRING,

TERMINAL

16.1 All wiring connections, terminal boards, fuses and links shall be suitable for tropical atmosphere. Any wiring liable to be in contact with oil shall have oil resisting insulation and the bared ends of stranded wire shall be sweated together to prevent creepage of oil along with wire. 16.2 There shall be no possibility of oil entering connection boxes used for cables or wiring. 16.3 Panel connections shall be neatly and squarely fixed to the panel. All instruments and panel wiring shall be run in PVC or non-rusting metal cleats of the limited compression type. All wiring to a panel shall be taken from suitable terminal boards. 16.4 Where conduits are used, the runs shall be laid with suitable falls, and the lowest parts on the-run shall be external to the boxes. All conduit runs shall be adequately drained and ventilated. Conduits shall not be run at or below ground level. 16.5 When 415 volt connections are taken through junctions boxes or marshalling boxes they shall be adequately screened and 415 "VOLTS DANGER" notices must be affixed to the outside of the junction boxes or marshalling boxes. 16.6 All box wiring shall be in accordance with relevant IS. All wiring shall be of stranded copper of 660 V grade and size not less than 4.00 sq mm for CT leads and not less than 2.5 sq mm for other connections. 16.7 All wires on panels and all multicore cables shall have ferrules which bear the same number at both ends. 16.8 At those points of interconnections between the wiring carried out by separate contractors, where a change of number cannot be avoided, double-ferrules shall be provided on each wire. The change of numbering shall be shown on the appropriate diagram of the equipment. 16.9 The same ferrule number shall not be used on wires in different circuits on the same panels. 16.10 Ferrules shall be of yellow insulating material and shall be provided with glossy finish to prevent the adhesion of dirt. They shall be clearly and durably marked in black and shall not be affected by damp or oil.

32


16.11 Stranded wires shall be terminated with tinned Ross Courtney terminals, claw washers or crimped tubular lugs. Separate washers shall be used for each wire. The size of the washer shall be suited to the size of the wire terminated. Wiring shall in general be accommodated on the sides of the box and the wires for each circuit shall be separately grouped. Back of panel wiring shall be arranged so that access to the connecting stems of relays and other apparatus is not impeded. 16.12 Wires shall not be jointed or tied between terminal points. 16.13 Wherever practicable, all circuits in which the voltage exceeds 125 volts, shall be kept physically separated from the remaining wiring. The function of each circuit shall be marked on the associated terminals boards. 16.14 Where apparatus is mounted on panels, all metal cases shall be separately earthed by means of copper wire or strip having a cross-section of not less than 2 sq mm where strip is used, the joints shall be sweated. 16.15 All wiring diagram for control and relay panel shall preferably be drawn as viewed from the back and shall show the terminal boards arranged as in service. All diagrams shall show which view is employed. 16.16 Multicore cable tails shall be so bound that each wire may be traced without difficulty to its cable. 16.17 The screens or screen pairs of multicore cables shall be earthed at one end of the cable only. The position of the earthing connections shall be shown clearly on the diagrams. 16.18 All terminal boards shall be mounted obliquely towards the rear doors to give easy access to terminations and to enable ferrule numbers to be read without difficulty. 16.19 Terminal board rows should be spaced adequately not less than 100 mm apart to permit convenient access to wires and terminations. 16.20 Terminal boards shall be so placed with respect to the cable gland (at a minimum distance of 200 mm) as to permit satisfactory arrangement of multicore cable tails. 16.21 Terminal boards shall have pairs of terminals for incoming and outgoing wires. Insulating barriers shall be provided between adjacent connections. The height of the barriers and the spacing between terminals shall be such as to give adequate protection while allowing easy access to terminals. The terminals shall be adequately protected with insulating dust-proof covers. 16.22 No live metal shall be exposed at the back of the terminal boards. 16.23 All fuses shall be of the cartridge type. 16.24 Fuses and links shall be labelled.

General

33

17.0 TESTS 17.1 Tests shall be carried out to evaluate the performance of the material and appliance generally as per the provision of IS : 2026 and as detailed out in Section' J' of this specification. 17.1.1 Where customers' inspection is specified, not less than 15 days notice shall be given to the customer in order that he may be represented. Four copies of test certificates will be supplied. 17.2 Tests are not required to be performed on bought out equipments like oil coolers, oil actuated relays, etc., at the works of the transformer manufacturer. Furnishing test certificates from the original equipment manufacturer works shall be deemed to be satisfactory evidence. Inspection of tests at the sub-contractors works will be arranged by the supplier wherever required. 17.3

Tanks

17.3.1 Routine Tests (a)

Fabrication stage:

(a1)

The tank shall be tested for leakage by being completely filled with air at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower. The pressure shall be maintained for a period of minimum one hour during which time no leakage shall occur. The equivalent air pressure corresponding to oil pressure calculated at the base of the tank to be considered for air pressure test. Permanent deflection of flat plates shall be measured on one tank of each design, if specified by customer, after the excess pressure has been released and shall not exceed the figures specified below: Horizontal length of flat plate (total length of tank wall) in mm Up to and including 750 751 to 1250 1251 to 1750 1751 to 2000 2001 to 2250 2251 to 2500 2501 to 3000 Above 3000

Permanent deflection (in mm) 5 6.5 8 9 11 12.5 16 19

(a2)

The conservator shall be tested for leakage by being completely filled with air at 35 kN/m2 . The pressure shall be maintained for a period of one hour during which time no leakage shall occur.

(a3)

The radiators shall be tested for leakage by placing them horizontally in a tank filled with clean water and applying air pressure 2 kg/cm2 for atleast 15 minutes during which time no leakage shall occur.


34 (a4)

The pipes shall be tested for leakage by applying air pressure of 4 kg/cm2 for 15 minutes during which time no leakage shall occur.

(b)

Transformer assembly stage Oil pressure test to be conducted on tank with turret and all other accessories as assembled for routine test by filling completely with oil at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower. The pressure to be maintained for eight hours during which time no leakage shall occur.

17.3.2 Type Tests (a)

Vacuum test (at fabrication stage) When required by customer, one transformer tank of each design shall be subjected to the specified vacuum as in clause 6.1.3. The tanks designed for full vacuum (760 mm of mercury at sea level or the barometric reading at the location of test) shall be tested at a maximum internal pressure of 3.33 kN/m2 (25 mm of Hg) for one hour i.e., 76025=735 mm of Hg at sea level and (Barometric reading -25) mm Hg at other location. The permanent deflection of flat plates after vacuum has been released shall not exceed the values specified in clause 17.3.1 (al) without affecting the performance of the transformer.

(b)

Pressure test When specified, one transformer tank of each design with its active part as assembled for type test (i.e., including pipe work and cooling equipment and excluding PRV and conservator when air cell is provided) shall be subjected to a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower measured at the base of the tank and will be maintained for 8 hours during which time no leakage shall occur. Before conducting the pressure test, the following are to be taken care of : (i)

Pressure relief valve/relief vent are to be removed and the opening blanked.

(ii)

Transformer and tap changer conservators are to be disconnected.

(iii)

Divertor switch compartment of tap changer to be connected with transformer tank for equalising the pressure on both sides.

(iv)

Oil should be completely filled and all trapped air released.

A list of transformer-accessories and routine test certificate required for them is given at Appendix III.

General

18.0

35

QUALITY ASSURANCE

The supplier should include a quality assurance programme (QAP) that will be used to ensure that the transformer design, materials, workmanship, tests, service capability, maintenance and documentation, will fulfil the requirements stated in the contract documents, standards, specifications and regulations. The QAP should be based on and include relevant parts to fulfil the requirements of ISO-9001. A quality plan describes: •

Lists of activities involved in design, procurement of raw materials and components, manufacture, stage inspection and final testing, preparation for despatch, delivery, installation and commissioning.

•

The identification reference of all documentation, standards, procedures, works instructions, drawings, test methods, acceptance criteria etc.

(Typical QAP format is attached for illustration as Appendix I) 19.0 A list of guaranteed technical particulars and additional technical particulars are given in Appendix II. A list of standards for transformers is given in Appendix IV.

QUALITY ASSURANCE PLAN (FOR ILLUSTRATION ONLY)

Appendix I

36 Manual on Transformers

General

37


38

Appendix II GUARANTEED TECHNICAL AND ADDITIONAL TECHNICAL PARTICULARS I.

Guaranteed Technical Particulars

1.

Name of the manufacturer and country of origin:

2.

Installation indoor/outdoor:

3.

Reference standard:

4.

Continuous ratings under service conditions specified in IS : 2026: (a)

Type of cooling:

(b)

Rating (MVA)

: HV

IV

LV

(i) With ONAN coolin g : (ii) With ONAF cooling : (iii) With OFAN cooling : (iv) With OFAF cooling : (v) With OFWF cooling : (vi) With ODAN cooling : (vii) With ODAF cooling : (viii) With ODWF cooling : (c)

(d) (e) (f)

Rated Voltage : (i) HV kV : (ii) IV kV : (iii) LV kV : Rated frequency Hz: Number of Phases: Current at rated no load voltage and on principal tap: (i) HV Amps : (ii) IV Amps : (iii) LV Amps :

5.

Connections: (i) HV : (ii) IV : (iii) LV :

6.

Connection symbol:

7.

Temperature rise: (a)

(b)

(c)

Temperature rise of oil above reference peak ambient temperature (By thermometer) ( o C) (i) At full ONAN rating (o C): (ii) At full OFAF/ODAF/OFWF/ODWF rating (o C): Temperature rise of winding above reference peak ambient temperature (By resistance method) (o C) (i) At full ONAN rating: (ii) At full OFAF/ODAF/OFWF/ODWF rating: Limit of hot spot temperature for which the transformer is designed (oC) Over the maximum yearly weighted average ambient temperature.

General 8.

Type of tap changing switch: (i) Off circuit switch/links: (ii) On load:

9.

Tappings on windings HV/IV/LV for: (i) Constant flux/variable flux/combined regulation: (ii) Location (Line/Central/Neutral) end of winding: (iii) Number of steps: (iv) Variation of (HV/IV/LV):

10.

(i) (ii) (iii)

No load loss at rated voltage and frequency at principal tap (kW): No load loss at the voltage corresponding to the highest tap (kW): Tolerance, if any, on the above values:

11.

(a)

Load loss at rated output, rated frequency and corrected for 75°C winding temperature at: Principal tap (kW): Highest tap (kW): Lowest tap (kW): Tolerance, if any, on the above values.

(i) (ii) (iii) (b) 12.

(a) (b)

39

Auxiliary losses at rated output, normal ratio, rated voltage, rated frequency and ambient temperature (kW): Tolerance, if any, on the above values.

13.

Total losses at normal ratio inclusive of auxiliary equipment losses (kW) :

14.

Positive sequence impedance on rated MVA base at rated current and frequency at 75°C winding temperature between : HV-IV HV-LV IV-LV (i) Principal tap per cent: (ii) Highest tap per cent: (iii) Lowest tap per cent: Zero sequence impedance at reference temperature of 75° C at principal tap per cent: Reactance at rated MVA base at rated current and frequency per cent: Regulation at full load and 75°C winding temperature expressed as a percentage of normal voltage: (i) At Unity Power Factor per cent: (ii) At 0.8 Power Factor (Lagging) per cent: Efficiency at 75°C winding : Temperature as derived from guaranteed loss figures and at Unity power factor (a) At full load per cent: (b) At 3/4 load per cent: (c) At 1/2 load per cent:

15. 16. 17.

18.

19.

(i) (ii)

20.

Time in minutes for which the transformer can be run at full load without exceeding the maximum permissible temperature at reference ambient temperature when: (a) (b) (c)

21.

Maximum efficiency per cent: Load at which maximum efficiency occur (per cent of full load):

Power supply to fans is cutoff but the oil pumps are working: Power supply to oil pumps is cut -off but fans are working: When power supply to both fans and pumps is cut off:

Short time rating for 2 seconds of: (a)

HV winding:


40 (b) (c)

IV winding: LV winding:

22.

Permissible over loading: (a) HV winding: (b) IV winding: (c) LV winding:

23.

Terminal arrangement: (i) High voltage (HV): (ii) Intermediate voltage (IV): (iii) Low voltage (LV): (iv) Neutral-HV/IV/LV: (v) Tertiary:

24.

Insulating and cooling medium:

25.

Test Voltage:

HV IV LV

(i)

Lightening impulse withstand test voltage (kV peak):

(ii)

Power frequency with-stand test voltage dry as well as wet for I minute (kV rms):

(iii)

Switching impulse withstand test voltage (kV peak):

26.

Partial discharge level at 1.5Um/ √ 3 kV rms (PC):

27.

Noise level when energised at normal voltage and frequency without load (db):

28.

External short circuit withstand capacity (MVA) and duration (seconds):

29.

Over flux withstand capability of the transformer:

II.

Additional Technical Particulars (These figures are indicative only. These shall not form the basis for upward or downward revision of prices).

1.

Details of Core: (a) Type of core construction: (b) Type of core joints: (c) Flux density at rated voltage and frequency and at principal tap Tesla: (d) Magnetising current at normal ratio and frequency: (i) 85 per cent of rated voltage: (ii) 100 per cent of rated voltage: (iii) 105 per cent of rated voltage: (In case kVA ratings of windings are different, this may be specified in terms of magnetising kVA) : (e) Power factor of magnetising current at normal voltage ratio and frequency: (f) (i) Material of core laminations: (ii) Thickness of core laminations (mm): (g) (i) Whether core construction is without core bolts: (ii) Insulation of core bolt: (iii) Insulation of core bolt washers: (iv) Insulation between core laminations: (v) Core bolt insulation withstand voltage for 1 min ute (kV rms): (vi) Are the core bolts grounded. If so how:

General (h) (i)

2.

(i) (ii) (iii) (i) (j)

Material of core clamping plate: Thickness of core clamping plate: Insulation of core clamping plate: Describe location/Method of core grounding: Details of oil ducts in core:

Details of windings: (a) Type of winding: (b) Material of the winding conductor: (c) Maximum current density of windings (at rated current) (i) (ii) (iii) (iv) (d) (e) (f) (g) (h) (i)

(j)

(k)

(l)

(m) 3.

(d) (e)

HV

IV

LV

Current density (A/cm2 )

H.V. : I.V. : L.V. : Regulating:

Whether HV windings are interleaved: Whether winding are preshrunk: Whether adjustable coil clamps are provided for HV and LV windings: Whether steel rings used for the windings if so, whether they are split: Whether electro-static shields are provided to obtain uniform voltage distribution in the HV windings: Insulating material used for: (i) HV winding: (ii) IV winding: (iii) LV winding: (iv) Regulating winding: Insulating material used between: (i) HV and IV, winding: (ii) IV and LV winding: (iii) LV winding and core: (iv) Regulating winding and earth: Type of axial coil supports: (i) H.V. winding: (ii) I.V. winding: (iii) L.V. winding: Type of radial coil supports: (i) H.V. winding: (ii) I.V. winding: (iii) L.V. winding: Maximum allowable torque on coil clamping bolts:

Bushings: (a) Make and type: (b) (i) Rated voltage class kV: (ii) Rated current (Amps): (c)

41

HV IV LV Neutral

Lightning impulse withstand test voltage (1.2 x 50 microsecond) (kV peak): Switching surge with-stand test voltage (kV peak): Power frequency with-stand test voltage (i) Wet for 1 minute (kV rms) (ii) Dry for 1 minute (kV rms)

: : :


42 (f) (g) (h) (i) (j) (k) (l) 4.

Partial discharge level Creepage distance in (mm) Creepage distance (protected) Whether test tap is provided Quantity of oil in bushing and specification of oil used (kg) Weight of assembled bushing (kg) Minimum clearance height for removal of bushing (mm)

Minimum clearance (mm):

: : : : : : : : In Air Between Phase to phases ground

(i) (ii) (iii) 5.

6.

Approximate weight (a) Core with clamping

: kg :

(b) (c) (d) (e)

: kg : : kg : : kg : : kg :

8.

Coil with insulation Core and winding Oil required for first filling Tank and fittings wi th accessories

(f) Untanking weight (g) Total weight with oil and fittings Details of Tank (a) Type of tank (b)

7.

HV IV LV

: kg : : kg : :

Approximate thickness of sheet (i) Sides (ii) Bottom

(mm) (mm)

: :

(mm) (mm) (torr) (torr)

:

(c) (d)

(iii) Cover (iv) Cooling Tubes/Radiators Vacuum recommended for hot oil circulation Vacuum to be maintained during oil filling in transformer tank

(e) (f) (g) (h)

Vacuum to which the tank can be subjected without distortion No. of bi-directional wheels provided Track gauge required for the wheels. Transverse Axis Type of pressure relief device/explosion

(torr) : : Longitudinal Axis

vent and pressure at which it operates Conservator : (a) Total volume (Litres) (b) Volume between the highest and lowest visible oil levels (litres)

:

(c) Power required by heaters (if provided) (kW) Oil Quality (i) Governing standard

: :

: :

: :

General (ii)

(iii) (iv) (v)

Specific resistance at 27°C

43 (ohm-cms) :

90°C Tan delta Water content (ppm) Dielectric strength (Breakdown voltage) (kV)

(vi)

9.

Characteristic of oil after ageing test : (a) Specific resistance at 27°C 90°C (b) Tan delta (c) Sludge content (d) Neutralisation Number (vii) Details of oil preserving equipment offered Radiator (i) Overall dimensions 1 x b x h (mm) (ii) Total weight with oil (kg) (iii) Total weight without oil (kg) (iv) Thickness of Radiator tube (mm) (v) Types of mounting

: : : :

(vi) 10.

11. 12.

Vacuum withstand capability

(ohm-cms) : : : : : : : : : : :

Cooling System : Fan Motor Pump Motor (a) Make and type : (b) No. of connected units : (c) No. of standby units : (d) Rated power input : (e) Capacity (cu m/min.) or (litres/min.) : (f) Rated voltage (Volts) : (g) Locked rotor current (Amp) : (h) Efficiency of motor at full load (Per cent) : (i) Temp. rise of motor at full load (o C) : (j) BHP of driven equipment : (k) Temperature range over which control is adjustable (o C) : (l) Whether the fan and/or pumps suitable for continuous operation at 85 per cent of their rated voltage : (m) Estimated time constant in hours for : (i) Natural cooling : (ii) Forced air cooling : Gas and Oil operated relay make and type : Temperature Indicators Oil temperature : Winding Indicator temperature Indicator (i) Make and type : (ii) (iii) (iv)

Permissible setting ranges for alarm and trip Number of contacts Current rating of each contact

: : :


44 (v) 13.

Whether remote indicators provided. If so, whether equipment required at purchasers control room is included

Approximate overall dimension (a) Length (b) Breadth (c) Height

: : mm : mm : mm

: : :

14.

(i) (ii)

15.

Shipping details : (a) Approximate weight of heaviest package : kg : (b) Approximate dimension of largest package : Transformer will be transported with oil/gas Size of rail recommended for the track : Details of bushing current transformers (HV IV LV Neutral) (i) Quantity : (ii) No. of cores : (iii) Ratio : (iv) V.A. burden : (v) Accuracy class : (vi) Knee point voltage : (vii) Magnetising current at knee point voltage : (viii) Secondary resistance : Lifting jacks : (i) Governing standard : (ii) No. of jacks in one set : (iii) Type and make : (iv) Capacity (tonnes) : (v) Pitch (mm) : (vi) Lift (mm) : (vii) Height in closed position (mm) : (viii) Mean diameter of thread (mm) : Marshalling kiosk : (i) Make and type : (ii) Details of apparatus proposed to be housed in the kiosk. : Details of anti-earth-quake device provided, if any : Separate conservator and Buchholz relay provided : Tap Changing Equipment (These details refer to the basic : rating of O.L.T.C. as guaranteed by O.L.T.C. manufacturers) (a) Make : (b) Type : (c) Power flow-unidirectional/bi-directional/restricted bi-directional : (d) Rated voltage to earth (kV) : (e) Rated current (Amps) : (f) Step voltage (Volts) : (g) Number of steps :

16. 17. 18.

19.

20.

21. 22. 23.

Minimum clearance height for lifting core and winding from tank Minimum clearance height for lifting tank cover

:

: mm : : mm :

General

24.

45

(h)

Control-Manual/Local Electrical/Remote Electrical

:

(i)

Voltage control-Automatic/Non-Automatic

:

(j)

Line drop compensation provided/Not provided

:

(k)

Parallel operation

:

(l)

Protective devices

:

(m)

Auxiliary supply details

:

(n)

Time for complete tap change (one step) sec.

:

(o)

Divertor selector switch transient time (cycles)

:

(p)

Value of maximum short circuit current (Amps)

:

(q)

Maximum impulse withstand test voltage with 1.2/50 micro-seconds: full wave between switch assembly and ground (kV peak)

(r)

Maximum power frequency test voltage between switch assembly and earth (kV rms)

:

(s)

Maximum impulse withstand test voltage with 1.2/50 microseconds across the tapping range (kV peak)

:

(t)

Approximate overall dimensions of tap changer (In case of separate tank type) (mm)

:

(u)

Approximate overall weight (In case of separate tank type) (kg)

:

(v)

Approximate mass of oil (In case of separate tank type) (kg) :

(w)

Particulars of the O.L.T.C. control panel for installation in the control room

:

Driving mechanism box

:

(i)

Make and type

:

(ii)

Details of apparatus proposed to be housed in the box.


46

Appendix III LIST OF TRANSFORMER ACCESSORIES AND TEST-CERTIFICATES REQUIRED FOR THEM Sl. No.

Accessory

1.

Condenser Bushing

Ref. Std.

1. 2. IS 2099

2.

IS Bushings IS 2099

3.

Test -certificates required

OLTC IS 8468

6. 7.

Appearance, construction and demensional check. Test for leakage of internal filling at a pressure of 1.0 kg/cm2 for 12 h. Insulation resistance measurement with 2000 V megger. Dry power frequency voltage withstand test. Dry power frequency voltage withstand test for test tap insulation. Partial discharge measurement upto 1.5µn / √3 kV. Measurement of tangent delta and capacitance.

1. 2. 3.

Appearance, construction and dimensional check. Insulation resistance measurement with 2000 V megger. Dry power frequency voltage withst and test.

1.

Oil tightness test for the diverter switch oil chamber at an oil pressure of 0.5 kg/cm2 at 100°C for 1 h. Mechanical operation test. Operation sequence measurement. Insulation resistance measurement using 2000 V megger. Power frequency voltage withstand test on diverter switch to earth and between even and odd contacts. Power frequency voltage withstand test on tap selectorbetween stationary contacts, between max. and min. taps, (between phases and supporting frames, between phases. Operation test of complete tap changer. Operation and dielectric test of driving mechanism.

3. 4. 5.

2. 3. 4. 5. 6. 7. 8.

4.

Winding Temperature Indicator

1. 2. 3. 4. 5. 6.

Calibration test. Dielectric test at 2 kV for one minute. Accuracy test for indication and switch setting scales. Test for adjustability of switch setting. Test for switch rating. Measurement of temperature rise with respect to the heater coil current.

5.

Oil Temperature Indicator

1. 2. 3. 4. 5.

Calibration test. Dielectric test at 2 kV for one minute. Accuracy test for indication and switch setting scales. Test for adjustability of switch setting. Test for switch rating.

6.

Pressure Relief Valve

1.

Functional test with compressed air to check bursting pressure, indication flag operation and switch operation. Dielectric test at 2 kV for one minute. Switch contact testing at 5A 240 V AC.

2. 3. 7.

Cooling Fan

IS 2312

1. 2. 3. 4.

Insulation resistance measurement. Dielectric test at 2 kV between winding and body for 1 minute. Operation check. Appearance, construction and dimensional check.

General 8.

Transformer Oil Pump

IS 325 & IS 9137

1. 2. 3. 4. 5. 6.

9.

Insulation resistance measurement. Cold resistance measurement at ambient temperature. Motor efficiency at full load. No load voltage, current, power input, frequency and speed. Locked-rotor readings of voltage, current and power input. Water pressure test for pump casing at 5 kg/cm2 for 10 minutes at ambient temperature. Transformer oil pressure test for the pump set assembly at 2 kg/cm2 for 30 minutes at 80 o C. Measurement of head, discharge, current, power input to motor and overall efficiency of the pump set at rated voltage. Appearance, construction and dimensional check.

1. 2. 3. 4.

Observation of flow with respect to requirement. Switch contact test at 5A 240 V AC. Dielectric test at 2 kV for one minute. Appearance, construction and dimensional check.

1.

5. 6. 7. 8. 9.

Leak test with transformer oil at a pressure of 3 kg/cm2 for 30 minutes at ambient temperature for relay casing. Insulation resistance measurement with 500 V megger. Dielectric test at 2 kV for 1 minute. Elements test at 1.75 kg/cm2 for 15 minutes using transformer oil at ambient temperature. Loss of oil and surge test. Gas volume test. Mechanical strength test. Velocity calibration test. Appearance construction and dimensional check.

7. 8.

9.

Oil Flow Indicator/Water Flow Indicator

10.

Buchholz Relay

IS 3637

2. 3. 4.

11.

Oil Level Indicator

1. 2. 3. 4. 5.

Test for oil levels. Switch operation for low level alarm. Switch contact test at 5A 240 V AC. Dielectric test at 2 kV for 1 minute. Appearance, construction and dimensional check.

12.

Pressed Steel Radiators

1. 2.

Air pressure test at 2 kg/cm2 under water for 15 minutes. Appearance, construction and dimension check.

13.

OLTC Control Cubicle/Cooler Control Cubicle

1. 2. 3.

Appearance, construction and dimensional check. Electrical operation. Insulation resistance measurement using 500 V megger at ambient temperature. Dielectric test at 2 kV for 1 minute

4. 14.

Bushing Current T ransformers

IS 2705

47

1. 2. 3. 4. 5.

Appearance, construction and dimensional check. Polarity check. Measurement of insulation resistance. High voltage power frequency test. Determination of ratio error and phase angle of measuring and protection BCTs. 6. Determination of Turns ratio error for PS class BCT. 7. Determination of composite error for protective class BCT. 8. Interturn insulation withstand test. 9. Exciting current characteristic test. 10. Secondary winding resistance measurement. 11. Knee-Point Voltage, measurement for PS class BCT.


48 15.

Off Circuit Tap Changer

1. 2. 3. 4. 5.

Construction and dimensional check. Mechanical operation check. Insulation resistance measurement using 2000 V megger. Millivolt drop test of contacts. High voltage power frequency withstand test by applying appropriate voltages to live parts to earth, between maximum and minimum taps, between change over contact and intermediate bearing, between adjacent t aps, between tap changer contact and intermediate bearing and between tap changer contact and current take off terminal.

16.

Oil to Water Heat Exchanger

1. 2.

Test certificates for the materials of construction. Manufacturers in process inspection records for all parts, sub-assemblies, accessories and complete assembly. Shell side pressure test at 10 kg/cm2 with transformer oil at a temperature of 70°C + 10°C for 6 h. Water side pressure test at 5 kg/cm2 with water at amdient temperature for 6 h. Appearance, construction and dimensional check. Calibration test. Alarm contact setting test.

3. 4. 17.

Pressure Gauges/ Differential Pressure Gauges

1. 2. 3.

Appendix IV LIST OF STANDARDS FOR TRANSFORMERS (Refer latest edition if IS, CBIP, IEEMA, BS, IEC, ANSI) I.

Transformer Specifications.

II.

Installation.

III.

Operation.

IV.

Maintenance.

V.

Instruments & Equipment.

VI.

Accessories and Consumables.

VII.

Raw Materials and Components

General I.

49

TRANSFORMER SPECIFICATIONS

IS 2026 (Part 1) IS 2026 (Part 2) IS 2026 (Part 3) IS 2026 (Part 4) IS 2026 (Part 5) -

IEC 60076.1

BS 171 (Part 1) BS 171 (Part 2) BS 171 (Part 3) BS 171 (Part 4) -

: IEC 60076.2 : IEC 60076.3 : IEC 600616 : IEC 60076-3-1

: General : : Temperature rise : : :

: Insulation levels and dielectric tests. : Terminal and tapping markings : External clearance in air.

:

IS 11171 : IS 10028 (Part 1) : IS 1180 (Part 1) : IS 1180 (Part 2) :

IEC 60076-5

BS 171 (Part 5) : -

IEC 726 (1982) -

Outdoor distribution transformers

: Ability to withstand short circuits. : Dry Type : Selection : Non-sealed type : Sealed type

ANSI C 57.12.10 -

:

Safety requirements 230 kV and below 833/958 through 8333/10417 kV A Single phase, and 750/862 through 60,000/80,000/100,000 kV A three phase, without load tap changing and 3750/4687 through 60,000/80,000/100,000 kV A load tap changing.

ANSI

C 57.12.13 -

:

Conformance requirements for liquid - Filled transformers used in unit installations including unit substations.

ANSI C 57.12.20 -

:

Transformers - Overhead - Type distribution transformers 500 kVA and smaller : High voltage, 34500 volts and below; Low voltage 7970/13800 Y Volts and below;

ANSI

C 57.12.21 -

:

Pad-Mounted compartmental type self-cooled single phase distribution Transformers with High Voltage Bushings High voltage 34500 Grd Y /19920 volts and below; low voltage 240/120; 167 kV A and smaller.

ANSI C 57.12.22 -

:

Transformers pad-mounted compartmental -Type self-cooled three-phase distribution transformers with high voltage bushings 2500 kVA and smaller. High voltage 34500 Grd Y 19920 volts and below. Low voltage 480 volts and below.

ANSI C 57.12.23 -

:

Transformers underground -Type self-cooled, single phase distribution transformers with separable, insulated High voltage connecters high-voltage connectors: High voltage (24940 Grd Y/14400 V and below) and low voltage 240/120 V, 167 kVA and smaller.

ANSI C 57.12.24 -

:

Transformers underground type three-phase distribution transformers 2500 kVA and smaller: High voltage 34500 Grd Y/19920 volts and below; Low voltage, 480 volts and below.

ANSI C 57.12.25 -

:

Pad-Mounted compartmental type self-cooled single phase distribution transformers with separable insulated high voltage connectors. High-voltage 34500 Grd Y/19920 volts and below; Low voltage 240/120; 167 kV A and smaller, requirements for.


50 ANSI C 57.12.26 -

:

Pad-Mounted compartmental type, self-cooled, three-phase distribution transformers for use with separable insulated high voltage connectors. Highvoltage 34500 Grd Y/19920 volts and below; 2500 kV A and smaller requirements for.

ANSI C 57.12.27 -

:

Liquid filled distribution transformers used in pad-mounted installations including unit substations, conformance standard for.

ANSI/C 57.12.00 IEEE

:

Liquid immersed distribution Power and regulating transformers.

ANSI C 57.12.28 -

:

Switchgear and transformers pad-mounted equipment - enclosure integrity.

ANSI C 57.12.29 -

:

Switchgear and transformers pad-mounted equipment - enclosure integrity for coastal environments

ANSI C 57.12.50 -

:

Distribution transformers 1 to 500 kVA, single-phase and 15 to 500 kVA, three-phase with high-voltage 601-34500 volts, low voltage 120-600 volts. Ventilated Dry-Type.

ANSI C 57.12.51 -

:

Dry-type power transformers 501 kVA and larger, three-phase with high voltage 601 to 34500 volts, low-voltage 208 Y/120 to 4160 volts requirements for ventilated.

ANSI C 57.12.52 -

:

Dry-type power transf ormers 501 kVA and larger, three-phase with highvoltage. 601 to 34500 volts, low voltage 208 Y/120 to 4160 volts, requirements for sealed.

ANSI C 57.12.55 -

:

Dry-type transformers in unit installations, including unit substations conformance standard.

ANSI C 57.12.57 -

:

Ventilated dry type network transformers 2500 kVA and below, Three phase, High voltage 34500 volts and below low-voltage 216 Y/125 and 480 Y/277 volts, requirement.

ANSI C 57.12.70 -

:

Terminal markings and connections for distribution and power transformers.

ANSI/ C 57.12.40 IEEE

:

Secondary network transformers sub way and vault types. (liquid immersed)

ANSI/ 1585 UL

:

Class - 2 and Class - 3 Transformers.

IS 5216 (Part 1) -

:

Guide for safety procedures and practices in electrical work.

IS 5216 (Part 2) -

:

Guide for safety procedures and practices in electrical work (life saving technique).

IS 3043 - BS 7430 -

:

Code of practice for earthing.

IS 2266 -

:

Steel wire ropes for general engineering purposes.

IS 6132 (Part 1) -

:

Shackles: Part 1 General requirements.

IS 6132 (Part 2) -

:

Shackles: Part 2 Dimensions of dee shackles.

IS 6132 (Part 3) -

:

Shackles: Part 3 Dimensions of bow shackles.

IS 3832 -

:

Hand operated chain pulley blocks.

IS 4190 -

:

Eye bolts with collars.

IS 4552 (Part 1) -

:

Automotive vehicles -portable jacks for automobiles - Part I mechanical jacks.

II.

INSTALLATION

General IS 4552 (Part 2 ) -

:

Automotive vehicles -portable jacks for automobiles - Part 2 Hydraulic jacks.

IS 12735 -

:

Wire rope slings - safety criteria and inspection procedure for use.

IS 5 :

:

Colours for ready mixed paints and enamels.

IS 1447 (Part 1) -

:

Code of practice for painting of ferrous metals in buildings -Part 1 Pre-treatment.

IS 1447 (Part 2) -

:

Code of practice for painting of ferrous metals in Building – Part 2 painting.

IS 1255 -

:

Code of practice for installation and maintenance of power Cables up to and including 33 kV rating.

IS 732 -

:

Code of practice for electrical wiring installations.

IS 10028 (Part 2) -

:

Installation - Transformers.

ANSI C 57.12.57 -

:

Ventilated dry type network transformers 2500 kVA and below, three phase, High voltage 34500 volts and below low-voltage 216 Y/125 and 480 Y/2 77 volts, requirement.

ANSI C 57.12.70 -

:

Terminal markings and connections for distribution and power transformers.

ANSI/ C 57.12.40 IEEE

:

Secondary network transformers sub way and vault types. (liquid immersed)

ANSI/ 1585 UL

:

Class - 2 and Class - 3 Transformers.

51


52

CBIP Technical Report No. 1 (Revised-1999) Manual on Transformers: Section O

:

Specification for Fire protection of Power Transformers

Section M

:

Specification for Protective Schemes for Power and Distribution transformers.

BS 381 C -

:

Colours for identification, coding and special purposes.

BS 4800 -

:

Schedule of paint colours for building purposes.

BS 5252 -

:

Frame work for colour for Co -ordination for building purposes.

BS 5252 F -

:

Colour matching fan.

BS 7664 -

:

Specification for undercoat and finishing paints.

BS 5493 -

:

Code of practice for protective coating of iron and steel structures against corrosion.

IEC 60990

:

Methods of measurement of touch current and protective conductor current.

ANSI/IEEE C 57.13.3 -

:

Grounding of instrument transformers secondary circuits.

ANS/IEE C 57.12.11 -

:

Guide for installation of oil immersed transformers (10 MVA and larger 69 to 287 kV rating)


:

Guide for installation of oil immersed EHV transformers 345 kV and above

ANSI/IEEE C 57.105 -

:

Application of transformers connection's in three phase distribution systems


:

Guide for liquid-immersed transformer through fault current duration.


:

Guide for Installation of oil immersed transformers (10 MVA and larger 69 to 287 kV rating)


:

Seismic guide for power transformers and reactors.

ANSI/IEEE 979 -

:

Fire protection, guide for substation

ANSI/IEEE 242 -

:

Recommended practice for protection and co-ordination of industrial and commercial power system.

ANSI/IEEE 142 -

:

Grounding of industrial and commercial Power system.

ANSI/IEEE C 37.91-

:

Protective relay application to power transformers.

IEEE C 57.93 -

:

IEEE guide for installation of liquid immersed power transformers

General

53

III. OPERATION IS 3961 (Part 1) -

:

Recommended current ratings for cables. Part 1 paper insulated lead sheathed cables.

IS 3961 (Part 2) -

:

Recommended current ratings for cable Part 2 PVC insulated and PVC sheathed heavy duty cables.

IS 3961 (Part 4) -

:

Recommended current ratings for cables Part 4 Polyethylene insulated cables.

IS 3961 (Part 5) -

:

Recommended current ratings for cables Part 5 PVC insulated light duty cables.

IS 5819 -

:

Recommended short circuit ratings of high voltage PVC cables.

IS 6600 - /BS 7735 /IEC 60354 -

:

Guide for loading of oil immersed transformers.

IS 10561 - /BS 5953 Part 1-/IEC 606-

:

Application guide for power transformers.

IS 8478-/BS 5611-/IEC 542-

:

Application guide for on load tap changers.

IS 4004-

:

Application guide for non-line or resistor type surge arrestors for alternating current system.

IS 4850 -

:

Application guide for expulsion-type lightning arrestors.

IS 4201 -

:

Application guide for current transformers.

IS 4146 -

:

Application guide for voltage transformers.

IS 5547 -

:

Application guide for capacitor voltage transformers.

IS 3638 -

:

Application guide for gas operated relays.

IS 3842 (Part 1) -

:

Application guide for electrical relays for ac systems. Part 1 over current relays for feeders and transformers.

IS 10864 -

:

Recommendation for heat exchanger gasket.

Section N - Specification for Voltage Control of Power Transformers CBIP Technical Report No. 74 - Loading Capability of Power Transformers IEC 60905 -

:

Loading guide for dry type power transformers.

ANSI/IEEE C 57.19.100-

:

Guide for application of power apparatus bushings.


:

Guide for loading power apparatus bushings.

ANSI/IEEE C 57.19-

:

Distribution Transformers Rated 500 kVA and less with 55°C or 65°C Average winding Rise, Guide for loading mineral oil-immersed overhead and pad-mounted.

ANSI/IEEE C 57.92-

:

Power transformers upto and including 100 MVA with 55 o C or 65°C winding Rise. Guide for loading mineral oil immersed


54 ANSI/IEEE C 57.95 -

:

Regulations, guide for loading liquid immersed step-voltage and induction voltage.

ANSI/IEEE C 57.96 -

:

Distribution and power transformers guide for loading dry-type.

ANSI/IEEE C 57.110-

:

Recommended practice for establishing transformer capability when supplying non sinusoidal load currents.


:

Mineral-oil immersed power transformers rated in excess of 100 MVA guide for loading.


:

Guide for transformers directly connected to generators.

IEEE C 57.91 -

:

Guide for loading mineral oil-immersed transformers.

IS 10028 (Part 3) -

:

Maintenance of transformers.

IS 1866 - 1983/BS 5730 -

:

Code of practice for maintenance and supervision of mineral insulating oil in equipment.

IS 6855 -

:

Method of sampling for liquid dielectrics.

IS 9434 -

:

Guide for sampling and analysis of free and dissolved gases and oil-filled electrical equipments.

IS 10593 -

:

Method of evaluating the analysis of gases in oil filled electrical equipment in service.

IV. MAINTENANCE

1.

CBIP Technical Report No. 62 Guide for Testing of Transformers by Sampling and Analysis of Free and Dissolved Gases.

2.

CBIP Technical Report No. 63 Failure Analy sis of Distribution Transformers.

3.

CBIP Technical Report No. 72 Investigation into Causes of Failure of Power Transformers.

BS 5800 -

:

Guide for the interpretation of the analysis of gases in transformers and other oil filled electrical equipment in service.

BS 5874 -

:

Methods for determination of the electric strength of insulating oils.

IEC 60706 - 4 Part 4

:

Section 8. Maintenance and maintenance support planning.

IEC 60422 -

:

Supervision and maintenance guide for mineral insulating oils in electrical equipments.

IEC 60475 -

:

Method of sampling liquid dielectrics.

IEC 60628 -

:

Gassing of insulating liquids under electrical stress and ionization.

IEC 60733 -

:

Determination of water in insulating oil and in oil impregrated paper and pressboard.

IEC 60599 -

:

Interpretation of the analysis of gases in transformers and other oil filled electrical equipment in service.

IEC 60567 -

:

Sampling of gases and oil from oil filled electrical equipment and for the analysis of free and dissolved gases.

General

55


:

Dry type transformer through -fault current duration guide.


:

Guide for the interpretation of gases generated in oil-immersed transformers.


:

Guide for acceptance and maintenance of insulating oil in Equipment.


:

Guide for acceptance of silicone insulating fluid and its maintenance in transformers.


:

Guide for reporting failure data for power transformers and shunt reactors on electric utility power systems.


:

Guide for acceptance and maintenance of less flamable-hydrocarbons fluids and maintenance in transformers.


:

Guide for failure -investigation, documentation and analysis for power transformers and shunt reactors.

ANSI/IEEE 637 -

:

Guide for reclamation of insulating oil and criteria for its uses.

IEEE 62 -

:

Guide for diagnostic field testing of electric power apparatus-part 1 : Oil filled power transformers, regulators, and reactors.

V.

INSTRUMENTS AND EQUIPMENT

IS 3151 -

:

Earthing transformers.

IS 5553 (Part 3) -

:

Reactors Parts 3 - Current limiting reactor and neutral earthing reactors.

IS 5553 (Part 6) -

:

Reactors Part 6 - Earthing transformers.

IS 3151 :

:

Earthing transformer.

IS 3070 (Part 1) -

:

Lightning arrestors for-alternating current systems. Part 1 Non linear resistor type lightning arrestors.

IS 3070 (Part 2) -

:

Lightning arrestors for alternating current system. Part 2 Metal Oxide lightning arrestors without gaps.

IS 3156 (Part 1) -

:

Voltage Transformers. Part 1 General requirements.

IS 3156 (Part 2) -

:

Voltage Transformers. Part 2 Measuring voltage transformers.

IS 3156 (Part 3) -

:

Voltage Transformers. Part 3 Protective voltage transformers.

IS 3156 (Part 4) -

:

Voltage Transformers. Part 4 Capacitor voltage transformers.

IS 6034 -

:

Insulating oil conditioning plants.

IS 2992 -

:

Insulation resistance tester, hand operated (magnets generator type).

IS 10656 -

:

Insulation resistance tester (electric type).

IS 11994 -

:

Portable insulation resistance tester (mains operated).

IS 9223 -

:

Portable earth resistance meter.


56 IS 3624 -

:

Pressure and vacuum gauge.

BS 5310 Part 1 -

:

General requirements and tests.

BS 5310 Part 2 - A -

:

Hand crimping tools (fixed die sizes A to E) for radio frequency connectors and concentric contacts.

BS 5310 Part 2 B -

:

Hand crimping tools (removable and interchangable dies, sizes A to G and Q to S for radio frequency connectors and concentric contacts.

BS 5310 Part 3 A -

:

Hand crimping tools for contacts of electrical connectors.

BS 7740 -

:

Specification for portable equipment for earthing or earthing, and short circuiting.

IEC 186 -

:

Voltage transformers.

VI.

ACCESSORIES AND CONSUMABLES

IS 2551 -

:

Danger notice plates.

IS 8923 -

:

Warning symbol for dangerous voltage.

IS 12776 -

:

Galvanized strand for earthing.

IS 104 -

:

Ready mixed paint, bushing, zinc chrome priming.

IS 2074 -

:

Ready mixed paint, air drying, red-oxide-zinc chrome, priming.

IS 2932 -

:

Enamel synthetic, exterior (a) undercoating (b) finishing.

IS 13213 -

:

Polyurethane full gloss enamel.

IS 13238 -

:

Epoxy based zinc phosphate primer.

IS 14209 -

:

Epoxy enamel, two component, glossy.

IS 692 -

:

Paper insulated lead-sheathed cables for electricity supply.

IS 694 -

:

PVC insulated cables for working voltages up to and including 1100 V

IS 1554 (Part 1) -

:

PVC insulated (heavy duty) electric cables: Part 1 for working voltages upto and including 1100 V.

IS 1554 (Part 2) -

:

IS 7098 (Part 1) -

:

PVC insulated (heavy duty) electric cables. Part 2 for working voltage from 3.3 kV upto and including 11 kV. Cross linked polyethylene insulated PVC sheathed cables. Part 1 for working voltage upto and including 1100 V.

IS 7098 (Part 2) -

:

Cross linked polyethylene insulated PVC sheathed cables. Part 2 for working voltages from 3.3 kV upto and including 33 kV.

General

57

IS 7098 (Part 3) -

:

Cross-linked polyethylene insulated thermoplastic sheathed cables. Part 3 for working voltages from 66 kV upto and including 220 kV.

IS 8308 -

:

Compression type tubular in -line connectors for aluminium conductors of insulated cables.

IS 8309 DIN 46329.07.83

:

Compression type tubular terminal ends for aluminium conductor of insulated cables.

IS 9147 -

:

Cable sealing bores for oil immersed transformers suitable for paper insulated lead sheathed cables for highest system voltages from 12 kV upto and including 36 kV.

IS 9537 (Part 1) -

:

Conduits for electrical installations Part 1 General requirements.

IS 9537 (Part 2) -

:

Conduits for electrical installations Part 2 Rigid steel conduits.

IS 9537 (Part 3) -

:

Conduits for electrical installations Part 3 Rigid plain conduits of insulating materials.

IS 9537 (Part 4) -

:

Conduits for electrical installations Part 4 Pliable self-recovering conduits of insulating materials.

IS 12943 -

:

Brass glands for PVC cables.

IS 8468 - BS 4571 -

:

On load tap changers.

IS 2705 (Part 1) -

:

Current transformers (General)

IS 2705 (Part 2) -

:

Current transformers. Part 2 Measuring current transformers.

IS 2705 (Part 3) -

:

Current transformers. Part 3 Protective current transformers.

IS 2705 (Part 4) -

:

Current transformers. Part 4 Protective current transformers for special purpose application.

IS 4253 (Part 2) -

:

Cork composition sheets: Part 2 Cork and rubber.

IS 6838 -

:

Dimensions for 'O' rings and grooves for vacuum flanges.

IS 9975 (Part 1) -

:

'O' rings Part 1 Dimensions.

IS 9975 (Part 2) -

:

'O' rings Part 2 Material selection and quality acceptance criteria.

IS 9975 (Part 3) -

:

'O' rings Part 3 Seal housing dimensions and tolerances.

IS 9975 (Part 4) -

:

'O' rings Part 4 Terminology and definition of terms.

IS 11149 -

:

Rubber gaskets.

IS 1363 (Part 1) -

:

Hexagon head bolts, screws and nuts of product grade C. Part 1 Hexagon head bolt.

IEC 60214 -


58 IS 1363 (Part 2) -

:

Hexagon head bolts, screws and nuts of product grade C. Part 2 Hexagon head screws.

IS 1363 (Part 3) -

:

Hexagon head bolts, screws and nuts of product grade C. Part 3 Hexagon nuts.

IS 1364 (Part 1) -

:

Hexagon head bolts, screws and nuts of product grades A & B. Part 1 Hexagon head bolts.

IS 1364 (Part 2) -

:

Hexagon head bolts, screws and nuts of product grades A & B. Part 2 Hexagon head screws.

IS 1364 (Part 3) -

:

Hexagon head bolts, screws and nuts of product grades A & B. Part 3 Hexagon nuts.

IS 1364 (Part 4) -

:

Hexagon head bolts, screws and nuts of product grades A & B. Part 4 Hexagon thin nuts -(chamfered).

IS 1364 (Part 5) -

:

Hexagon head bolts, screws and nuts of product grades A & B. Part 5 Hexagon thin nuts (unchamfered).

IS 1365 -

:

Slotted counter sunk head screws.

IS 2016 -

:

Plain washers.

IS 2388 -

:

Slotted grub screws.

IS 3063 -

:

Fastners - single coil rectangular section spring washers.

IS 335 -

:

New insulating oils.

IS 12463 -

:

Inhibited mineral insulating oils.

IS 9700 -

:

Activated alumina.

IS 1747 -

:

Nitrogen.

IS 3639 -

:

Fittings and accessories for power transformers.

IS 778 -

:

Copper alloy gate, globe and check valves for water works purposes.

IS 780 -

:

Sluice valves for water works purposes.

IS 5312 (Part 1) -

:

Swing check type reflux (non-return) valves Part 1 single door patern.

IS 11699 -

:

Steel plug valves for petroleum Petrochemical and allied industries.

IS 11792 -

:

Steel ball valves for the Petroleum and allied industries.

IS 3637 -

:

Gas operated relays.

IS 2312 -

:

Propeller type ac ventilating fans.

IS 3538 -

:

Electric axial flow fans.

IS 4503 -

:

Shell and tube type heat exchangers.

General

59

IS6088- DIN 425561.01.75

:

Oil to water heat exchangers for transformers.

IS 3401 -

:

Silicagel.

IS 5561 -

:

Electric power connectors.

IS 2099 - BS 223-

:

Bushings for alternating voltages above 1000 V.

IS 2544 -

:

Porcelain post -insulator for systems with nominal voltage greater than 1000 V.

IS 3347 (Part l/sec 1) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 1 upto and including 1 kV bushings, sec. 1 porcelain parts.

IS 3347 (Part l/sec 2) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 1 upto and including 1 kV bushings, sec. 2 metal parts.

IS 3347 (Part 2/sec 1) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 2. 3.6 kV bushings sec 1. Porcelain parts.

IS 3347 (Part 2/sec 2) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 2. 3.6 kV bushings sec. 2. metal parts.

IS 3347 (Part 3/sec 1) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 3. 12 and 17.5 kV bushings, sec. 1 porcelain parts.

IS 3347 (Part 3/sec 2) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 2. 12 & 17.5 kV bushings, sec. 2 Metal parts.

IS 3347 (Part 4/sec 1) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 4. 24 and 27 kV bushings sec. 1. Porcelain parts.

IS 3347 (Part 4/sec 2) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 4. 24 and 27 kV bushings sec. 2. Metal parts.

IS 3347 (Part 5/sec 1) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 5. 36 kV bushings. sec. 1 Porcelain parts.

IS 3347 (Part 5/sec 2) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 5. 36 kV bushings. sec. 2. Metal parts.

IS 3347 (Part 6/sec 1) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 6. 72.5 kV bushings. sec. 1. Porcelain parts.

IS 3347 (part 6/sec 2) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 6. 72.5 kV bushings. sec. 2. Metal parts.

IS 3347 (Part 7/sec 1) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 7. 123 kV bushings, sec. 1. Porcelain parts.

IS 3347 (Part 7/sec 2) -

:

Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 7. 123 kV bushings, sec. 2. Metal parts.

IS 3347 (Part 8/sec 1) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 8. 52 kV bushings, sec. 1. Porcelain parts.


60 IS 3347 (Part 8/sec 2) -

:

Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. Part 8. 50 kV bushings sec. 2. Metal parts.

IS 4257 (Part 1) -

:

Dimensions for clamping arrangements for porcelain transformer bushings. Part 1. For 12 kV to 36 kV bush ings.

IS 4257 (Part 2) -

:

Dimensions for clamping arrangements for porcelain transformer bushings. Part 2. For 72.5 and 123 kV bushings.

IS 5621 -

:

Hollow insulators for use in electrical equipment.

IS 7421 -

:

Specifications for low voltage bushings

IS 7648 -

:

Silicone compound for application on high voltage porcelain insulators.

IS 8603 (Part 3) -

:

Dimensions for porcelain transformer bushings for use in heavily polluted atmospheres. Part 3. 36 kV bushings.

IS 12676 -

:

Dimension for oil impregnated paper insulated condenser bushings.

IS 13134 -

:

Guide for the selection of insulators in respect of pollution conditions.

IS 13305 -

:

Permissible limits of visual defects for insulating porcelain for electrical circuits.

IS 13312 -

:

Dimension of porcelain oil filled transformers bushings (rated 52 kV) in medium polluted atmospheres.

IEEMA 9 -

:

Pressed steel radiators.

BS 2562 -

:

Cable boxes for transformers and reactors.

BS 6435 -

:

Specification for unfilled enclosures for the dry termination of HV cables for transformer and reactors.

BS 223 -

:


BS 7616 -

:

Bushings for liquid filled transformer above 1 kV and upto 36 kV.

BS 1780 -

:

Specification for bo urdon tube pressure and vacuum gauges.

BS 5235 -

:

Dial type expansion them1ometer.

BS 6121

:

Mechanical cable glands

BS 6121 Part 1 -

:

Metallic glands.

BS 6121 Part 2 -

:

Polymeric glands.

BS 6121 Part 3 -

:

Special corrosion resistant glands.

BS 6121 Part 5 -

:

Selection, installation and inspection of cable glands used in electrical installations.

BS 7215 -

:

Separable insulated cable connector systems above 1 kV and upto 36 kV.

General

61

BS 11 -

:

Specification for railway rails (24.8 kg/m)

BS 148 -

:

Specification for unused mineral insulating oils for transformer and switchgear.

BS EN 60867 -

:

Insulating liquids-unused liquids based on synthetic arromatic hydrocarbonic.

IEC 185

:

Current transformers.

IEC 60137

:

Bushings for alternating voltages above 1000 V

IEC 233

:

Tests on hollow insulators for use in electrical equipment.

IEC 60296

:

Specification for unused mineral insulating oils for transformers and switchgear.

ANSI/IEEE 32 -

:

Neutral grounding devices standard requirements-terminology test procedures.

ANSI/IEEE C 57.13-

:

Instrument transformers.


:

Field testing of relaying current transformers.


:

Conformance test procedures for instrument transformers.


:

General requirements and test procedures for outdoor power apparatus bushings.


:

Outdoor apparatus bushings performance characteristics and dimensions for.

IEEE C 57.131 -

:

Standard requirements for load tap changers.

SECTION B

Specifications for Three Phase 11 kV/433 - 250V Class Distribution Transformers (upto and including 100 kVA)

64


Specifcations for Three Phase 11 kV/433-250V Class Distribution Transformers (upto and including 100 kVA)

65

SECTION B Specifications for Distribution Transformers (upto and including 100 kVA) 1.0

SCOPE

1.1 This section covers, technical requirements/parameters of distribution transformers of rating upto and including 100 kVA, 11 kV 3 phase and does not purport to include all the necessary provisions of a contract. 1.2

Standard Ratings

The standard ratings shall be 16, 25, 63 and 100 kVA. For general requirement reference shall be made to sections A & J of this manual. 2.0

STANDARDS

2.1 The materials shall conform in all respects to the relevant Indian / International Standard Specification, latest, amendments thereof, some of them are listed below: Indian Standard

Title

International & Internationally recognised standard

ISS - 2026/1977 ISS - 1180

IEC 60076

IS 12444 ISS - 3347/1967 ISS - 335/1983

Specification for Power Transformer Outdoor Distribution Transformer upto and including 100 kVA Specification for Copper Wire Rod Specification for Porcelain Transformer Bushing Specification for Transformer Oil

ISS 5/1961 ISS - 2099/1973 ISS - 7421/1974 ISS - 3347 ISS - 5484 ISS - 9335 ISS - 1576 ISS / 6600/1972

Specification for Colours for Ready Mixed Paints Specification for High Voltage Porcelain Bushings Specification for Low Voltage Bushings Specification for Outdoor Bushings Specification for Al Wire Rods Specification for Insulating Kraft Paper Specification for Insulating Press Board Guide for Loading of Oil Immersed Transformers

ASTM B-49 DIN 42531,23,3 BS 148, D-1473, D-1533- 1934 IEC Pub 296-1969

DIN 42531 to 33 ASTM B - 233 IEC 60554 IEC 60641 IEC 60076

3.0 SERVICE CONDITIONS The Dis tribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows, as per IS 2026 (Part - I) Latest Revision/International Standards tabulated above : (i)

Location

:

___

(ii)

Max ambient air temperature (deg.C)

:

___


66 (iii)

(iv)

(v)

(vi)

4.0

Min. ambient air temperature (deg.C)

:

___

Max average daily ambient air temperature (deg.C)

:

___

Max. yearly weighted average ambient temperature(deg.C)

:

___

Max. altitude above mean sea level (meters)

:

___

STANDARD RATINGS

Transformers shall be suitable for outdoor installation with three phase, 50 Hz, 11 kV system in which the neutral is effectively earthed and these should be suitable for service under fluctuations in supply voltage upto plus 10% to minus 15%. The transformer shall conform to the following specific parameters : Sl. no. 1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13.

Item Continuous rated capacity System highest voltage Rated voltage HV Rated voltage LV BIL Frequency No. of phases Connection HV Connection LV

Vector group Type of cooling Percentage impedance at 75o C Permissible temperature rise over ambient (i) Of top oil measured by thermometer (ii) Of winding measured by resistance 14. Minimum clearances in air (a) HV phase to phase/ phase to earth (mm) (b) LV phase to phase/ phase to earth (mm) 15. Tap changer

Specification 16 kVA 12 kV 11 kV 433 V 75 kV peak 50 Hz +/- 5% Three Delta Star (Neutral brought out) and directly earthed Dyn-11 ONAN

25 kVA

63 kVA

100 kVA

-----------------------4.5----------------------35 Deg.C 40 Deg.C. --------------As per IS-1180 latest -------------------------255 / 140----------------------75 / 40--------– not provided


5.0

TECHNICAL REQUIREMENTS

5.1

Core

67

5.1.1 Material - CRGO 5.1.2 The core shall be stacked / wound type, of high grade cold rolled grain oriented steel laminations having low loss and good grain properties, coated with hot oil proof insulation, bolted together and to the frames firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core losses with continuous working of the transformers. 5.1.3 Core Clamping for CRGO Stacked Core l l

MS Channel shall be used on top and bottom Core channel on LV side to be reinforced at equidistance, if holes / cutting is done for LT lead in order to avoid bending of channel.

l

MS channels shall be painted with varnish or oil-resistant paint.

l

Clamping and Tie-rods shall be made of HT steel and shall be parkarised

5.1.4 Core Clamping for CRGO Wound Core l

l l

Core clamping shall be with top and bottom U-shaped core clamps made of sheet steel clamped with HT steel tie rods for efficient clamping. MS core clamps shall be painted with varnish or oil-resistant paint. Suitable provision shall be made in the bottom core clamp / base plate of the transformer to arrest movement of the active part.

5.1.5 The transformer core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12.5% without injurious heating at full load conditions and shall not get saturated. The Bidder shall furnish necessary design data in support of this situation. 5.1.6 No load current shall not exceed 3% of full load current and will be measured by energising the transformer at 433 volts, 50 Hz on the secondary. Increase of voltage of 433 volts by 12.5% shall not increase the no load current beyond 6% of full load current. 5.2

Windings

5.2.1 Material: Super enamel covered copper conductor / double paper covered copper conductor. 5.2.2 LV winding shall be in even layers so that neutral formation is at top. 5.2.3 HV coil single wound or crossover coils over LV coil shall be wound. 5.2.4 Inter layer insulation shall be electrical grade insulation Kraft paper/Epoxy dotted paper.


68

5.2.5 Proper bonding of inter layer insulation with the conductor shall be ensured. 5.3

Oil

The insulating IS 335 / IEC 296 5.4

oil

shall

comply

with

the

requirements

of

relevant

standards

Insulation Material

5.4.1 Material: Electrical Grade Insulation Kraft Paper 5.4.2 All spacers, axial wedges / runners used in windings shall be made of pre-compressed Pressboard−solid, conforming to type B 3.1 of IEC 641-3-2. In case of cross-over coil winding of HV all spacers shall be properly sheared and dovetail punched to ensure proper locking. All axial wedges / runners shall be properly milled to dovetail shape so that they pass through the designed spacers freely. Insulation shearing, cutting, milling and punching operations shall be carried out in such a way, that there is no burr or dimensional variations. 5.5

Tank

The transformer tank can be with radiator fins/ rounded or elliptical cooling tubes or made of corrugated panels. 5.5.1 For Rectangular/Octogonal Plain Tank The transformer tank shall be of robust construction rectangular/octogonal in shape and shall be built up of tested MS sheets. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish/hot oil resistant paint. The four walls of the tank shall be made of Two “L” shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.8 kg/cm2 for 10 minutes. The tank shall be reinforced by angle welded on all the outside walls on the edge of the tank to form two equal compartments. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for octogonal tank and 760 mm of mercury for round tank, shall not be more than 5 mm upto 750 mm length and 6.5 mm upto 1250 mm length. The tank shall further be capable of withstanding a pressure of 0.8 kg/sq cm (g) and a vacuum of 0.3 kg/sq cm (g) without any deformation.


69

The radiators can be tube type or fin type or pressed steel type to achieve the desired cooling and the same shall be capable of giving continuous rated output without exceeding the specified temperature rise.. 4 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall. 4 Nos. of welded heavy duty pulling lugs of MS plate 8 mm thick (min) shall be provided to pull the transformer horizontally. Top cover fixing bolts of Galvanised Iron adequately spaced and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers and one spring washer. 5.5.2 Corrugated Tank Corrugated tanks may be offered for 63kVA and 100 kVA . The transformer tank shall be of robust construction corrugation in shape and shall be built up of CRCA sheets of 1.2mm thick. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank, with CCA (core-coil assembly), HV & LV bushings mounted on Top cover. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnis h / hot oil resistant paint. Corrugation panel shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 2 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced are to be provided. Top cover fixing bolts of galvanized iron and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II / nitrile rubber shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers and one spring washer. Tanks with corrugations and without conservator shall be tested for leakage test at a pressure of 0.15kg/sq cm measured at the top of the tank.


70

5.5.3 Sealed Transformer with Radiators In this type of construction tank is designed to have have cover welded to the curb of tank. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure. The tank should be of stiff construction able to withstand pressure of 2 atmospheres . 5.6 Conservator On Transformers of 100 kVA rating with rectangular plain tank the provision of conservators is obligatory. For other ratings manufacturer may adopt their standard practice. Conservator is not required in transformers with corrugated tank. When a conservator is provided, oil gauge and the plain or dehydrating breathing devise shall be fixed to the conservator which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations. In addition the cover of main tank shall be provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank. The inside diameter of the pipe connecting the conservator to the main tank should be within 20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump for collection of impurities. The minimum oil level (corresponding to -5 deg C) should be above the sump level. 5.7

Surface Preparation and Painting

5.7.1 For surface preparation refer to section A of this Manual 5.8

Bushings

The bushings shall conform to the relevant standards specified and shall be outdoor type. The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. Arcing horns or lightning arrestors shall be provided on HV bushings. For 11 kV, 17.5 kV class bushings and for 0.433 kV, 1.1 kV class bushings shall be used. Bushings with plain sheds as per IS-3347 shall be mounted on the side of the Tank and not on top cover. A minimum phase to phase clearance of 75 mm for LV (upto 1.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushing mounted on the transformer. The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank.


5.9

71

Terminal Connectors

The LV bushing and HV bushing stems shall be provided with suitable terminal connectors so as to connect the jumper without disturbing the bushing stem. Connectors shall be with eye bolts so as to receive 55 sq.mm. conductor for HV. 5.10 Terminal Markings High voltage phase windings shall be marked both in the terminal boards inside the tank and on the outside with capital letter 1U, 1V, 1W and low voltage winding for the same phase marked by corresponding small letter 2U, 2V, 2W. The neutral point terminal shall be indicated by the letter 2N. Neutral terminal to be brought out and connected to local grounding terminal by an Earthing strip. 5.11 Current Transformers (for 63 and 100 kVA ratings only) l

CT’s shall be provided if required on secondary side.

l

Current transformer shall be mounted inside the tank on LV side of the transformer.

l

The current transformers shall be comply with IS : 2705.

l

All secondary leads of bushing mounted CT’s shall be brought to a terminal box near each bushing. The CT terminals shall have shorting facility. CT should not get saturated upto 200% of rated current. Transformer rating

63 kVA

100 kVA

CURRENT RATIO

100/5 A

150/5 A

CLASS

0.5

0.5

BURDEN

20 VA

20 VA

APPLICATION

METERING

METERING

ISF

5

5

5.12 Fittings The following standard fittings shall be provided : (a)

Rating and terminal marking plates non-detachable

(b)

Earthing terminals with lugs - 2 Nos.

(c)

Lifting lugs for main tank & top cover

(d)

Pulling lugs - 4 Nos

(e)

HV bushings - 3 Nos.


72 (f)

LV bushings - 3 Nos.

(g)

Neutral bushings – 1 No.

(h)

Terminal connectors on the HV/LV bushings

(i)

Arcing horns or 9 kV 5kA lightning arrestors on HT side - 3 no.

(j)

Thermometer pocket with cap - 1 No.

(k)

Air release device

(l)

Stiffener angle 40 x 40 x 5 mm and vertical strip of 50 x 5 mm flat

(m)

Radiators

(n)

Pris matic oil level guage

(o)

Drain cum sampling valve

(p)

Oil filling hole having M30 thread with plug and drain valve on the conservator

(q)

Silicagel breather

(r)

Pressure relief device or explosion vent.

(s)

Base channel 75 x 40 mm

5.13 Fasteners All bolts, studs, screw threads, pipe threads, bolt heads and nuts shall comply with the appropriate Indian Standards for metric threads, or the technical equivalent. Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. All nuts and pins shall be adequately locked. Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanising, except high tensile steel bolts and spring washers which shall be electro-galvanised/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided whereever necessary.


73

5.14 Mounting Arrangement The under base of all transformers shall be provided with two 75 x 40 mm channels 460 mm long with holes to make them suitable for fixing on a platform or plinth 5.15 Overload Capacity The transformers shall be suitable for loading as per IS: 6600. 6.0.

TESTS

6.1.

Routine Tests

•

Ratio, polarity and phase sequence.

•

No load current and losses at rated frequency, rated voltage and at 90% & 110% voltage.

•

Load loss at rated current and normal frequency

•

Impedance voltage test

•

Resistance of windings

•

Insulation resistance

•

Induced over voltage withstand test.

•

Separate source voltage withstand test.

6.2

Special Tests

•

Lightening impulse chopped on the tail.

•

Short circuit test.

6.3

Additional Tests

•

Neutral current measurement

•

Air pressure test : As per C1.22.5 of IS - 1180 / part-I/1989

•

Magnetic balance test

•

Transformer tank shall be subjected to specified vacuum. The tank designed for vacuum shall be tested at an internal pressure of 0.35 kg/cm2 absolute (250 mm of Hg) for one hour. The permanent deflection of flat plates after the vacuum has been released shall not exceed the values specified below: Horizontal length of flat plate (in mm)

Permanent deflection (in mm)

Upto & including 750

5.0

751 to 1250

6.5


74 •

Transformer tank together with its radiator and other fittings shall be subjected to pressure corresponding to twice the normal pressure or 0.35 kg/cm2 whichever is lower, measured at the base of the tank and maintained for an hour. The permanent deflection of the flat plates after the excess pressure has been released, shall not exceed the figures for vacuum test.

•

The pressure relief device shall be subject to increasing fluid/air pressure. It shall operate before reaching the test pressure as specified in the above clause. The operating pressure shall be recorded. The device shall seal-off after the excess pressure has been released.

•

Oil samples (one sample per lot) to comply with IS 1866.

•

Single phase LV excitation current at all three phases (for reference)

6.4

Type Tests to be Conducted on one Unit In addition to the tests mentioned above the following tests shall be conducted.

•

Temperature rise test.

•

Lightening impulse withstand voltage test : -

Oil samples (before and after short-circuit and temperatures rise test) for each tested transformer.

SECTION B1

Specifications for Single Phase 11 kV / 250V and 11 kV/√ √ 3 / 250V Distribution Transformers (10, 15 & 25 kVA Ratings)

76


Specifcations for Singlephase 11 kV / 250V and 11 kV/√3 / 250V Distribution Transformers (10, 15 & 25 kVA Ratings)

77

SECTION B1 Specifications for Outdoor Type Single Phase Distribution Transformers 1.0

SCOPE

1.1 This section covers oil immersed naturally cooled 11 kV / 250V and 11 kV/ √3 / 250V single phase distribution transformers, but does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to sections A to J of the Manual. 1.2

Standard Ratings

The Standard Ratings of 1φ Transformers shall be 10, 15 & 25 kVA. 2.0

STANDARDS

2.1 The materials shall conform in all respects to the relevant Indian Standard Specifications with latest amendments/edition thereof : Indian Standard

Title

International and Internationally recognized standard

ISS-2026 (Part -I to IV)

Specification for Power Transformer

IEC-60076

ISS-1180 (Part 1& 2) 1989

Outdoor Three Phase Distribution Transformer

ISS-3347/1967

Specification for Porcelain Transformer Bushings

ISS-7421/1974

Specification for Low Voltage Bushings

ISS - 12444

Specification for Copper Wire Rods

ASTM B - 49

ISS-335/1983

Specification for Transformer Oil

BS 148/ASTM D1275, D1533, IEC Pub 296-1969

ISS-3070/1974

Specification for Lightning Arresters

IEC 99-1

ISS-6600/1972

IEC 60076-7 (IEC 354)

ISS-2099/1973

Guide for Loading of Oil Immersed Transformers High Voltage Porcelain Bushings

ISS 9335

Specification for Insulating Kraft Paper

IEC 60554

ISS 1576

Specification for Insulating Press Board

IEC 60641

ISS 5/1961

Specification for Colours for Ready Mixed Paints

DIN 42531,2,3

IEC 60137


78 3.0

SERVICE CONDITIONS

The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows as per IS 2026 (Part-I) latest revision. International Standards tabulated above. (i) (ii) (iii)

Location Max ambient air temperature (deg.C) Min. ambient air temperature (deg.C)

: : :

–––– –––– ––––

(iv)

:

––––

:

––––

(vi)

Max. average daily ambient air temperature (deg.C) Max. yearly weighed average ambient temperature (deg.C) Max. altitude above mean sea level (m)

:

––––

4.0

STANDARD RATINGS

(v)

The transformers shall be suitable for outdoor installation with Single phase, 50 c/s 11 kV systems in which the neutral is effectively earthed and they should be suitable for service under fluctuations in supply voltage upto plus 10% to minus 15%. The transformer shall conform to the following specific parameters. Rated HV side value (11 kV or 11/√ √ 3) Sl. No. Item 1. Continuous rated capacit y 2. System voltage (max)

10 kVA, 15 kVA, 25 kVA 12 kV

3. 4. 5. 6.

Rated voltage HV Rated voltage LV BIL Frequency

11/√3 or 11 kV 250 V 75 kV (Peak) 50 Hz + 3%

7. 8. 9. 10.

No. of phases Type of cooling Tap changing arrangement Percentage impedance at rated frequency and 75o C

Single ONAN Not provide d

11.

Permissible temperature rise over ambient (i)

12.

Less than or equal to 4% with tolerance as per IS : 2026

Of top oil measured by thermometer

35 deg.C

(ii) Of winding measured by resistance

40 deg.C

Minimum clearances in air

As per IS-1180

(a)

HV phase to phase/ phase to earth (mm) 255/140

(b)

LV phase to phase/ phase to earth (mm) 75/40


5.0


5.1

Winding Connection and Terminal Arrangements

79

For 11 kV transformers both ends of primary winding shall be brought out through HV bushings. For 11 kV/ √3 transformers, neutral end of the primary HV winding shall be bought out for connecting to ‘Neutral’ supply wire through 1.1 kV bushing. There shall be provis ion for connecting ‘Neutral’ terminal, to local ‘Earth’ by way of a tinned Copper strip, of adequate size and dimension. The secondary winding shall be connected to two LV bushings. 5.1

Core

5.1.1 Core Material Transformer core shall be shell type or core type wound core construction using high quality CRGO steel with heat resistant insulating coating. The core shall be properly stress relieved by annealing. The transformer shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12.5% without injurious beating. The operating flux density shall be such that there is a clear safe margin over the over fluxing limit of 12.5%. 5.2

Winding

HV and LV windings shall be wound from copper conductors covered with DPC (double paper covered) / Enamel. The windings shall be progressively wound in LV-HV coil design for better voltage regulation and mechanical strength. 5.3

Oil

The insulating oil shall comply with the relevant standards/specifications. 5.4

Insulation Material

The inter layer insulation shall be of epoxy resin bond paper. The core/coil assembly shall be securely held in position to avoid any movement under short circuit conditions. 5.5.

Tank

The oil volume inside the tank shall be such that even under the extreme operating conditions, the pressure generated inside the tank does not exceed 0.4 kg/sq cm positive or negative. There must be sufficient space from the core to the top cover to take care of oil expansion. The tank cover shall have plasticised surface at the top to guard against bird faults. Alternately, suitable insulating shrouds shall be provided on the bushing terminals. The Transformer tank shall be of robust construction round in shape and shall be built up of tested CRCA / MS sheet.

80


The tank shall be capable of withstanding a pressure of 1 kg/cm2 (g) and a vacuum of 760 mm of Hg for 30 minutes without any permanent deflection (Air pressure test shall be conducted as per IS -1180) The L - seam joint, C - seam joint and all fittings and accessories shall be oil tight and no deflection / buldging should occur during service. The circular base plate edges of the tank should be folded upward, for at least 25 mm, to have sufficient overlap with vertical sidewall of the transformer. Tank shall have permanent lugs for lifting the transformer bodily and there shall be facilities for lifting the core coil assembly separately. The transformer tank and the top cover shall be designed in such a manner as to leave no external pockets in which water can lodge. The transformer shall be provided with two mounting lugs suitable for fixing the transformer to a single pole by means of 2 bolts of 20 mm diameter as per ANSI C 57.12.20-1988. Both mounting lugs are made with steel of min. 5 mm thickness. Minimum Oil level mark shall be embossed inside the tank. Jump proof lips shall be provided for upper mounting lug. Mounting lugs faces shall be in one plane. The top cover shall be fixed to the tank through clamping only. HV bushing pocket shall be embossed to top side of the top cover so as to eliminate ingressing of moisture and water. The edges of the top cover shall be formed, so as to cover the top end of the tank and gasket. Nitrile / neoprene rubber gaskets conforming to latest IS 4253 part-II shall be provided between tank and top cover. The space on the top of the oil shall be filled with dry air or nitrogen. The nitrogen plus oil volume inside the tank shall be such that even under extreme operating conditions, the pressure generated inside the tank does not exceed 0.4 kg/cm2 positive or negative. The nitrogen shall conform to commercial grade of the relevant Standard. 5.6


For surface preparation refer to section ‘A’ of this Manual.


5.7

81

Bushing Terminals

HV terminal shall be designed to directly receive ACSR conductor upto 7/2 59 mm (without requiring the use of lug) and the LV terminals shall be suitable for directly receiving LT cables (aluminium) ranging from 10 sq mm to 25 sq mm both in vertical and horizontal position and the arrangements should be such as to avoid bimetallic corrosion. Terminal connectors must comply as per IS : 5561. 5.8

Bushings

The bushings shall conform to the relevant standards specified. For HV, 12 kV class bushings shall be used and for LV, 1.1 kV class bushings shall be used. The HV bushings shall be fixed to the top cover of the transformer and the LV bushings shall be fixed to transformer on sides and in the same plane. The bushing rods and nuts shall be of brass. The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. The HV bushings shall not have arcing horns. 5.9

Rating and Terminal Plates

There shall be a rating plate on the transformer containing the information given in the relevant Indian Standard. The HV winding terminals shall be marked 1.1 and 1.2 for 11 kV/√3 HV winding. 1.2 terminal will be connected to neutral. In case of 11 kV HV winding the terminal shall be marked as 1.1 – 1.2. The corresponding secondary terminal shall be marked as 2.1 and 2.2. 5.10 Pressure Release Device The transformer shall be equipped with a self sealing pressure release device designed to operate at a pressure of 8 PSI (0.564 kg/ cm2 ). 5.11 Fittings The following standard fittings shall be provided : (a)

Rating and terminal marking plates.

(b)

Earthing terminals – 2 No.

(c)

Lifting lugs – 2 No.

(d)

HV bushings.

(e)

Bird guard.


82 (f)

LV bushings.

(g)

HV & LV terminal connectors.

(h)

HV side Neutral ‘Earthing’ strip;

(i)

LV earthing arrangement.

(j)

Metal oxide lightning arrestors (9kV, 5kA)

(k)

Top cover fixing clamps.

(l)

Pressure relief device.

(m)

Mounting lugs - 2 Nos.

(n)

Any other fitting necessary for satisfactory performance of the manufacture.

5.12 Fasteners All bolts, studs, screw threads, pipe threads, bolt heads and nuts shall comply with the appropriate Indian Standards for metric threads, or the technical equivalent. Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. All nuts and pins shall be adequately locked. Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanising, except high tensile steel bolts and spring washers which shall be electro-galvanised/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided where necessary. 6.0

TESTS

6.1

Routine Tests

•

Ratio, polarity tests.

•

No load current and loss at rated voltage and frequency.

•

Load loss at rated current and normal frequency.

•

Impedance voltage test.


•

Resistance of windings.

•

Insulation resistance.

•

Induced over voltage withstand test.

•

Separate source voltage withstand test.

6.2

Special Tests

•

Lightening impulse chopped on the tail.

•

Short circuit test

6.3

Additional Tests

•

Oil samples test (one sample / lot) to comply with IS 1866

•

Air pressure Test: As per clause 24.5.1 of IS-1180/ part-II.

6.4

Type Tests to be conducted on one Unit

In addition to the tests mentioned above the following tests shall be conducted. •

Temperature rise test.

•

Lightening/Impulse withstand voltage test.

•

Oil samples (before and after short circuit and temperature rise test).

83

SECTION C

Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class)

86



87

SECTION C Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 1.0

SCOPE

1.1 This section covers, Three phase distribution transformers of above 100 kVA to 3000 kVA, 11 & 33 kV (outdoor & indoor use) but does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to sections A and J respectively of the Manual. 1.2

Standard Ratings kVA 160

Voltage Ratio

200 250 315 400 500 630 800 1000

11000/433V

1250 1600 2000 2500 3000 315 400 500 630 800 1000 1250 1600 2000

33000/433V

2500 3000

The above ratings are also applicable for 22/.433 kV transformers.


88 2.0

STANDARDS

2.1 The materials shall conform in all respects to the relevant Indian / International Standard Specification, with latest amendments thereof, some of them are listed below: Indian Standard ISS -2026/1997 ISS - 1180

Title

International & Internationally recognised standard

IS 12444 ISS -3347/1967 ISS-335/1983

Specification for Power Transformer Outdoor Distribution Transformer upto and including 100 kVA Specification for Copper Wire Rod Specification for Porcelain Transformer Bushing Specification for Transformer Oil

ISS - 5/1961 ISS - 2099/1973 ISS - 7421/1974 ISS - 3347 ISS - 5484 ISS - 9335 ISS - 1576 ISS - 6600

Specification for Colours for Ready Mixed Paints Specification for High Voltage Porcelain Bushings Specification for Low Voltage Bushings Specification for Outdoor Bushings Specification for Al Wire Rods Specification for Insulating Kraft Paper Specification for Insulating Press Board Guide for Loading of Oil Immersed Transformers

3.0

IEC 60076 ASTM B-49 DIN 42531,23,3 BS 148, D-1473, D-1533- 1934 IEC Pub 296-1969

DIN 42531 to 33 ASTM B - 233 IEC 60554 IEC 60641 IEC 60076

SERVICE CONDITIONS

3.1 The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows as per IS 2026 (Part - I) Latest Revision/International Standards tabulated above : (i)

Location

:

–––

(ii)

Max ambient air temperature (deg.C)

:

–––

(iii)

Min. ambient air temperature (deg.C.)

:

–––

(iv)

Max average daily ambient air temperature (deg.C)

:

–––

(v)

Max. yearly weighted average ambient temperature (deg.C)

:

–––

(vi)

Max. altitude above mean sea level (m)

:

–––

4.0

TAPPINGS AND TAP CHANGING

4.1 Tappings shall be provided on the higher voltage winding for variation of HV Voltage from plus 5% to minus 10% in steps of 2.5%. 4.2 Tap changing shall be carried out by means of an off circuit externally operated self positioning tap switch when the transformer is in deenergised condition. Switch position No. 1 shall correspond to the maximum voltage tapping. Each tap change shall result in variation of 2.5% in voltage. Provision shall be made for locking the tap changing switch handle in position.


89

4.3 For ratings greater than 500 kVA On-load tapchanger may be provided for variation of HV voltage from plus 5% to minus 15% in steps of 1.25%. 5.0

TYPE OF COOLING

The transformers shall be oil immersed with natural oil circulation type-ONAN. 6.0

INSULATION LEVELS

Voltage 433 11000 33000

7.0

Impulse voltage (kV Peak) -75 170

Power frequency (kV) 3 28 70

WINDING CONNECTIONS

HV-------Delta LV ---------Star Vector Symbol -----Dyn11 8.0

LOSSES AND IMPEDANCE (Subject to Tolerance as per IS 2026)

Rating 160 200 250 315 400 500 630 800 1000 1600 2000 2500 3000 315 400 500 630 800 1000 1600 2000 2500 3000

Voltage ratio (volts) 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 33000/433 33000/433 33000/433 33000/433 33000/433 33000/433 33000/433 33000/433 33000/433 33000/433

No load loss (watts) 375 450 530 650 750 900 1030 1200 1400 2000 2300 2600 3200 760 920 1100 1400 1550 1800 2500 3000 3400 3800

For 22/.433 kV transformers losses of 33/.433 kV shall be applicable.

Load loss (watts) 2100 2450 2850 3500 4200 5000 5800 8000 10400 14000 19000 25000 32000 5000 6200 7000 8500 10500 12000 16000 21000 28000 34000

Impedance % 4 4 4 4 4 4 4 5 5 6.25 6.25 6.25 6.25 4 4 4 4 5 5 6.25 6.25 6.25 6.25


90 9.0


9.1

Core

9.1.1 Material - CRGO The core shall be stacked type generally of high grade cold rolled grain annealed steel lamination having low loss and good grain properties, coated with hot oil proof insulation, bolted together and to the frames firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core losses with continuous working of the transformers. 9.1.2 Core Clamping for CRGO Stacked Core MS channel or plate shall be used on top and bottom Channel frames on LV side to be reinforced at equidistance, if holes / cutting is done for LT lead in order to avoid bending of channel. MS channels/plate frames shall be painted with hot oil-resistant varnish or paint. 9.1.3 The transformers core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 10% without injurious heating at full load conditions and shall not get saturated. 9.1.4 No load current shall not exceed 3% of full load current for ratings below 315 kVA and shall not exceed 1.5% of full load current for ratings above 315 kVA. Increase in secondary voltage of 433 volts by 10% shall not increase the no load current beyond 6% of full load current for ratings below 315 kVA and 4% of full load current for ratings above 315 kVA. 9.2

Windings

9.2.1 Material: conductor.

Super enamel covered/Double paper covered (DPC) Copper round/strip

9.2.2 LV winding shall be of strip type copper conductor/copper foil type. 9.2.3 HV coil is wound over LV coil as crossover coils or continuous disc coils. 9.2.4 Inter layer insulation shall be Kraft paper/Epoxy dotted paper. Proper bonding of inner layer insulation with the conductor shall be ensured. 9.3

Oil

The insulating oil shall comply with the requirements of relevant standards IS 335. 9.4

Temperature Rise

The temperature rise over ambient shall not exceed the limits described below: Top oil temperature rise measured by thermometer

:

50 deg.C

Winding temperature rise measured by resistance

:

55 deg.C


9.5

91

Insulation Material

Material: Electrical grade insulation Kraft paper. All spacers, axial wedges / runners used in windings shall be made of pre-compressed Pressboard−solid, conforming to type B 3.1 of IEC 641-3-2. In case of cross-over coil/continuous disc winding of HV all spacers shall be properly sheared and dovetail punched to ensure proper locking. All axial wedges / runners shall be properly milled to dovetail shape so that they pass through the designed spacers freely. Insulation shearing, cutting, milling and punching operations shall be carried out in such a way, that there is no burr or dimensional variations. 10.0

TANK

10.1

Rectangular Plain Tank

The transformer tank shall be of robust construction rectangular in shape and shall be built up of tested MS sheet. The internal clearance of tank shall be such that it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings. All joints of tank and fittings shall be oil tight and no buldging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. The four walls of the tank shall be made of two “L” shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.8 kg/cm2 for 10 minutes. The tank shall be reinforced by welded angle on all the outside walls on the edge of the tank to form two equal compartments. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for rectangular tank and 760 mm of mercury for round tank shall not be more than 5 mm upto 750 mm length and 6 mm upto 1250 mm length. The tank shall further be capable of withstanding a pressure of 0.8 kg/sq cm (g) and a vacuum of 0.3 kg/sq cm (g) without any deformation. Pressed steel radiators / circular cross section cooling tubes/ elliptical tubes shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 4 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall. Top cover fixing shall be with galvanised iron bolts and 6 mm Neoprene bonded cork gasket conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer.


92 10.2

Corrugated Tank

The transformer tank shall be of robust construction corrugated in shape and shall be built up of CRCA sheets of 1.2 mm thickness. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank with CCA (core-coil assembly), HV & LV bushings mounted on Top cover. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. Corrugated panel shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 2 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced shall be provided. Top cover fixing shall be with GI (Galvanised Iron) bolts and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer. Tanks with corrugations and without conservator shall be tested for leakage test at a pressure of 0.15kg/cm2 measured at the top of the tank. 10.3

Sealed Transformer with Radiators

In this type of construction, tank is designed to have cover welded to the curb of tank. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure. The tank should be of stiff construction, able to withstand pressure of 2 atmospheres. 10.4

Conservator

The provision of conservator is obligatory for plain Tanks mentioned in clause 10.1 above i.e., where circular tubes or elliptical tubes or pressed steel radiators are used for oil circulation. Conservator is not required for corrugated tanks. When a conservator is provided, oil gauge and the plain or dehydrating breathing devise shall be fixed to the conservator, which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations. In addition, the cover of main tank shall be provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank. The inside diameter of the pipe, connecting the conservator to the main tank, should be within 20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump for


93

collection of impurities. The minimum oil level (corresponding to -5 deg.C.) should be above the sump level. 10.5


For surface preparation refer to section A of this Manual. 11.0

TERMINALS

The terminals arrangement alternatives are given below : kVA All ratings

Voltage 11000

All ratings

33000

160/200 250 315,400,500 630,800 1000,1250 1600 2000,2500 3000

433 433 433 433 433 433 433

Details of Terminals 17.5 kV porcelain Bushings as per IS 3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables. 36 kV porcelain Bushings as per IS3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables. 4p1G air filled cable box suitable for3 1/2core 135mm 2 PVC aluminium cable 4p1G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 4p2G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 4p4G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 4p6G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 4p21G air filled cable box suitable for 1 core 1000mm2 PVC aluminium cable 4p28G air filled cable box suitable for 1 core 1000mm2 PVC aluminium cable

Note : (a) Alternatively 433V terminal could be provided wth 1.1kV bushings as per IS:3347 for normally polluted atmosphere. (b) Alternatively 433V terminal could be provided wth 1.1kV bushing suitable for bus duct connections. (c)

Alternatively 433V terminal could be provided wth 1.1kV epoxy bushings in cable box or bus duct.

(d) P & G denote ‘Pole’ and ‘Gland’ respectively. (e) Epoxy may be used as the filling medium instead of bitumen compound.

(i)

The bushings shall conform to the relevant standards specified and shall be outdoor type. The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. The tests as per IS 2099 / IS 7421 shall be conducted on the transformer bushings.

(ii)

For 0.433/11 kV/33 kV service voltage, 1.1/17.5/36 kV class bushings shall be used. Bushings of plain sheds as per IS-3347 shall be mounted on the tank/cover. For 1.1 kV class indoor transformers, 1.1 kV class epoxy busbar bushings /porcelain bushings can be used. Bushings in HV cable box as per BS2562 may be used for compound filled cable box or termination in air with boots covering the live terminals.

(iii)

Dimensions of the bushings shall conform to the Standards specified.

(iv)

A minimum phase to phase clearance of 75 mm for LV (upto 1.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushings mounted on the transformer.

(v)

The bushings shall be fixed on the sides with pockets in the same plane or on the top cover.


94 (vi)

Brazing of all inter connections, jumpers from winding to bushing shall have crosssection larger than the winding conductor.

(vii)

The design of the cable box internal bushing for LV shall be such as to provide adequate earth clearance and creepage distance as stipulated in the standards specified. All other tests as per relevant standards shall be applicable.

(viii) The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. (ix)

HV and LV bushings shall be mounted on top cover in case of corrugated tank.

11.1

Terminal Clearances

The minimum clearance shall be as under: Voltage Medium

433 11000 33000

Clearance phase to phase (mm) terminal chamber Open Closed

Air Air * Compound Air * Compound

40

25

280 –– 351 ––

165 75 351 125

Clearance phase to earth (mm) terminal chamber Open Closed 40

20

140 102 –– 60 320 222 –– 100

* Clearances given against compound filled cable box are applicable for same cable box in air if terminals insulated with boots are used and cable is of XLPE type.

For outdoor bare bushings the LV and HV bushing stems shall be provided with suitable terminal connectors so as to connect the jumper without disturbing the bushing stem. 11.2

Terminal Markings

High voltage and low voltage phase windings shall be marked both in the terminal boards inside the tank and on the outside with capital letter 1U, 1V, 1W and low voltage winding for the same phase marked by corresponding letter 2U, 2V, 2W. The neutral point terminal shall be indicated by the letter 2N. 11.3 Fittings to be Provided The following fittings shall be provided for transformers with conservator : (a) Rating and terminal marking plates. (b)

Two earthing terminals (studs and bolts should be properly galvanized and conform to IS:1363 and IS:1367.

(c)

Two lifting lugs to lift core assembly.

(d)

Two lifting lugs to lift complete transformer

(e)

Lifting lugs for tank cover.


95

(f)

Thermometer pocket in accordance with IS: 3580.

(g)

Air release plug on the transformer tank to release air trapped inside the tank when filling oil through conservator.

(h)

Conservator tank shall have inter connection pipe projection, 20 mm above bottom of the conservator so as to create a sump for collection of impurities. It shall have 30 mm dia drain valve, oil filling hole with cap on the top of the conservator.

(i)

Oil level gauge with toughened glass with “minimum” marking.

(j)

De-hydrating breather.

(k)

One drain cum sampling valve.

(l)

One filter valve on the upper side of the tank

(m)

Unidirectional flat rollers.

(n)

Inspection hole

For sealed transformers with radiators and nitrogen cushion, the following accessories are recommended : (a)

Oil level guage

(b)

Pressure guage

(c)

Oil temperature indicator and winding temperature indicator (optional).

(d)

One drain cum sampling valve

(e)

One filter valve on upper side of tank.

11.4 Fasteners Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. Wherever possible, bolts shall be fitted in such a manner that in the event of failure of locking, resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanizing, except high tensile steel bolts and spring washers which shall be electro-galvanized/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts, nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided where necessary.

SECTION D

Specifications for Power Transformers of Voltage Class below 145 kV

98



99

SECTION D Specifications for Power Transformers of Voltage Class below 145 kV 1.0

SCOPE

1.1 This section covers technical requirements/parameters for power transformers for voltage below 132 kV. This part specification does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to Section ‘A’ and ‘J’ respectively of this Manual. For 110 kV/100 kV class transformers reference can be drawn from section E for appropriate rating of transformer. 1.2

Standard Rating

1.2.1 66 kV Class Transformers There-phase power rating MVA

Voltage ratio kV

Cooling

6.3

66/11

ONAN

8.0

66/11

ONAN

10.0

66/11

ONAN

12.5

66/11

ONAN/ONAF

20.0

66/11

ONAN/ONAF

ONAN rating-60 percent of ONAF.

1.2.2 33 kV Class Transformers Three-phase power rating MVA

Voltage ratio kV

Cooling

1.0

33/11

ONAN

1.6

33/11

ONAN

3.15

33/11

ONAN

4.0

33/11

ONAN

5.0

33/11

ONAN

6.3

33/11

ONAN

8.0

33/11

ONAN

10.00

33/11

ONAN


100 1.2.3 11 kV Class Transformers

(A)

(B)

Rating MVA

Voltage ratio kV

Impedance voltage percent

Cooling

3.15

11/6.6

6.25

ONAN

4

11/6.6

7.15

ONAN

3.15

11/3.3

6.25

ONAN

4

11/3.3

7.15

ONAN

5

11/3.3

7.15

ONAN

6.3

11/3.3

7.15

ONAN

2.0

WINDING CONNECTIONS AND VECTOR GROUP

2.1

Transformers of 11 kV and 33 kV Class

2.2

HV

— Delta

LV

— Star

Vector Group

— Dy 11

Transformers of 66 kV Class HV

— Star

LV

— Star

Vector Group

— Yyo

Note : No Tertiary Winding is required for 66 kV Class Transformers.

3.0

TAPPINGS

3.1 OLTC is not recommended for 11 kV and below 5 MVA. For other kV class transformers, this may be provided for higher ratings, if required. In case of off-circuit tap changer, the tappings shall be such as to provide for a voltage adjustment on the high voltage of + 3 percent to – 9 percent in steps of 3 percent, the tappings being located on the high voltage winding. In case of on-load tap changer, the tappings shall also be on the high voltage winding. A voltage adjustment of high voltage of + 5 to – 15 percent in 16 equal steps is recommended. With transformers having OLTC, these tappings may be used to get 10 percent over-voltage on low voltage side at no-load. When under this condition the high voltage side experiences an over-voltage, the tappings shall be changed so that the over-excitation is limited to 10 percent only.


4.0

101

INSULATION LEVELS Highest Voltage for equipment kV rms

Rated lighting impulse withstand voltage kV peak

Rated Short duration power frequency withstand voltage kV rms

3.6

40

10

7.2

60

20

12

75

28

36

170

70

72.5

325

140

66 kV Windings will be with graded insulation.

5.0

TEMPERATURE RISE

For the purpose of standardisation of maximum temperature rises of oil and windings, the following ambient temperatures are assumed : Cooling medium

: Air

Maximum ambient temperature

: 500 C

Maximum daily average ambient temperature

: 400 C

Maximum yearly weighted average temperature

: 320 C

With the above ambient temperature condition the temperature rises are as given below: Oil 0 C 50

6.0

Winding 0 C 55

TERMINALS

3.3 kV-3.6 kV porcelain bushings with plain sheds as per IS : 3347. 6.6 and 11 kV–17.5 kV porcelain bushings with plain sheds as per IS : 3347. 33 kV-36 kV procelain bushings with plain sheds as per IS : 3347. 66 kV-72.5 kV condenser bushings. Transformers shall be fitted either with bushing insulators or cable boxes as required by the purchaser. 7.0

FITTINGS AND ACCESSORIES

(a)

Rating and diagram plate.

(b)

2 Nos. earthing terminals.


102 (c)

Cover lifting lugs.

(d)

Lifting lugs.

(e)

Skids and pulling eyes on both directions.

(f)

Oil-filling hole and cap.

(g)

Jacking pads.

(h)

Pocket on tank cover for thermometer.

(i)

Air release devices.

(j)

Conservator with oil filling hole, cap and drain plug-size 19 mm nominal pipe (3/4 in. BSP/M 20).

(k)

(i)

Plain oil level gauge for all transformers upto and including 1.6 MVA.

(ii)

Magnetic type oil gauge for transformers above 1.6 MVA, with low oil level alarm contact.

(l)

Silica gel breather with oil seal.

(m)

Pressure relief device.

(n)

Valves :

(o)

(i)

Drain valve with plug or blanking flanges. The same can be used for filtering purpose.

(ii)

A sampling device or sampling facility on drain valve.

(iii)

1 No. filter valve on upper side of transformer tank.

Buchholz relay with alarm and trip contacts with one shut-off valve on conservator side (i) Size of Buchholz relay up to 10 MVA -50 mm (ii) 10 MVA and above-80 mm

(p)

Oil temperature indicator with one electrical contact shall be provided with antivibration mounting.

(q)

Winding temperature indicator with two electrical contacts for alarm and trip purposes. Switching of fans shall be done by winding temperature indicator for all transformers having ONAF rating. The winding temperature indicator shall be provided with anti-vibration mounting.

(r)

Tank mounted weather-proof marshalling box for housing control equipment and terminal connectors. Wiring up to marshalling box with PVC SWA PVC copper cables 660/1100 volts grade.


(s)

103

Rollers-4 Nos. Gauge Rating

Type Shorter axis

1

Up to 5 MVA

Flat, uni-directional

2 3

6.3 MVA Flanged, bi-directional 10 MVA and above Flanged, bi-directional

As per manufacturer’s practice, however, not to exceed 1000 mm 1435 mm 1435 mm 1676 mm 1676 mm

(t) (u)

Inspection cover. Cooling accessories ONAN/ONAF cooling (i) Radiators with shut-off valves and air release plugs. (ii) Fans. (iii) Filter valves. (iv) Drain and sampling device. (v) Air release device.

8.0

STANDARD LOSSES AT 750 C Three-phase power rating MVA

(a)

(b)

(c)

Longer axis

No-load loss (kW)

Load loss (kW)

Percentage impedance (%)

3.15

2.9

20

6.25

4.0

3.2

27

7.15

5.0

3.9

31

7.15

6.3

4.5

37

7.15

1.00 1.60 3.15 4.00

1.8 2.1 3.2 4.0

8 14 22 24

5 6.25 6.25 7.15

5.00 6.30 8.00 10.00

4.6 5.4 6.1 7.2

27 33 44 53

7.15 7.15 8.35 8.35

66 kV Transformers 6.3 8.0 10.0

6.0 7.1 8.4

40 48 57

8.35 8.35 8.35

12.5 20.0

9.7 13.0

70 102

8.35 10.00

11 kV Transformers

33 kV Transformers

Note : The above losses are standardised on the basis of the optimised designs from various manufacturers.

SECTION E


106



107

SECTION E Specifications for 145 kV Class Power Transformers 1.0

SCOPE

1.1 This section covers technical requirements/parameters for power transformers of 145 kV class. This part specification does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to Sections A and J respectively of this Manual. For 100/110 kV class transformers reference can be drawn from this section for appropriate rating of the transformers. 1.2

Standard Ratings

1.2.1 Two Winding Transformers

(i)

Three-phase power rating MVA

Voltage ratio kV

Impedance voltage per cent

Cooling

(a) 16 16 16 (B) 16 25 31.5

132/33 132/33 132/33 132/11 132/11 132/11

10 10 12.5 10 10 12.5

ONAN/ONAF ONAN/ONAF ONAN/ONAF ONAN/ONAF ONAN/ONAF ONAN/ONAF

Connections

:

Vector Group

:

Vector Group :

HV-Star with neutral directly earthed LV-Star with neutral directly earthed YNynO Alternatively HV-Star with neutral directly earthed LV-Delta YNd11

(ii) Tappings : On-load tappings at the neutral end of HV winding for HV variation. Tapping Race + 5 to –15 per cent in 16 equal steps. (iii) ONAN rating for 16 MVA, 25 MVA and 31.5 MVA transformers shall be 10, 16 and 20 MVA respectively. 1.2.2 Interconnecting Auto-Transformers Three-phase power rating MVA

Voltage ratio kV


Cooling

50

132/66

10

ONAN/ONAF

63

132/66

10

ONAN/ONAF


108 (i)

Connections Vector Group

Note : (i)

: HV and LV – Star auto with neutral directly earthed. : Y Nao

No stabilising winding upto 100 MVA for 3-Phase, 3 limbed core type construction.

(ii) Tappings : On-load for the variation of 66 kV voltage from - 5 to + 15 per cent in 16 steps. (iii) ONAN rating shall be 60 per cent of ONAF rating.

1.2.3 Generator Transformers Three-Phase Power Rating MVA

Voltage ratio


140

11/138

12.5

140 250

13.8/138 15.75/138

12.5 14.5

(i)

Connection :

Cooling ONAN/OFAF or ODAF or OFWF/ODWF --do-ONAN/OFAF/ ODFAF or OFWF/ODWF

HV-Star with neutral directly earthed LV-Delta

Vector Group : YNd11 (ii)

Tappings : Off-circuit taps on HV for HV variation from + 21 /2 to-71 /2 percent in 21 /2 per cent steps or On-load tap changer on HV for HV variation from + 5 per cent to – 10 per cent in 1.25 per cent steps.

(iii)

ONAN rating in case of ONAN/OFAF cooling shall be 60 per cent of OFAF rating. OFAF/ODAF = 100 per cent.

(iv)

The standardised ratings are for three-phase units only. If single phase units are required due to transport limitations, then these ratings will be one-third of the threephase unit.

2.0

INSULATION LEVELS Highest voltage for equipment kV rms

Rated lightning impulse withstand voltage kV peak

Rated short duration power frequency withstand voltage kV rms

12

75

28

17.5

95

38

36

170

70

72.5

325

140

550

230

145


3.0

COOLING EQUIPMENT

(a)

ONAN/ONAF

(b)

ONAN/OFAF or ODAF

(c)

OFWF or ODWF

4.0

TEMPERATURE RISE

109

— 1-100 per cent tank or separately mounted cooling system consisting of radiators and fans and one standby fan — 2-50 per cent group and 2 standby fans, one in each 50 per cent group — 2-50 per cent groups 2-100 per cent pumps, one of which will be standby. 2-standby fans one in each 50 per cent group — 3-50 per cent group with independent pump and fans out of which one group to act as standby. — 2-100 per cent heat exchangers out of which one is standby.

For the purpose of standardisation of maximum temperature rises of oil and winding the following ambient temperatures considering the transformer to be operating at extreme tap position incurring extra copper losses. Cooling medium

Air

Maximum ambient temperature Maximum daily average ambient temp. Maximum yearly weighted average temp.

0

50 C 40 0 C 32 0 C

Water 30 0 C 25 0 C

With the above ambient temperature conditions temperature rises considering the transformer to be operating at extreme tap position incurring extra copper losses are as given below : External Cooling Medium Air

Part

Water

Winding (measured by resistance) o C

55, when the oil circulation is natural or forced nondirected. 60, when the oil circulation is forced directed.

60, when the oil circulation is natural or forced non-directed. 65, when the oil circulation is forced directed.

Top oil (measured by thermometer) o C

50

55

5.0

TERMINAL BUSHINGS

(a)

Two-winding and auto-transformers The terminal bushings shall be as per section-P of the manual.

(b)

Generator transformer LV side :

HV side

:

LV bushings shall be mounted on turrets suitable for connection to busbars in isolated phase bus ducts. As per Section-P.


110 6.0

FITTINGS AND ACCESSORIES

(a)


(b)


(c)

Lifting bollards.

(d)

Jacking pads.

(e)

Haulage lugs.

(f)


(g)


(h)

Conservator with oil filling hole, cap and drain valve.

(i)

Magnetic type oil level gauge with low oil level alarm contacts of 0.5 A, 220 V DC rating.

(j)

Silica gel breather with oil seal-2 Nos. of 100 per cent for 140 and 250 MVA ratings.

(k)

Pressure relief vent, or spring operated pressure relief devices.

(l)

Valves (I) For transformers up to 31.5 MVA (i)

Drain valve with plug or blanking flange. (The same can be used for filter purposes).

(ii)

1 No. filter valve at top of transformer tank.

(II) For transformers above 31.5 MVA (i)

Oil valve between each cooler and main tank.

(ii)

Drain valve.

(iii)

2 Nos. filter valves, one on top and another at bottom on diagonally opposite corners.

(iv)

2 Nos. sampling valves at top and bottom of main tank.

The sampling valve shall be provided with provision for fixing PVC pipe. (m)

Valve schedule plate for transformers above 31.5 MVA.

(n)

Buchholz relay with alarm and trip contacts of 0.5A, 220 V DC rating and one shut-off valve size 80 mm.

(o)

(i)

Oil temperature indicator with maximum-pointer and one electrical contact.

(ii)

Oil temperature indicator with maximum pointer and two sets of contacts for above 31.5 MVA.


111

(p)

Winding temperature indicator with ‘maximum pointer’ and 3 sets of contacts for ONAN/ONAF and 4 sets of contacts for ONAF/OFAN or ODAF and 2 sets of contacts for OFWF/ODWF.

(q)

Repeater dial of winding temperature indicator for remote indication for transformers above 16 MVA. For transformer above 50 MVA, the remote indication shall be a separate measuring system.

(r)

Rollers. Gauge Rating

Type

Shorter axis

Longer axis

1.

Two winding and auto transformers

Flanged bi-directional with locking and bolting device.

1676 mm

1676 mm

2.

Generator transformers

Flanged, bi-directional with locking and bolting device.

2 rails with 1676 mm gauge


Alternatively 3 rails with 1676 mm gauge between adjacent rails. Alternatively 4 rails in two pairs with 1676 mm gauge for each pair and centre distance betweem pair 3486 mm

(s)

Inspection cover.

(t)

Wiring upto marshalling box with PVC copper cables, 660/1100 volts grade.

(u)

Tank mounted/floor mounted weather-proof marshalling box for housing control equipment and terminal connections.

(v)

On-load tap changing gear with remote control panel as required.

(w)

Cooling accessories : (I)

ONAN/ONAF Cooling

(i)

Requisite number of radiators with top and bottom shut-off-valves, air release plug and drain plug.

(ii)

Fans.

(iii)

For header mounted radiator 2 Nos. valves, one at top header and other at bottom header to be used for filtration and oil filling. Bottom valve to be used as drain valve also.

(iv)

Drain and samp ling device.

(v)

Air release device.


112 (II)

(III)

(IV)

ONAN/ONAF-OFAF/ODAF Cooling (i)

Requisite number of radiators with shut-off-valves.

(ii)

Fans.

(iii)

Oil pumps.

(iv)

Oil flow indicator with one alarm contact.

(v)

For header mounted radiators 2 Nos. valves, one at top header and other at bottom header to be used for filtration and oil filling. Bottom valve to be used as drain valve also.

(vi)

Drain-cum-filter valve for cooling system.

(vii)

Air release plug of 19 mm nominal pipe size (3/4 in BSP).

OFAF/ODAF Cooling (i)

OFAF coolers with integral fans.

(ii)

Oil pumps.

(iii)


(iv)

Brass encased thermometers.

(v)

Drain plug and air release devices.

OFWF/ODWF Cooling. (i)

Oil/Water heat exchangers.

(ii)

Oil pumps.

(iii)


(iv)

Water flow indicator with one alarm contact.

(v)

Pressure gauges.

(vi)


(vii)

Differential pressure gauge with one alarm contact.

(viii) Reflux valves (Non-return). (ix)


SECTION F


114



115

SECTION F Specifications for 245 kV Class Power Transformers 1.0 SCOPE 1.1 This section covers technical requirements/parameters for power transformers of 245 kV voltage class but does not purport to include all the necessary provisions of contract. For general requirements and tests, reference shall be made to Sections A and J respectively of this Manual. 1.2 Standard Ratings 1.2.1 Two Winding Transformers (A) Three-phase power rating MVA

Voltage ratio kV


50

220/66

12.5

100

220/66

12.5

(i)

Cooling ONAN/OFAF or ONAN/ODAF ONAN/OFAF or ONAN/ODAF

Connections

:

HV—Star with neutral effectively earthed. LV—Star with neutral effectively earthed.

Stabilising winding

:

(ii) Vector Group (iii) Tapping

: :

(iv) Cooling

:

No stabilishing winding upto 100 MVA for 3-phase, 3 limbed core type construction. YNyno. On-load tappings at the neutral end of HV for HV –10 per cent in 16 equal steps. ONAN : 60 per cent OFAF : 100 per cent ODAF : 100 per cent The rating under ONAF condition although not guaranteed should be about 80 per cent.

(B) Three-phase power rating MVA

Voltage ratio kV


50

220/33

12.5

100

220/33

15.0

(i) Connections :

HV—Star with neutral effectively earthed. LV—Delta. Alternatively star with neutral effectively earthed.

Vector Group

Yd 11. Alternatively YNyno.

:

Cooling ONAN/OFAF or ONAN/ODAF ONAN/OFAF or ONAN/ODAF


116 Stabilising

:

No stabilising winding for YNyno upto 100 MVA for 3-phase, 3 limbed core type construction.

(ii) Tappings

:

On load tappings at the neutral end of HV for HV variation from + 10 to –10 per cent in 16 equal steps.

(iii) Cooling

:

ONAN OFAF ODAF

: : :

60 per cent 100 per cent 100 per cent

Note : The rating under ONAF condition although not guaranteed shall be about 80 per cent.

1.2.2 Auto-Transformers Three-phase power rating MVA

Voltage ratio

Percentage impedance voltage

100

220/132

12.5

ONAN/OFAF or ONAN/ODAF

160

220/132

12.5


200

220/132

12.5


(i) Connections (ii) Stabilising winding (iii) Vector Group (iv) Tappings

: : : :

(v) Cooling

Cooling

HV and LV — Star auto with neutral effectively earthed. Delta. YNaOd1. On-load for the variation of 132 kV voltage from —5 to + 15 per cent in 16 equal steps. ONAN : 60 per cent OFAN/ODAF : 100 per cent.

Note : (1) In case auto -transformers are provided with L.V. winding (Tertiary Winding) for loading purpose then the MVA rating, voltage rating, percentage impedance between HV winding to L.V. winding and IV winding to L.V. winding shall be specified by the customer. The minimum rated lightning impulse withstand voltage level shall be 170 kV peak. Rated short duration power frequency voltage shall be 70 kV. (2) Rating under ONAF condition although not guaranteed shall be about 80 per cent.

1.2.3 Generator Transformers for Thermal Stations Three-phase power rating MVA

Voltage ratio

Percentage impedance voltage

Cooling

140

11/235

12.5

ONAN/OFAF or ODAF or OFWF/ ODWF

140

13.8/235

12.5

ONAN/OFAF or ODAF or OFWF

250

15.75/235

14.0

315

15.75/235

14.0

ONAN/OFAF or ODAF or OFWF/ ODWF -do-


117

(i) Connections

:

HV — Star with neutral effectively earthed. LV — Delta.

(ii) Vector Group

:

YNd 11.

(iii) Tappings

:

Off-circuit taps on HV for HV variation from + 21 /2 to –71 /2 percent in 21 /2 per cent steps or On-load tap changer on HV for HV variation from + 5 per cent to –10 per cent in 1.25 per cent steps.

(iv) The standardised ratings are for three phase units only. If single phase units are required due to transport limitations then these ratings will be one-third of the three-phase unit. (v)

Cooling

:

ONAN

: 60 per cent

OFAF/ODAF

: 100 per cent.

2.0 INSULATION LEVELS Highest voltage for equipment kV rms 12.0 17.5 36 72.5 145 245

Rated lightning impulse withstand voltage kV peak 75 95 170 325 550 950

Power frequency rated short duration withstand voltage kV rms 28 38 70 140 230 395

3.0 COOLING EQUIPMENT (a)

ONAN/OFAF or ODAF

– 2-50 per cent groups 2-100 per cent pumps one of which will be standby, 2-Standby fans one in each 50 per cent group, or 3-50 per cent groups with independent pumps and fans out of which one group to act as standby

(b)

OFWF or ODWF

– 2 -100 per cent heat exchangers out of which one is standby.

4.0 TEMPERATURE RISE For the purpose of standardisation of maximum temperature rises of oil and winding, the following ambient temperatures are assumed: Cooling medium

Air

Water

Maximum ambient temperature

0

50 C

30 0 C


40 0 C

25 0 C


32 0 C

—


118

With the above ambient temperature conditions, temperature rises are as given below: Part

Air o C

External cooling-medium Water o C

Windings (measured by resistance)

55, when the oil circulation is natural or forced nondirected. 60, when the oil circulation is forced directed.

60, when the oil circulation is natural or forced nondirected. 65, when t he circulation is forced directed.

Top oil (measured) by thermometer)

50

55

5.0 TERMINAL BUSHINGS (a) Two windings and Auto-Transformers. The terminal bushings shall be as per Section-P. (b) Generator Transformer LV Side : HV Side

LV bushings shall be mounted on turrets suitable for connection to busbars in isolated phase bus ducts. :

As per Section-P.

6.0 FITTINGS AND ACCESSORIES (a)


(b)


(c)

Lifting bollards.

(d)

Jacking pads.

(e)

Haulage lugs.

(f)


(g)


(h)

Conservator with oil filling hole, cap drain valve.

(i)

Magnetic type oil gauge with low oil level alarm contacts of 0.5 A, 220 V DC.

(j)

Silicagel breather with oil seal-2 Nos.

Note : In addition to the silicagel breather more advanced oil preservation system like air dryers, molecular sieve flexible membrane can also be considered.

(k)

Required Nos. of pressure relief vents or spring operated pressure relief devices.

For transformer above 50 MVA, the remote indication shall be a separate measuring system. (l)

Valves (i)

Oil valve between each cooler and main tank


119

(ii)

Drain valve

(iii)

2 Nos. filter valves on diagonally opposite corners.

(iv)

2 Nos. sampling valves at top and bottom of main tank. The sampling valve shall be provided with provision for fixing PVC pipe.

(m) Valve schedule plate. (n)

Buchholz relay with alarm and trip contacts of 0.5A, 220 V DC and one shut-off valve on conservator side, size 80 mm.

(o)

Oil temperature indicator with maximum-pointer and two sets of contacts.

(p)

Winding temperature indicator with maximum pointer and 3 sets of contacts of ONAN/ONAF and 4 sets of contacts for ONAF/OFAN or ODAF and 2 sets of contacts for OFWF/ODWF.

(q)

Repeater dial of winding temperature indicator for remote indication.

(r)

Rollers. Gauge

1

Rating

Type

Shorter axis

Longer axis

2

3

4

5

1.

Two winding and Flanged auto-transformers bidirectional with locking and bolting device.

1676 mm

1676 mm

2.

Generator Transformers


2 rails with 1676 mm guage

Flanged bidirectional With locking and Bolting device.

Alternatively 3 rails with 1676 mm gauge between adjacent rails. Alternatively 4 rails in two pairs with 1676 mm gauge for each pair and centre distance between pair 3486 mm.

(s)

Inspection cover.

(t)

Wiring up to marshalling box with PVC copper cables, 660/1100 volts grade.

(u)

Tank mounted/floor mounted weather-proof marshalling box for housing control equipment and terminal connections.

(v)

On-load tap changing gear with remote control panels as required.


120 (w)

Cooling accessories. (I)

(II)

(III)

ONAN/OFAF or ODAF Cooling (i)

Requisite number of radiators with shut-off-valves.

(ii)

Fans.

(iii)

Oil pumps.

(iv)


(v)

For header mounted radiators 2 Nos. valves, one at top header and other at bottom header to be used for filtration and oil filling. Bottom valve to be used as drain valve also.

(vi)

Drain-cum-filter valve for cooling system size.

(vii)

Air release plug of size 19 mm nominal pipe (3/4in. BSP).

OFAF/ODAF Cooling (i)

OFAF/Coolers with integral fans.

(ii)

Oil pumps.

(iii)


(iv)


(v)

Drain plug and air release devices.

OFWF/ODWF Cooling (i)

Oil / Water heat exchangers

(ii)

Oil pumps.

(iii)


(iv)

Water flow indicator with one alarm contact.

(v)

Pressure gauges.

(vi)


(vii)

Differential pressure gauge with one alarm contact.

(viii) Reflux valves (Non-return) (ix)


SECTION G


122



123

SECTION G Specifications for 420 kV Class Power Transformers 1.0

SCOPE

1.1 This section covers technical requirements/parameters for power transformers of 420 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, tests and loss capitalisation reference shall be made to sections A, J & L respectively of this Manual. 1.2

Standard Ratings

1.2.1 Generator Transformers Three phase rating MVA 250 or 315

600 Bank

Voltage ratio 15.75 /420

21/420

Tapping Percent range impedance per cent voltage A-Off circuit taps + 2.5% to – 7.5% 14.5 B-Onload taps + 5% to - 10% A-Off circuit taps + 2.5% - 7.5% 15.0 B – On load taps + 5% to - 10%

Cooling

ONAN/OFAF or OFAF or OFWF or ONAN/ ODAF or ODAF or ODWF ONAN/ OFAF or OFAF or OFWF or ONAN/ ODAF or ODAF or ODWF

Note : The ratings of generator transformers for hydro generating sets have not been standardized as the sizes of these sets depend upon site characteristics. The purchaser shall specify the type of cooling required.

Other Parameters (i)

Connections

HV Star neutral effectively earthed, LV delta

(ii)

Connections symbol

YNd11

(iii)

Tappings

Full power tappings on HV winding for HV voltage variation. Tap changing shall be by : (a)

Off-circuit tap changer, tapping range + 2.5% to - 7.5% in steps of 2.5 per cent alternatively


124 (b)

On-load tap changer, tapping range + 5% to - 10% in steps of 1.25 per cent. (iv) Three-phase rating should be understood as three phase bank ra ting and not necessarily three-phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one-third of the three phase bank rating may be specified. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF, or ODAF rating. Rating under ONAF condition shall be about 80 per cent. (vi) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (vii) Short circuit level Transformer shall be suitable for connection to for 420 kV system the system having the following short circuit and duration levels and duration: 40 and 63 kA for one second. (viii) Terminal Bushings

(a)

LV Terminals: Oil - sealed / Oil communicating Kondenser type bushings mounted on turrets suitable for connections to busbars in isolated phase busducts which shall have spacings of 1250 mm for 250 MVA three-phase unit and 1500 mm for each 200 MVA single-phase unit of a 600 MVA three-phase bank.

(b)

HV Terminals -Line End: 420 kV oil filled condenser bushing. No arcing horns shall be provided. For details refer section P. Neutral End: 17.5 kV porcelain bushing. No arcing horns shall be provided.

1.2.2 Auto - Transformers The purchaser may specify auto-transformer with constant ohmic value of impedance or constant percentage impedance as given below: Standard Ratings (a) Auto – Transformers (Constant Percentage Impedance) Three-phase HV/IV/LV MVA

Voltage ratio

Tapping range per cent

Percent impedance voltage

100/100/33.3

400/132/33

+ 10% to – 10% 16 steps of 1.25%

12.5

27

12

ONAN/OFAF

200/200/66.7

400/132/33

+ 10% to – 10% 16 steps of 1.25%

12.5

36

22


250/250/83.3

400/220/33

+ 10% to – 10% 16 steps of 1.25%

12.5

45

30


315/315/105

400/220/33

+ 105 to – 10% 16 steps of 1.25%

12.5

45

30


500/500/166.7

400/220/33

+ 10% to – 10% 16 steps of 1.25%

12.5

45

30


630/630/210

400/220/33

+ 10% to – 10% 16 steps of 1.25%

12.5

45

30


HV-IV

Cooling

HV-LV IV-LV


125

(b) Auto-Transformers (Constant Ohmic Impedance) Three-Phase HV/IV/LV

Voltage Ratio

Tapping Range per cent

MVA

Per cent impedance voltage

Cooling

HV-IV HV-LV IV-LV (min)* (min)*

100/100/33.3

400/132/33

200/200/66.7

400/132/33

250/250/83.3

400/220/33

315/315/105

400/220/33

500/500/166.7

400/220/33

630/630/210

400/220/33

+ 10 to –10% 16 steps of 1.25% + 10 to –10% 16 steps of 1.25% + 10 to –10% 16 steps of 1.25% + 10 to –10% 16 steps of 1.25% + 10 to – 10% 16 steps of 1.25% + 10 to – 10% 16 steps of 1.25%

12.5

45

30

ONAN/OFAF

12.5

45

30

12.5

60

45

12.5

60

45

12.5

60

12.5

60

ONAN/OFAF ONAN/ODAF ONAN/OFAF ONAN/ODAF ONAN/OFAF ONAN/ODAF ONAN/OFAF ONAN/ODAF ONAN/OFAF ONAN/ODAF

45 45

* No limit is specified on higher side.


Connections

HV Star auto with neutral IV effectively earthed LV Delta

(ii)

Connection symbol YNa0d11

(iii)

Full power tappings shall be provided on series winding for the variation of voltage on HV side. The tap changers shall be suitable for bi-directional flow of rated power. The tap changers shall be in accordance with Section N.

(iv)

Three-phase rating should be understood as three phase bank rating and not necessarily three phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one third of the three phase bank ratings may be specified.

(v)

ONAN rating shall be guaranteed at 60 per cent of the OFAF or ODAF rating. Rating under ONAF condition although not guaranteed shall be about 80 per cent.

(vi)

For these transformers the temperature rise of the top oil refers to the specified loading combination for which the total losses are highest. Individual winding temperature rises shall be considered relative to that specified loading combination which is the most severe for the particular winding under consideration.

(vii)

Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu.

(viii) The specified percentage impedance voltage is at principal tapping and on the MVA base corresponding to HV/IV rating. Tolerance on percentage impedance voltage shall be as under:


126

Pairs of windings

Tolerance

HV-IV (normal tap) HV-IV (max & min tap) For constant percentage impedance auto-transformer HV-LV IV-LV

± 10 per cent ± 15 per cent ± 15 per cent ± 15 per cent

1.2.3 Short Circuit Level Transformer shall be suitable for connection to the system having the following short circuit level: 420 kV 40 and 63 kA (rms), 1 second 245 kV 40 kA (rms), 1 second 145 kV 40 kA (rms), 1 second 1.2.4 Terminals (a)

LV Terminals: 52 kV oil-filled condenser bushings. The bushing shall be arranged in a line with 1000 mm spacing to allow mounting of phase to phase barriers. No arcing horns shall be provided.

(b)

IV Terminals: 145/245 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided.

(c)

HV Terminals – Line End: 420 kV oil-filled condenser bushing. No arcing horns shall be provided. For details refer section P. Neutral End: 17.5 kV porcelain bushing. No arcing horns shall be provided.

1.2.5 Insulation Levels 1.2.5.1 LIGHTNING IMPULSE AND POWER FREQUENCY VOLTAGE TEST LEVEL Highest voltage for equipment kV (rms) 12 17.5 24 145 245 420

Rated lightning impulse withstand voltage kV (peak) 75 95 125 550 950 1300 All 1425 for GTs

Rated power frequency, short duration withstand voltage kV (rms) 28 38 50 ----

1.2.5.2 Rated switching surge kV (peak) 1050 withstand voltage for 400 kV terminal. 1.2.5.3 Partial discharge at 1.5 x Um/ 3 Single- phase method pico-coulomb 500.


127

Notes (i)

Insulation of tertiary winding of auto -transformer should be adequate to withstand the transferred surge voltage appearing across them due to an impulse striking on HV or IV terminals. Therefore, 33 kV LV winding shall be designed for a minimum lightning impulse withstand voltage of 250 kV (peak) and short duration power frequency withstand voltage of 95 kV (rms).

(ii)

The shunt reactor or capacitors connected to the LV side would be required to be frequently switched on and off. The LV winding should be capable of withstanding the stresses as may be caused by frequent switching.

1.2.5.4 TEMPERATURE RISES Air - cooled transformers (a) (b)

Temperature rise of top oil Measured by thermometer Temperature rise of winding Measured by resistance: - When oil circulation Natural or forced non-directed - When oil circulation is Forced directed

Water-cooled transformers

50 °C

55 °C

55 °C

60 °C

55 °C

65 °C

Notes (i)

For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance, the following ambient temperatures are assumed : Air

Water

Cooling medium ambient temperature

50°C

30°C


40°C

25°C


32°C

--

(ii) (iii) (iv) (v)

Maximum yearly weighted temperature is based on ambient temperature cycle and its duration. Wherever ambient temperatures are higher than those sp ecified above, the temperature rises, reduced by corresponding amount, shall be specified. Guaranteed temperature rise limits are valid for all the tappings. The above temperature rises are applicable to transformers required for operation at an altitude not exceeding 1000 metres above sea level.

1.3 Cooling 1.3.1 ONAN/OFAF or ONAN/ODAF

Two 50 per cent banks. One number of pump and one standby pump in each bank. Adequate number of fans and one standby fan in each 50 per cent bank.

1.3.2 OFAF or ODAF

Adequate number of coolers with one cooler as standby

1.3.3 OFWF or ODWF

Two 100 per cent coolers.


128 Notes (i)

The transformer shall be filled up with mineral oil, conforming to IS: 335-1993. Some of the essential parameters are specified in Annexure 1, Section G1.

(ii)

For auto transformers 100 per cent cooling equipment should be capable of dissipating losses occurring in all the three windings, at any tap. It is required only in case specifically called fort.

1.4 Bushings 1.4.1 The voltage and current ratings, basic insulation level and creepage distances of the bushings shall be in accordance with the following table: Sl. No.

Voltage rating kV (rms)

Current rating (Amps)

Creepage distance (mm)

Basic insulation level (kVP)

1.

420

10500

1425

2.

245

6125

1050

3.

145

3625

650

4.

52

800 1250 800 1250 2000 800 1250 800 2000 3150

1300

250

Notes (i)

In case of very heavy polluted atmosphere, guidance may be taken from Annexure II, Section G1.

(ii)

The 800 amps as well 1250 amps bushings shall be suitable for draw lead type or draw rod type assembly and it shall be possible to replace the 800 amps bushing with 1250 amps bushing or vice versa.

1.4.2 The Dimensions of Bushings are as per Section P. 1.5

Clearances of Line Terminals in Air

Clearances in air between live parts and to earthed structures for LV terminals of generator transformers and auto-transformers shall be determined as per spacings given in clause 1.1.1 (viii) and 1.2.1.4 respectively. The clearances for HV and IV terminals shall be as tabulated below: Highest voltage for equipment kV (rms) 145 245 420

Clearances Phase to phase (mm) 1220 2000 4000

Phase to earth (mm) 1050 1800 3500

Air clearances of 3500 mm between phase to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned.


129

1.6

Fittings and Accessories

(a) (b)

Rating and diagram plate. Two earthing terminals.

(c) (d) (e)

Lifting bollards. Jacking pads. Haulage lugs.

(f) (g) (h)

Pocket on tank cover for thermometer. Air release devices. Conservator with oil- filling hole, cap and drain valve (size: 25 mm.)

(i)

Magnetic type oil gauge with low oil level alarm contacts (ratings: 0.5 Amp, 220 Volts DC.) Dial size 250 mm. Silicagel breather with oil seal.

(j) (k) (l)

Air cell type oil preservation system. Required number of pressure relief device capable of resealing after release of pressure.

(m)

Valves Oil valves between cooler and main tank Drain valve preferably with padlocking arrangement (size 100 mm). Two filter valves (size: 50 mm) on diagonally opposite ends - one at top and other at bottom preferably with padlocking arrangement on bottom valve. - Two sampling valves (size: 15 mm) at top and bottom of main tank.

(n) (o) (p)

Oil flow indicator with alarm contacts (ratings, 0.5 Amps, 220 Volts DC) with each pump. Valve schedule plate. Buchholz relay with alarm and trip contacts (Ratings: 1.0 Amp 220 Volts DC) shall have : One number shut-off valve (Size: 80 mm) on conservator side Test cock Gas collection box and gas check valve at ground level. Copper tube interconnection between gas collection box and relay shall also be provided. In transformers, for installation in areas subject to high seismic forces, i.e., horizontal acceleration of 0.3 g or more at and above a frequency of 8 Hz pitot type or reed type of gas and oil relay shall be used.

(q)

Dial type oil temperature indicator with ‘maximum-reading’ pointer and two sets of contacts (ratings, 5 Amps, 220 Volts DC).

(r)

1 No. dial type winding temperature indicator for a two winding transformer and one dial type windings temperature indicator for each winding of a multi winding transformer with ‘maximum-reading’ pointer and two sets of contacts rating : 5 Amps,


130

220 Volts DC (for OFAF/ODAF and OFWF/ODWF) and four sets of contacts (for ONAN/ODAF/OFAF). (s) (t) (u)

Remote indication for each winding temperature shall be through a separate measuring system. Cover lifting lugs. Provision for mounting bi-directional flanged rollers with locking and bolting device for rail gauge specified below:

Type of construction

Shorter axis

Longer axis

Single-phase

Two rails with 1676 mm gauge


Three-phase

2/3/4 rail combination according to layout and size of the transformer


(v)

Weather proof marshalling box for housing control equipment and terminal connections.

(w)

Wiring up to marshalling box with PVC SWA copper cables of 650/1100 volt grade.

(x)

Cooling accessories. I

ONAN/OFAF or ONAN/ODAF cooling (i) Requisite number of radiators provided with: -

One shut off valve on top (size: 80 mm) One shut off valve at bottom (size: 80 mm) Air release device on top

Drain and sampling device at bottom Lifting lugs (ii) Fans (iii) Oil pumps with shut off valve on both sides (if required for ONAN cooling pumps can be by-passed using by-pass pipes and valves). (iv) Expansion joints, one each on top and bottom cooler pipe connections. (v) Air release device and oil drain plug on oil pipe connections. II

OFAF or ODAF cooling (i) OFAF coolers with integral fans (ii) Oil pumps with shut-off valves on both sides. (iii) Brass encased thermometers. (iv) Air release devices and oil pipe connections. (v) Lifting lugs.

III OFWF or ODWF Cooling (i) Oil/water heat exchangers with segregated oil and water headers


131

(ii) Oil pumps with shut-off valves on both sides. (iii) Water flow indicator with alarm contacts (ratings: 0.5 Amp, 220 Volts DC). (iv) Brass encased thermometer. (v) Pressure gauges. (vi) Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg./cm2 (vii) Reflux valve if required as per scheme. (viii) Drain and sampling device on cooler pipe connection. 2.0

TESTS

2.1

Type Tests Type tests shall be done as per Section J. Temperature rise test shall be carried out at the lowest tap.

l l

2.2 l l

Additional Type Tests For vacuum and pressure tests on tank refer section A. Relief Device

The pressure relief device of each size shall be subjected to increasing oil pressure. It shall operate before reaching the test pressure specified in the test at C1 17.3.2 (b) of section A. The operating pressure shall be recorded. The device shall seal-off after the excess pressure has been relieved. 2.3 Routine Tests Routine tests shall be done as per Section J. 2.4 Additional Routine Tests Following additional routine tests shall also be conducted: l Magnetic Circuit Test After assembly each core shall be tested for 1 minute at 2000 Volts ac between all bolts side plates and structural steel work. Immediately prior to the dispatch of the transformer from the manufacturer’s works the magnetic circuit shall be pressure tested for 1 minute at 2000 volts ac between the core and earth. l

For Oil Leakage Test on Transformer Tank Refer Section A.

2.5

Special Tests

Special tests other than type or routine tests as agreed between the manufacturer and purchaser shall be carried out as per Section J. In addition the following tests shall also be conducted. 2.5.1 Capacitance Test Capacitance test to determine the capacitance between each winding to earth shall be made by ampere turn bridge method.

SECTION G1


134



135

SECTION G1 Specifications for 800 kV Class Power Transformers 1.0

SCOPE

1.1 This section covers power transformers of 800 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, tests and loss capitalisation reference shall be made to Sections A, J & L respectively of this Manual. 1.2

Standard Ratings

1.2.1 Generator Transformers Single phase rating MVA

Voltage ratio kV

200

21/ 765 √3

260

24/ 765 √3

Tapping range

Percent impedance voltage

Cooling

± 5% in 8 steps with off circuit taps/ links

15% (with ± 5% tolerance) at principal tap

OFAF/OFWF ODAF/ODWF

Three single-phase units will form a bank of 3-phase. Note : The purchaser shall specify the type of cooling required before purchase.


Maximum flux density – 1.9 tesla.

(ii)

Withstand capability for 25% above the rated voltage – 1 minute

(iii)

Withstand capability for 40% above the rated voltage – 5 seconds

(iv)

Connections - HV star neutral effectively earthed, LV delta

(v)

Connections symbol - YNd11 in 3-phase bank.

(vi)

Tappings - Full power tappings on HV winding for HV voltage variation.

(vii)

Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu.

(viii) Short circuit level Transformer shall be suitable for connection to the system having the following short circuit level: 800 kV – 40 kA (rms) for 1 second (ix)

Terminal bushings


136 (a)

LV Terminals: 36 kV, 12500 Amps. Oil filled condenser type bushings mounted on turrets, suitable for connections to bus bars in isolated phase bus ducts which shall have spacing of 1500 mm for each 200 MVA single-phase unit of a 600 MVA three-phase bank. For 260 MVA single-phase unit of a 780 MVA three-phase bank, 1 no. 36 kV, 16000 Amps. rating.

(b)

HV Terminals -Line End: 800 kV, 1250 Amps. oil filled condenser bushing with test tap. No arcing horns shall be provided. For details refer section P. Neutral End: 36 kV porcelain bushing. No arcing horns shall be provided.

(x)

Temperature rises (a) Top oil measured by Thermometer

-

400 C

(b) Winding rise measured by Resistance method

-

450 C

(c) Maximum design ambient temperature (also refer para 1.4 note i)

-

500 C

1.2.2 Auto - Transformers Single-phase rating HV/IV/LV MVA

Voltage ratio

Tapping range

kV

210/210/70

765/400/33 √3 √3

333.33/333.33/111.1

-do-

500/500/167.67

-do-

Per cent impedance voltage at principal tap HV-IV

± 5% in 20 steps

HV-LV

Cooling

IV-LV

12.5

60

40

-do-

14.0

65

45

ONAN/ONAF/ OFAF or ODAF Alternatively ONAN/ONAF1/ ONAF2 -do-

-do-

14.0

65

45

-do-

tolerance tolerance tolerance ±10% ±15% ±15% Note : Three single-phase units will form a bank of 3-phase.

Other Parameters (i) Maximum flux density – 1.9 Tesla. (ii) Withstand capability for 25% above the rated voltage – 1 Minute (iii) Withstand capability for 40% above the rated voltage – 5 Seconds (iv) Connections - HV/IV Star auto with neutral effectively earthed LV Delta (v)

Connection symbol - YNa0,d11

(vi)

ONAN rating shall be guaranteed at 60 per cent of the OFAF or ODAF rating. Rating under ONAF condition although not guaranteed shall be about 80 per cent. Alternatively ONAN/ONAF1/ONAF2 (60%/ 80%/ 100%) cooling with 2 x 50% or with radiator bank and 4-33 1/3% with unit caters can be used.


(vii)

137

For these transformers the temperature rise of the top oil refers to the specified loading combination for which the total losses are highest. Individual winding temperature rises shall be considered relative to that specified loading combination which is the most severe for the particular winding under consideration.

(viii) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (ix)

Short circuit Level Transformer shall be suitable for connection to the system having the following short circuit level: 800 kV – 40 kA (rms) for 1 second 420 kV - 40 and 63 kA (rms) for 1 second

(x)

Terminals (a) LV Terminals: 52 kV oil-filled condenser bushings. The bushing shall be arranged in a line with 1000 mm spacing. No arcing horns shall be provided. (b) IV Terminals: 420 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. (c) HV Terminals: 800 kV oil-filled condenser bushing with test tap. No arcing horns shall be provided. Neutral End: 17.5 kV porcelain bushing. No arcing horns shall be provided.

(xi)

Temperature Rises: (a) Top oil measured by thermometer

-

400 C

(b) Winding rise measured by resistance method

-

450 C

(c) Maximum design ambient temperature

-

500 C

(Also refer to Notes under clause 1-4) 1.3

Insulation Levels

1.3.1 Impulse and Power Frequency Voltage Test Level for Transformer Windings. Highest voltage for equipment Um kV (rms) 17.5 (Neutral) 24 52 (LV of Auto Transformer) 420 800

Rated lightning impulse withstand voltage kV (peak)

Rated switching impulse Rated power frequency withstand voltage short duration withstand kV (peak) voltage kV (rms)

95 125 250 1300 1950

38 50 95 1050 1550


138 1.3.2

Notes (i)

(ii)

1.4

Partial discharge at 1.5 x Um/ 3 kV Single- phase method pico-coulomb 500 (max.). Insulation of tertiary winding of Auto -transformer should be adequate to withstand the transferred surge voltage appearing across it due to an impulse striking on HV or IV terminals. Therefore, 33 kV LV winding shall be designed for a minimum lightning impulse withstand voltage of 250 kV (peak) and short duration power frequency withstand voltage of 95 kV (rms). The shunt reactor or capacitors connected to the LV side would required to be frequently switched on and off. The LV winding should be capable of withstanding the stresses as may be caused by frequent switching.

Temperature Rises

Notes (i)

(ii) (iii) (iv) (v)

1.5

For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance, the following ambient temperatures are assumed. Air

Water

Cooling medium ambient temperature

50°C

30°C


40°C

25°C


32°C

--

Maximum yearly weighted temperature is based on ambient temperature cycle and its duration. Wherever ambient temperature are higher than those specified above, the temperature rises, reduced by corresponding amount, shall be specified. Guaranteed temperature rise limits are valid for all the tapping. The above temperature rises are applicable to transformers required for operation at an altitude not exceeding 1000 meters above sea level.

Cooling

1.5.1 ONAN/OFAF or ONAN/ODAF

Two 50 percent banks. One number of pump and one standby pump in each bank. Adequate number of fans and one standby fan in each 50 per cent bank.

1.5.2 ONAN/ONAF1/ONAF2

Two 50 percent banks or four 33.3% unit codes. Adequate number of fans and one standby fan in each 50 per cent bank or in each 33.3% bank.

1.5.3 OFAF or ODAF

Adequate number of coolers with one cooler as standby.

1.5.4 OFWF or ODWF

Two 100 per cent coolers.

Notes (i)

The transformer shall be filled up with mineral oil, conforming to IS: 335. Some of the essential parameters are specified in Annexure 1.

(ii)

For auto transformers 100 per cent cooling equipment should be capable of dissipating losses occurring in all the three windings, at any tap.


139

1.6 Bushings 1.6.1 The voltage and current ratings, basic insulation level and creepage distances of the bushings shall be in accordance with the following table: Voltage rating kV (rms)

Current rating (Amps)

Creepage distance (mm)

Basic impulse level (kVP)

1250 1250 2000 2000 5000

16,000 10500

2100 1425

1550 1050

1300

250

--

800 420 52

Switching impulse level (kVP)

Note (i)

In case of very heavy polluted atmosphere, guidance may be taken from Annexure II.

1.6.2 Partial Discharge Level Pico-Coloumbs : 10 (Max.) 1.7 Clearances of Line Terminals in Air Clearances in air between live parts and to earthed structures for LV terminals of generator transformers and auto - transformers shall be determined as per spacing given in clause 1.1.1 (ix) and 1.2.1 (x) respectively. The clearances for HV and IV terminals shall be as tabulated below: Highest voltage for equipment kV (rms)

Clearances Phase to phase (mm)

420 800

4000 6400

Phase t o earth (mm) 3500 5800

Air clearances between phase to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned. 1.8 Fittings and Accessories (a)


(b)

Two earthing terminals.

(c)

Lifting bollards.

(d)

Jacking pads.

(e)

Haulage lugs.

(f)


(g)


(h)

Conservator with oil- filling hole, cap and drain valve (size: 25 mm.)


140 (i)

Magnetic type oil gauge with low oil level alarm contacts (ratings: 0.5 Amp, 220 Volts DC.) Dial size 250 mm.

(j)

Silicagel breather with oil seal.

(k)

Air cell type oil preservation system.

•

In addition to provision of air cell in conservators for sealing of the oil system against the atmosphere, an on line insulating oil drying system shall be provided. This on line insulating oil drying system shall be designed for very slow removal of moisture that may enter the oil system or generated during cellulose decomposition.

(l) (m)

Required number of pressure relief device capable of resealing after release of pressure. Valves

•

Oil valves between cooler and main tank

•

Drain valve preferably with padlocking arrangement (size 100 mm).

•

Two filter valves (size: 50 mm) on diagonally opposite ends - one at top and other at bottom preferably with padlocking arrangement on bottom valve.

•

Two sampling valves (size: 15 mm) at top and bottom of main tank.

(n)

Oil flow indicator with alarm contacts (ratings, 0.5 Amps, 220 Volts D.C.) with each pump. Valve schedule plate.

(o) (p)

Buchholz relay with alarm and trip contacts (Ratings: 1.0 Amp. 220 Volts D.C.) shall have

•

One number shut-off valve (Size: 80 mm) on conservator side

•

Test cock

•

Gas collection box and gas check valve at ground level. Copper tube interconnection between gas collection box and relay shall also be provided. In transformers, for installation in areas subject to high seismic forces, i.e., horizontal acceleration of 0.3 g or more at and above a frequency of 8 Hz pitot type or reed type of gas and oil relay shall be used. Online dissolved gas monitoring device.

(q) (r) (s)

(t) (u)

Dial type oil temperature indicator with maximu m reading pointer and two sets of contacts (ratings, 5 Amps, 220 Volts D.C.). 1 No. dial type winding temperature indicator for a two winding transformer and one dial type windings temperature indicator for each winding of a multi winding transformer with ‘maximum reading’ pointer and two sets of contact ratings: 5 Amps, 220 Volts D.C. (for OFAF/ODAF and OFWF/ODWF) and four sets of contacts (for ONAN/ODAF/OFAF). Remote indication for each winding temperature shall be through a separate measuring system. Cover lifting lugs.


(v)

141

Provision for mounting bi-directional flanged rollers with locking and bolting device for rail gauge specified below: Type of construction Single-phase

Shorter axis 2/3 rail combination with 1676 mm gauge according to layout and size of Transformer

Longer axis Two rails with 1676 mm gauge

(w)

Weather proof marshalling box for housing control equipment and terminal connections.

(x)

Wiring up to marshalling box with PVC SWA copper cables of 650/1100 Volt grade.

(y)

Cooling accessories.

(I)

ONAN/OFAF or ONAN/ODAF cooling (i)

(II)

(III)

Requisite number of radiators provided with: -

One shut off valve on top (size: 80 mm)

-

One shut off valve at bottom (size: 80 mm)

-

Air release device on top

-

Drain and sampling device at bottom

-

Lifting lugs

(ii)

Fans

(iii)

Oil pumps with shut off valve on both sides (if required for ONAN cooling pumps can be by-passed using by-pass pipes and valves).

(iv)

Expansion joints, one each on top and bottom cooler pipe connections.

(v)

Air release device and oil drain plug on oil pipe connections.

OFAF or ODAF cooling (i)

OFAF coolers with integral fans

(ii)

Oil pumps with shut-off valves on both sides.

(iii)


(iv)

Air release devices and oil pipe connections.

(v)

Lifting lugs.

OFWF or ODWF cooling (i)

Oil/water heat exchangers with segregated oil and water headers

(ii)

Oil pumps with shut-off valves on both sides.

(iii)

Water flow indicator with alarm contacts (ratings: 0.5 Amp, 220 Volts DC).

(iv)

Brass encased thermometer.

(v)

Pressure gauges.


142 (vi)

Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg/cm2

(vii)

Reflux valve if required as per scheme.

(viii)

Drain and sampling device on cooler pipe connection.

2.0

TESTS

2.1

Type Tests

l

Type tests shall be done as per Section J.

l

Temperature rise test shall be carried out at the lowest tap.

2.2

Additional Type Tests

l

For vacuum and pressure tests on tank refer section A.

l

2.3

The pressure relief device of each size shall be subjected to increasing oil pressure. It shall operate before reaching the test pressure specified in the test at C1 17.3.2 (b) of section A. The operating pressure shall be recorded. The device shall seal-off after the excess pressure has been relieved. Routine Tests

Routine tests shall be done as per Section J. 2.4

Additional Routine Tests

Following additional routine tests shall also be conducted: l

Magnetic Circuit Test

After assembly each core shall be tested for 1 minute at 2000 Volts ac between all bolts side plates and structural steel work. Immediately prior to the dispatch of the transformer from the manufacturer’s works the magnetic circuit shall be pressure tested for 1 minute at 2000 Volts ac between the core and earth. l

For Oil Leakage Test on Transformer Tank Refer Section A.

2.5

Special Tests

Special tests other than Type or Routine tests as agreed between the manufacturer and purchaser shall be carried out as per Section J. In addition the following tests shall also be conducted. 2.5.1 Capacitance Test Capacitance test to determine the capacitance between each winding to earth shall be made by ampere turn bridge method. Note : This specification is generally in line with CEA specification for 800 kV class Power Transformers.


143

Annexure I CHARACTERISTICS AND PARAMETERS OF INSULATING OIL AS PER IS 335 – 1993 Sl. Characteristics No. 1. Appearance

Units

2. 3. 4. 5. 6. 7. 8. 9.

g/cm3 Cst N/m °C °C mg/KOH/gm

Density at 29.5°C (Max) Kinematic viscosity at 27°C (Max.) Interfacial tension at 27°C (Min.) Flash point Pensky Marten (closed) (Min.) Pour point (Max.) Neutralization value (total acidity), (Max) Corrosive sulphur Electric strength (breakdown voltage) (Min.) (a) New unfiltered oil

(b) After filtration 10. Dielectric dissipation factor (tan d ) at 90°C (Max.) 11. Water content (Max.) (in untreated and unfiltered oil) 12. Specific resistance (Min) at 90°C at 27°C 13. Oxidation stability (a) Neutralization value (max.) (b) Total sludge after oxidation (max.) 14. Ageing characteristics after accelerated ageing (open beaker method with Copper catalyst) (a) Resistivity (min) at 90°C at 27°C (b) Tan ä at 90°C (max.) (c) Total acidity (max.) (d) Sludge content by wt. (max.) 11. Presence of oxidation inhibitor

kV (rms) kV (rms)

Requirement The oil shall be clear and transparent and free from suspended matter or sediments. 0.89 27 0.04 140 -6 0.03 Non-corrosive 30 (If this value is not obtained the oil shall be filtered in laboratory) 60 0.002

ppm

50

ohm cm ohm cm

35 x 10 12 1500 x 10 12

mg kOH/gm

0.4 0.10%

ohm cm ohm cm

0.2 x 10 12 2.5 x 10 12 0.2 0.05 0.05% Oil shall not contain anti oxidation additives.

mg KOH/gm

Annexure II CREEPAGE DISTANCE (420 KV BUS HING) (INLINE WITH IS 2099 / IEC 137) Pollution level Light Pollution Medium Pollution Heavy Pollution Very Heavy Pollution

Creepage distance phase to earth (mm) 420 kV 800 kV 6720 12,800 8400 16,000 10500 20,000 13020 24,800

Note : Guidelines for selection in respect of polluted conditions as per IEC 815.

SECTION H

Specification for Earthing Transformers

SECTION H Specification for Earthing Transformers 1.0

SCOPE

1.1 This section covering specification for earthing transformers, does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to other sections of the Transformer Manual. 2.0

GENERAL

2.1 Unless otherwise modified in this section the earthling transformers shall comply with latest versions of IS 5553 (Part 6) and IS 2026. 2.2 Three phase earthing transformers provide an artificial neutral and are used for the following purposes : (a)

to earth an otherwise unearthed system

(b)

to connect single phase loads between lines and neutral

(c)

to connect an arc suppression coil

(d)

to limit fault current during a line to earth fault determined by the zero sequence impedance of earthing transformers and also by the possible addition of resistors and thereby permitting selective protection.

*Note : The provision of the earthing transformer does not necessarily make the system effectively earthed.

(e)

Earthing transformers with zigzag (inter-star) connected winding can have a star connected secondary winding to provide an auxiliary supply.

2.3 Construction of earthing transformer is similar to conventional oil filled transformer. Usually cooling specified is ONAN type. 3.0

WINDING CONNECTIONS

Earthing transformers are usually connected either in zigzag (inter-star) or star-delta. For stardelta transformer the secondary delta winding shall always be connected in closed delta. The neutral of star connected main winding is earthered. Earthing transformer which consists of a single winding connected in inter-star may also provided with, an auxiliary (secondary) winding. This secondary winding when provided shall be connected in star. (a)

Primary - Zigzag (interstar),

Secondary - Star

(b)

Primary - Star

Secondary - Delta

(c)

Primary - Zigzag (interstar)

3.1 For the purpose of fault current limitation resistors/reactors can be inserted either between primary neutral point and earth or in series with primary terminals of interster or star connected primary windings to adjust the zero sequence impedance (Figs. 1 and 2).

148


For star-delta connected earthing transformer the delta connected winding may be of the open type in order to permit the insertion of a resistor or reactor to adjust the zero sequence impedance. 3.2

Also connecting the resistor/reactors at the neutral end would be preferable.

4.0

TAPPINGS AND TAP CHANGING

4.1 For zigzag connected earthing transformer having auxiliary winding if tappings are required for voltage variation, it shall be provided on zigzag connected main winding. Equal and uniform number of tappings shall be provided on both zig and zag windings of main windings.

(a) Current-limiting resistor in neutral

(b) Current limiting resistor in line

Fig. l Interconnected star (zigzag) neutral earthing transformer

(a) Current limiting resistor in neutral

(b) Current limiting resistors in line

Fig. 2 3 phase star-delta neutral earthing transformer

Range of variation: +5 to -5% in steps of 2.5%


149

4.2 Tap changing shall be carried out by means of an off circuit externally operated selfpositioning switch (when the transformer is in de-energised condition. Position No. 1 shall correspond to maximum plus tappings. Provisions shall be made for locking the tap changing switch handle in position. 4.3

However, tappings are not preferred for earthing transformer.

5.0

INSULATION LEVEL

The insulation level for the line terminals of an earthing transformer shall correspond to those specified for transformers as per IS: 2026 (Part 3). 6.0

LOSSES AND IMPEDANCE

6.1

Losses

6.1.1 Only no-load losses should be specified for earthing transformer not provided with additional auxiliary windings. The tolerance on specified no. load losses will be subject to limits specified in IS: 2026. 6.1.2 Both no-load and load losses will be specified for earthing transformers provided with windings' suitable for supplying auxiliary loads. The load losses specified should be based on the rating of the auxiliary winding. These losses are also subject to tolerance in accordance with IS : 2026. 6.2

Impedance

6.2.1 Zero sequence impedance of each earthing transformer shall be specified in ohms per phase and this impedance will be subject to a tolerance of +20%. -0%. 6.2.2 When earthing transformers are provided with auxiliary winding impedance between the auxiliary winding and the main interstar (zigzag) winding must be specified and this impedance shall be subject to tolerance as per IS : 2026. However, if any difficulty arises to achieve both the specified zero sequence impedance of main winding and the percentage impedance between the main winding and auxiliary winding, in such cases either external resistors/reactors may be provided on main windings to adjust the zero sequence impedance or current limiting resistors/reactors may be provided on auxiliary side to limit the fault current on auxiliary side to the specified value. 7.0

CONTINUOUS AND SHORT TIME CURRENT RATING

7.1

Continuous Current

7.1.1 Rated Neutral Continuous Current Continuous neutral current is specified either in the case where phase unbalance of the system exists or when the earthing transformer is to be designed for connection of single phase loads between line and the neutral.


150 7.1.2 Rated Continuous Current

The current flowing through the line terminals continuously when a rated power of a secondary winding is specified. Note : The earthing transformer shall carry the specified neutral or rated continuous current and comply as regards the temperature rise with appropriate requirements of IS : 2026 when it is energised at rated voltage and frequency

7.2

Rated Short Time Current in the Neutral

The earthing transformer shall carry the specified neutral fault current for the specified duration without exceeding the winding temperature of 250oC for copper and a temperature of 200oC for aluminium. When an earthing transformer is designed for the neutral point to be connected to a current limiting impedance in the connection to earth, it should also be capable of withstanding, for a period of 5 seconds, the maximum earth fault current that can flow without the additional impedance in circuit. This safe guard is necessary should, for instance, the bushing of an earth resistor flash over. When earthing transformer are operated without external resistor, the rated short time current and zero sequence impedance shall have the following relationship : Ish =

3. Vph zo

Vph is the maximum permissible operating phase voltage zo is the zero sequence impedance per limb of earthing transformer Ish is the short time neutral current of the transformer 7.3

Ability to Withstand Rated Short Time Current

7.3.1 The earthing transformers shall be capable of withstanding the mechanical and thermal stresses caused by the rated short time current flowing for the specified duration. The thermal ability can be demonstrated by calculation using the following formula as per clause 9.1 of IS 2026 (Part 1) - 1977 θ1 = θ0 + a J2 t x 10-3 oC where θ1 is the highest average temperature attained by the winding due to short time current maintained over the specified duration and shall not exceed 250oC for copper winding and 200oC for aluminium winding. θo is the initial temperature in degree celsius


151

J is the short time current density in ampere per square millimetre t is the duration in seconds a is a function of 1/2 (θ2 + θ0), in accordance with Table 1. θ2 is the maximum permissible average winding temperature, 250oC for copper and 200oC for aluminium. 7.3.1.1 Where earthing transformers are used with external resistor/reactors to limit the earth fault current, the earthing transformer should also be able to withstand dynamically and thermally the maximum earth fault current without external resister/reactors for a period of 5 seconds. 7.3.1.2 For earthing transformers without secondary winding θ0 shall be taken as the sum of the maximum ambient temperature and manufacturers guaranteed average oil temperature rise of the earthing transformer under normal operating conditions. 7.3.2 For earthing transformer with loaded secondary windings θ0 shall be the sum of the appropriate maximum ambient temperature and the relevant temperature is specified in IS : 2026 measures by change in resistance. Table 1 ½ (θ0 + θ2)

a - function of ½ (θ1 + θ2) oC Copper windings Aluminium windings

140

7.41

16.5

160

7.80

17.4

180

8.20

18.3

200

8.59

19.1

220

8.99

-

240

9.38

-

7.3.3 Ability of earthing transformer to withstand mechanical stresses due to the rated short time current flowing in the windings under fault conditions shall be determined by tests described as per clause 8.6 of IS 5553 (part 6). 8.0

TESTS

8.1

Type Test

l

Impulse Voltage Withstand Test (IS : 2026 (Part III).

l

Heat run test (clause 4 of IS : 2026 (Part 2).

Applicable only in the case of earthing transformers having auxiliary winding.


152 8.2

Special test

l

Short circuit withstand test.

Clause 8.6 of IS : 5553 (Part 6). Note : After short circuit test, success of test shall be verified by usual inspection, by repetition of dielectric test at 75 per cent of its original value and by a check measurement of zero sequence impedance.

8.3

Routine Test

l

Measurement of winding resistance clause 16.2 of IS : 2026 (Part 1).

l

Measurement of insulation resistance (clause 16.0 of IS : 2026 (Part I).

l

Measurement of zero sequence impedance (clause 16.10 of IS : 2026 (Part I).

Note : Zero sequence impedance may be measured at any current between 25 per cent to 100 per cent rated short time neutral current and is expressed in ohms per phase. It shall be ensured that the applied current shall not exceed the current carrying capability of the winding or metallic constructional parts. l

Measurement of no load loss and no load current (clause 16.5 of IS : 2026 (Part I).

l

Dielectric tests (IS: 2026 (Part 3).

l

Separate source voltage withstand Test (Clause 10 of IS : 2026 (Part 3).

l

l

l

l

Induced over voltage test (clause 11 of IS : 2026 (Part 3). Applicable only in the case of earthing transformer with a secondary winding. Check of voltage vector relationship and polarity (clause 16.3 of IS : 2026 (Part 1). Measurement of voltage ratio (clause 16.3 of IS : 2026 (Part 1). Applicable only in the case of earthing transformer with a secondary (auxiliary) winding. Ratio measurement of zigzag connected earthing transformer with star connected auxiliary winding.

For a zigzag connected earthing transformer the zig and zag windings constituting one phase are physically wound on two different limbs of the core. Hence if a single phase supply is applied between line and neutral of interstar (zigzag) connected winding, the voltage induced in zig and zag winding, will be different. Due to this, voltage induced on secondary winding of same phase will not be the same as that defined by the per phase voltage ratio of the transformer. Thus voltage ratio measurement with single phase application will give misleading results if application and measurement is made on per phase basis. (ie between line and neutral). A vector diagram of a 33/0.435 kV ZNynl connected earthing transformer is given as Fig. 3. The zig and zag windings per limb are designed for 11 kV and LV is designed for 0.435/3kV. Here 3 phase voltage ratio is defined as the ratio of the HV line to neutral voltage to LV line to line voltage (IR-IN/2R-2Y) ie 33/3/0.435 = 43.799


153

Fig. 3 Vector and voltage relationship or a 33/0.435 kV ZNynl connected earthing transformer

For single phase application, the ratio IR-IN/2R-2Y will be : 11 kV x

2/0435 x 2 √3

= 33 /3 x 2 /0.435 / 3 x 2 = 33/ √ 3 / 0.435 kV = 43.799

i.e., for a zig zag connected earthing transformer to get actual design ratio with single phase application, the ratio measurement shall be made by applying line to neutral voltage (per phase) on interstar connected main winding and measuring the induced line to line voltage on corresponding star connected secondary windings or vice versa. Note : For ratio measurement with 3 phase application equal and balanced supply (w.r. to voltage and phase difference) shall be applied, otherwise ratio error will be high.

8.3.1 Measurement of impedance voltage short circuit impedance and loss (see clause 16.4 of IS : 2026 (Part I) 1977. 9.0

REFERENCES

l

IS : 5553 (Part 6)

l

IS : 2026 (Part 1,2,3,4 & 5)

l

ISC 289 section 6

l

IEC 76

l

CBIP Manual on Transformers, Section H - 1987 “Specification for Earthing Transformer”

l

The J & P Transformer Book, Tenth edition chapters 11, 20 and 21.

SECTION I


156



157

SECTION I Specifications for Valves for Transformers 1.0

SCOPE

1.1 This section covers specification for valves for transformers. This part specification does not purport to include all necessary provision of a contract. For general requirements reference shall be made to other sections of the Transformer Manual. 2.0

GENERAL

2.1 All valves in oil line shall be suitable for continuous operation with transformer oil (IS : 335) at 1000 C. 2.2 Gland packing/ gasket material shall be of teflon rope/nitrile rubber. In case of GM/CI (gum metal/cast iron) gate and globe valves, gland packing preferably of teflon rope shall be used to prevent oil seepage through gland. Asbestos or graphite (gun metal/cast IMM) packing material shall not be used as they are not compatible with hot transformer oil. 2.3 Inside surface of valves shall be clean and valve ends shall be suitably blanked at the time of despatch. Machined and flange surfaces shall be suitably protected against rusting and transit damage. Butterfly and radiator valves shall be covered in polythene bags before packing for despatch. 2.4 After testing, inside surface of all C.I. valves coming in contact with transformer oil shall be applied with one coat of oil resisting paint/ varnish. Inside surface of all valves in water lines shall be applied with two coats of paint conforming to IS: 9862, two coats of black Japan conforming to Type B of IS: 341 or any other suitable water resistant compound. Unless specified otherwise by the purchaser, outside surface of all valves with ferrous body shall be painted with two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no. 631 of IS: 5. Outside surface of all valves with non ferrous body shall be painted with one coat of etch primer followed by two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no. 631 of IS: 5. Outside surface of all valves with non ferrous body shall be painted with one coat of etch primer followed by two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no. 631 of IS: 5. After testing, inside surface of all C.I. valves coming in contact with transformer oil shall be applied with one coat of oil resisting paint/varnish. Inside surface of all valves in water lines shall be applied with two coasts of pant conforming to IS : 9862 or two coats of black Japan conforming to Type B of IS : 341. Outside surface of the valves shall be painted with two


158

coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS : 2932 and of distinct different shade (631 of IS – 5) to that of main tank surface. 2.5

All hardwares used may be electro-galvanized.

3.0

SIZE OF THE VALVE AND VALVE FLANGES

3.1 The size of the valve and valve flanges shall be as per Table 1. All flanges shall be drilled at off-centres. 3.2 Recommended size of the valves for drain, filter, sampling and air release applications shall be per Table 2. Table 1 Nominal size of valve (nominal pipe bore) mm

Diameter of flange mm

Diameter of bolt circle mm

No. of bolts

Bolt size

Diameter of bolt hole mm

15 25 50 80 100 150 200 250

95 115 150 185 215 285 340 395

65 85 115 145 180 240 295 350

4 4 4 4 4 8 8 12

M20 M12 M16 M16 M16 M20 M20 M20

14 14 18 18 18 23 23 23

Table 2 Transformer rating kVA Over

Upto and Size of drain Size of filter Size of sampling Size of vacuum including valve mm valve mm valve mm valve mm

—

1600

55

25

15

--

1600

10000

59

25

15

--

10000

50000

80

50

15

--

50000

100000

100

50

15

100

100000

—

100

50

15

100

4.0

TYPE OF VALVES

4.1

Gunmetal Globe Valves

4.1.1 15 mm valves used for oil sampling or air release shall be made of gunmetal gate valve (screw down stop) type generally conforming to class 1 of IS : 778 with inside screw stem. The oil sampling valve shall have provision to fix 15 mm PVC pipe. 4.1.2 The flanges shall be either drilled as per Table 1 (of this specification) or screwed as explained in Table 2 of IS : 778. 4.1.3 Preferred dimensions shall be as per Table 3.


159

Table 3 Size of valve mm

Face to face dimension mm

Thickness of flange mm

15 (Screwed)

57

—

15 (Flanged)

72

6.6

4.1.4 Tests (a) Pressure tests : Body

..15 kg/cm2

Seat

.. 10 kg/cm2

Duration .. 2 minutes (b) Seepage test in open condition : Test pressure.. 2 kg/cm2 Duration

.. 12 hrs.

Note : Transformer oil (IS : 335) or water at ambient temperature shall be used for pressure and oil seepage tests.

4.2

Gunmetal Gate Valves

4.2.1 25 mm valves used for oil filtering, oil drainage or in Buchholz pipe shall be gunmetal gate (screw down step) type generally conforming to class 1 of IS : 778) with inside screw stem. 4.2.2 The flanges shall be drilled as per Table 1. 4.2.3 Preferred dimensions shall be as per Table 4. Table 4 Size of valve mm 25 (flanged)

Face to face dimension mm 90

Thickness of flange mm 8.0

4.2.4 Tests as per clause 3.1.4 4.3

Cast Iron Taper Plug Valves

4.3.1 50 mm, 80 mm and 100 mm C.I. taper plug or gate valves may be used for oil filtering, oil draining. 4.3.2 Valves shall be self-lubricating, short pattern type, provided with valve operating wrenches. The lubricant used shall be compatible with transformer oil (IS : 335) at 1000 C. 4.3.3 Preferred dimensions shall be as per Table 5.


160 Table 5 Size of valve mm



50

178 (7”)

19

80

203(8”)

19

100

229 (9”)

22

4.3.4 Tests (a)

Pressure tests Body ..15 kg/cm2 Seat ..10 kg/cm2 Duration ..2 minutes

(b)

Seepage test in open condition Test pressure..2 kg/cm2 Duration ..12 hrs.

Note: Transformer oil (IS : 335) at ambient temperature shall be used for pressure and seepage tests.

4.4

C.I. Gate Valves

4.4.1 50 mm, 80 mm, 100 mm, 150 mm, 200 mm and 250 mm, C.I. gate valves may be used in cooling water pipe lines. 4.4.2 C.I. gate valve shall generally conform to class PNI of IS : 780 double flanged, inside screw, non-rising spindle, Solid wedge gate and with screwed (inside screw type or bolted bonnet, Gunmetal trim material, teflon rope gland packings and nitrile rubber gaskets. The valves shall be provided with open and shut indicator. 4.4.3 Preferred dimensions shall be as per Table 6. Table 6 Size of valve mm

Preferred face to face dimension mm

Thinckness of flange mm

50

215

16

80

230

18

100

255

20

150

280

22

200

318

25

250

355

26

4.4.4 Hydrostatic Pressure Tests Body

..15 kg/cm2 for 5 minutes

Seat

..10 kg/cm2 for 2 minutes


161

4.5 C.I. Check Valves 4.5.1 100 mm, 150 mm, 200 mm, and 250 mm C.I. check valves may be used in oil pipe lines of forced oil cooled transformers to avoid reverse flow and/or cross flow of oil where required. 4.5.2 C.I. Check valves shall be of horizontal swing check type genrally conforming to IS : 5312 (Part I), without bypass arrangement. The weight of the swing (flap) shall be such that full opening is available at a minimum oil velocity of 1.0 m/s at a pressure of 1.5 kg/cm2 . The pressure drop through the valve shall be less than 0.3 m of oil column (0.026 kg/cm2 ) with normal flow rates. The above requirement should be proved by calculations if test facilities not available. 4.5.3 Preferred dimensions shall be as per Table 7. Table 7 Size of valve mm



100

300

20

150

400

22

200

500

25

250

600

26

4.5.4 Testing as per CI. 7.1, 7.2 and 7.3 of IS : 5312 (Part 1). 4.6 Butterfly Valves 4.6.1 Cast iron butterfly valves shall be of the nominal sizes; 50, 80, 100, 150, 200 and 250 mm. These valves are used in oil pipe lines such as Buchholz relay pipe lines and as shut off valves between transformer main tank and cooler tank. 4.6.2 The body and cap of butterfly valves shall be made from any of the seven grades of grey iron casting conforming to IS : 210 except grade FG 150 and the disc shaft and stop pin shall be made from mild steel conforming to Fe 410-WA of IS : 2062 disc and body of the valve shall be in direct contact without any sealing ring. Valve is operated by a spanner and provision shall be given for locking in open/closed position. ‘OPEN’, ‘CLOSE’ markings on the body and an arrow mark on the tap for position indication shall be provided. To indicate the disc position in the cap removed condition, some suitable marking on the spindle head shall also be provided. 4.6.3 Preferred dimensions shall be as per Table 8. Table 8 Size of valve mm



50

40

16

80

40

16

100

40

16

150

40

16

200

40

16

250

45

22


162 4.6.4 Tests

The valves shall be tested with transformer oil at 1.5 kg/cm2 with the assembled valve in closed/open position, the leakage shall not be more than as specified below: (i)

Leakage through body when pressure is applied for thirty minutes

Nil

(ii)

Leakage around top spindle with cap fitted on

Nil

(iii)

Leakage around top spindle when cap is removed for 50, 80 and 100 mm valves-2 drops per minute (max.) for 150, 200 and 250 mm valves-8 drops per minute (max.)

(iv)

Leakage past diaphragm in closed position – for 50, 80 and 100 mm valves-6 drops per minute (max.) for 150, 200 and 250 mm valves-6 cc per minute (max.)

4.7

Radiator Valves

4.7.1 Radiator valves shall be of 80 or 100 mm size 4.7.2 Construction same as per clause 4.6.2 4.7.3 Preferred dimensions shall be as per Table 9. Table 9 Size of valve

Diameter of bolt circle mm

No. of bolt

mm

Size of flange square A/F mm

80

150

160

100

180

180

mm

Face to Thickness of face flange dimension mm mm

4

18

40

16

4

18

40

16

4.7.4 Tests : Same as per clause 4.6.4

Bolt hole size

SECTION J


164



165

SECTION J Test Requirements for Transformers 1.0

SCOPE

Test requirements, procedures and criteria for transformers are defined in national and international standards, i.e., IS 2026 and IEC Publication 60076 in general. This section describes specific requirements for performing tests specified in IEC Publication 60076, IS 2026 and other standards applicable to distribution, power and regulating transformers. It is intended for use as a guide and reference for testing of transformers and covers purpose, interpretation and explanation of specific tests pertaining to the transformer and procedure for correction when ideal test conditions can not be achieved. 2.0

NECESSITY OF TESTS ON TRANSFORMER

When all manufacturing processes have been completed, tests are performed on transformer at the manufacturer’s works to ensure the following purposes: •

To prove that the transformer meets the customer specifications and design expectations.

•

To check that the quality requirements have been met and that performance is within the tolerances guaranteed.

Tests performed for the former purpose are referred to as Type Tests and that for the latter purpose are referred to as Routine Tests (carried out on every unit manufactured). In addition, Special Tests may also be performed to obtain information useful to the user during operation or maintenance of the transformer. Transformer is important and vital equipment, it is therefore necessary to ensure its proper performance throughout its service life. Also during transportation, installation and service operation, the transformer may be exposed to conditions, which can adversely affect its reliability and useful life. It is therefore necessary to do field testing prior to dispatch as well as to carry out pre-commissioning tests to ensure good operating health of transformers.


166 3.0

TESTS

The general requirements and details of the various category of tests (Routine Tests, Type Tests and Special Tests) are in accordance with IEC Publication 60076 (latest edition). The Indian Standard IS: 2026 is under revision and is expected to be revised in line with IEC. The customer specific requirements are referred here as Additional Tests. The following tests are generally performed on the transformer at works which may also form part of the customer acceptance requirements : 3.1

Routine Tests

(a)

Measurement of winding resistance

(b)

Measurement of voltage ratio, polarity and check of voltage vector relationship

(c)

Measurement of no-load loss and excitation current

(d)

Measurement of short-circuit impedance and load loss

(e)

Measurement of insulation resistance

(f)

Tests on on-load tap-changers, where appropriate

(g)

Dielectric tests (i)

Switching impulse withstand voltage test, transformer winding Um > 170 kV

(ii)

Lightning impulse withstand voltage test, transformer winding Um > 72.5 kV

(iii)

Separate-source withstand voltage test

(iv)

Induced AC over voltage withstand test

3.2

Type Tests

(a)

Lightning impulse voltage withstand test, transformer winding Um

(b)

Temperature rise test

3.3

Special Tests

(a)

Lightning impulse test on neutral terminal

(b)

Long-duration induced AC voltage test (ACLD) transformer winding < 170 kV

(c)

Short-circuit withstand test

(d)

Measurement of zero-sequence impedances on three phase transformers

72.5 kV

72.5 < Um


167

(e)

Measurement of acoustic sound level

(f)

Measurement of the harmonics of the no-load current

(g)

Measurement of the power taken by the fan and oil pump motors

(h)

Test with lightning impulse chopped on the tail

(i)

Determination of capacitances and dissipation factor between winding-to-earth and between windings

(j)

Determination of transient voltage transfer characteristics

3.4

Additional Tests

(a)

Magnetic circuit (Isolation) test

(b)

Determination of capacitances and dissipation for condenser bushings >72.5 kV

(c)

Magnetic balance test on three-phase transformers

(d)

Dissolved gas analysis (DGA) of oil filled in the transformer before and after temperature rise test above 72.5 kV

(e)

Frequency response analysis ( FRA ) test > 170 kV (recommended)

(f)

Measurement of magnetization current at low voltage

(g)

Functional tests on auxiliary equipment

(h)

Tests on oil filled in transformer

(i)

Oil pressure test on completely assembled transformer

(j)

Dew point measurement before despatching

The dielectric tests may be routine, type or special tests depending upon the voltage rating, specific customer requirements and referred standards. 3.5

Recommended Field Tests

(a)

Dew point measurement for large transformer filled with dry air or nitrogen filled

(b)

Winding resistance measurement

(c)

Verification of vector group and polarity

(d)

Measurement of voltage ratio test

(e)

Measurement of magnetizing current

(f)

Magnetic balance test on three phase transformer

(g)

Magnetic circuit (isolation) test


168 (h)

Measurement of short circuit impedance at low voltage

(i)

Insulation resistance measurement

(j)

Measurement of capacitance and dissipation factor on transformers above 72.5 kV class

(k)

Dissolved gas analysis (DGA) on transformers above 50 MVA

(l)

Tests on oil filled in transformer as per IS 1866

The purpose, interpretation and explanation for specific conditions of the tests are briefly described below. The tests and their sequence shall be mutually agreed between the manufacturer and the user. 4.0

ROUTINE TESTS

4.1

Measurement of Winding Resistance

4.1.1 General Resistance measurement helps to determine the following : (a)

Calculation of the I2 R losses.

(b)

Calculation of winding temperature at the end of a temperature rise test.

(c)

As a bench mark for assessing possible damage in the field.

4.1.2 Determination of Cold Temperature The resistance is measured at ambient (cold) temperature and then converted to resistance at 75 0 C, for all practical purpose of comparison with specified design values, previous results and diagnostics. The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The following should be observed. 4.1.2.1 TRANSFORMER W INDINGS IMMERSED IN INSULATING LIQUID The temperature of the winding shall be assumed to be the same as the average temperature of the insulating liquid, provided: (a)

The windings are under insulating liquid without excitation and without current in the winding for three to eight hours (depending upon the size of the transformer) before the cold resistance measurement.

(b)

The temperature of the insulating liquid has stabilized, and the difference between top and bottom temperature does not exceed 5 o C.

4.1.2.2 TRANSFORMER W INDINGS WITHOUT INSULATING LIQUID The temperature of the winding shall be recorded as the average of several thermometers or thermocouples inserted between the coils, with care taken to see that their measuring probes


169

are as nearly as possible in actual contact with the winding conductors. It should not be assumed that the windings are at the same temperature as the surrounding air. 4.1.3 Resistance Measurement Methods The resistance of each winding shall be measured by any one of the following methods. If winding has tapping, then resistance shall be measured at all taps. 4.1.3.1 VOLTMETER-AMMETER M ETHOD This method can be employed if the rated current of the transformer winding is one ampere or more. The following steps are performed to conduct this test. (a)

Measurement is made with direct current, and simultaneous readings of current and voltage are taken.

(b)

To minimize observation errors: -

The measuring instruments shall have such ranges as will give reasonably large deflection.

-

The polarity of the core magnetization shall be kept constant during all resistance readings.

(c)

The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to exclude the resistance of current-carrying leads, their contacts and extra length of leads.

(d)

Readings shall be taken after the current and voltage have reached steady-state values.

(e)

Readings shall be taken with at least four values of current when deflecting instruments are used.

(f)

The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.

4.1.3.2 BRIDGE M ETHOD Bridge methods or high-accuracy digital instrumentation are generally preferred because of their accuracy and convenience. The current rating of the measuring instrument should not be very low for large inductive objects. In case of delta connected windings of a large rating transformer, the resistance meter should have adequate current rating.


170

For star connected windings with neutral brought out, the resistance may be measured by two methods : •

Between line and neutral

•

For small transformer with star connected windings, the resistance may be measured between phases (line to line), and then resistance of the individual windings shall be determined by dividing the value by 2. This will rule out the effect of the resistance of the neutral lead and bus bars which is significant in comparison to phase resistance of small transformers. However, for the delta connected windings, measurements shall be made between pairs of line terminals. In this case the resistance per winding will be 1.5 X measured resistance between the pair of line terminals. In case of open delta connected winding, the resistance can be measured across all the three windings in series and also individual winding resistance can be measured.

The following precautions shall be taken to minimize errors while performing the test as follows: (a)

Charged battery of sufficient capacity of at least 10 AH shall be used with the bridge to avoid errors due to drop in battery voltage during measurements.

(b)

To reduce the high inductive effect, it is advisable to use a sufficiently high current to saturate the core. Therefore the measuring instruments shall have high ranges as well as large deflection.

(c)

The polarity of the core magnetization shall be kept same during all resistance readings. A reversal in magnetization of the core can change the time constant and result in erroneous readings.

(d)

The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads.

(e)

To protect the voltmeter from off-scale deflections, the voltmeter should be disconnected from the circuit before switching the current on or off. To protect the personnel from inductive kick, the current should be switched off by a suitably insulated switch.

(f)

Readings shall not be taken until after the current and voltage have reached steadystate values.

(g)

The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy due to heating of the winding and thereby changing its temperature and resistance.


4.2

171

Measurement of Voltage Ratio, Polarity and Check of Voltage Vector Relationship

4.2.1 Ratio Test 4.2.1.1 GENERAL •

The turn ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low-voltage winding.

•

When the transformer has taps, the turn ratio shall be determined for all taps and for the full winding.

•

The ratio tests shall be made at rated or lower voltage and the voltage shall be applied to the winding with higher voltage rating.

•

In case of three-phase transformers, when each phase is independent and accessible, single-phase supply can be used; although, when convenient, three-phase supply may be used.

4.2.1.2 TOLERANCES FOR RATIO The tolerances for ratio shall be as specified in IS 2026 Part 1 and IEC 60076-1. Ratio Test Methods •

Various types of ratio test methods are given in IS: 2026 Part 1 and IEC 60076 -1. Out of these, Ratio Bridge method is most commonly adopted. In this method, the turn ratio on each tapping between pairs of winding shall be measured by a direct reading ratio meter. This method gives more accurate results as compared to other methods described in aforesaid standards.

•

The modern ratio bridge can also be used to test polarity, phase relation and phase sequence. More accurate results can be obtained using a ratio bridge that provides phase-angle correction.

4.2.2 Polarity and Vector Group Verification Polarity and phase-relation tests are of interest primarily because of their bearing on paralleling or banking two or more transformers. Phase-relation tests are made to determine angular displacement and relative phase sequence. Phase-relation or vector group verification test is performed on a three-phase transformer or on a bank of three single-phase transformers. The details of additive and subtractive polarity are given in IS: 2026-Part 1 and IEC 60076-1.


172

4.2.2.1 POLARITY BY ALTERNATING-VOLTAGE TEST For a single-phase transformer having a ratio of transformation of 30 to 1 or less, the polarity test shall be done as follows. The line terminal of high voltage winding (1.1) shall be connected to the adjacent line terminal low-voltage lwinding (2.1) as shown in Fig. 1

source

1.1

2.1

1.2

(a)

1.1

1.2

2.1

2.2

2.2 Fig. 1 Polarity by alternating voltage test

Any convenient value of alternating voltage shall be applied to the full high-voltage winding and readings shall be taken of the applied voltage and the voltage between the right-hand adjacent high-voltage and low-voltage leads. When the later reading is greater than the former, the polarity is additive. When the later reading is less than the former (indicating the approximate difference in voltage between that of the high-voltage and low-voltage windings), the polarity is subtractive. 4.2.2.2 VERIFICATION OF VECTOR GROUP The phasor diagram of any three-phase transformer that defines the angular displacement and phase sequence can be verified by connecting the HV and LV leads together to excite the unit at a suitably low three-phase voltage, taking voltage measurements between the various pairs of leads and then either plotting these values or comparing them for their relative order of magnitude with the help of the corresponding phasor diagrams. Typical check measurements are to be taken and their relative magnitudes are then compared. (For procedure refer clause 8.4 of this section). 4.3

Measurement of No-Load Loss and Excitation Current

4.3.1 General No-load (excitation) losses are those losses that are incident to the excitation of the transformer. No-load (excitation) losses include core loss, dielectric loss and conductor loss in the winding due to excitation current. These losses change with the change in excitation voltage.

V


173

Excitation current (no-load current) is the current that flows in any winding used to excite the transformer when all other windings are open-circuited. It is generally expressed in percent of the rated current of the winding in which it is measured. 4.3.1.1 NO-LOAD LOSS TEST The purpose of the no-load loss test is to measure no-load losses at a specified excitation voltage and a specified frequency. The no-load loss determination shall be based on a sinewave voltage. The average-voltage voltmeter method is the most accurate method for correcting the measured no-load losses to a sine-wave basis and is recommended. This method employs two-parallel-connected voltmeters; one is an average-responding (possibly rms calibrated) voltmeter; the other is a true rms -responding voltmeter. The readings of both voltmeters are employed to correct the no-load losses to a sine-wave basis, using equation given in paragraph for waveform correction of no-load losses. Losses are measured at 90%, 100%, 110%, of rated voltage for checking G.T.P. and for reference purpose. 4.3.1.2 VOLTAGE AND FREQUENCY FOR NO-LOAD LOSS TEST The operating and performance characteristics of a transformer are based upon rated voltage and rated frequency, unless otherwise specified. Therefore, the no-load loss test is conducted with rated voltage impressed across the transformer terminals, using a voltage source at a frequency equal to the rated frequency of the transformer under test, unless otherwise specified. For the determination of the no-load losses of a single-phase transformer or a three-phase transformer, the frequency of the test source should be within ± 0.5% of the rated frequency of the transformer under test. If the excitation frequency is beyond the specified tolerance, then the test voltage shall be adjusted to maintain the V/f ratio corresponding to the ratio of rated voltage and rated frequency. The voltage shall be adjusted to the specified value as indicated by the average-voltage voltmeter. Simultaneous values of rms voltage, rms current, electrical power and the average voltmeter readings shall be recorded. For a three-phase transformer the average of the three voltmeter readings shall be the desired nominal value of the voltage. The most difficult cases, both with regard to voltage wave shape distortion and power measurements usually arise when testing large single-phase transformers. 4.3.1.3 INSTRUMENT ERROR AT LOW POWER FACTOR At low power factors, such as those encountered while measuring the load losses and impedance voltage of power transformers, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks and accessories affect the load loss test results. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8


174

4.3.1.4 Correction of No-load Losses The eddy current component of the no-load loss varies with the square of the rms value of excitation voltage and is substantially independent of the voltage waveform. When the test voltage is held at the specified value as read on the average-voltage voltmeter, the actual rms value of the test voltage may not be equal to the specified value. The no-load losses of the transformer corrected to a sine-wave basis shall be determined from the measured value by means of the following equation: PO =Pm(1+d) d=(U1 -U) ⁄ U1

(as per IEC 60076-1)

where : Po = corrected no load loss Pm = measured no load loss U1 = average voltage X 1.11 U = RMS voltage The actual per unit values of hysteresis and eddy-current losses should be used if available. If actual values are not available, it is suggested that the two loss components be assumed equal in value, assigning each a value of 0.5 per unit for CRGO. The above equation is valid only for voltage with moderate waveform distortion. If waveform distortion in the test voltage causes the magnitude of the correction to be greater than 5%, then the test voltage waveform must be improved for an adequate determination of the no-load losses and currents. For large single phase transformers, it is expected that the difference between rms voltages and average voltage will be greater than 5%, which should be accepted in view of test voltage source limitation. The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltages and power. 4.3.2 Measurement of Excitation (no-load) Current The excitation (no-load) current of a transformer is the current that maintains the rated magnetic flux excitation in the core of the transformer. The excitation current is usually expressed in per unit or in percent of the rated line current of the winding in which it is measured. Measurement of excitation current is usually carried out in conjunction with the tests for no-load losses. RMS current is recorded simultaneously during the test for no-load losses using the average-voltage voltmeter method. This value is used in calculating the per unit or percent excitation current. For a three-phase transformer, the excitation current is calculated by taking the average of the magnitude of the three line currents. The tolerance for no-load current should be as per IS 2026 Part –1.


4.4

175

Measurement of Short-Circuit Impedance and Load Loss

4.4.1 General The load losses of a transformer are those losses incident to a specified load carried by the transformer. Load losses include I2 R loss in the windings due to load current and stray losses due to eddy currents induced by leakage flux in the windings, core clamps, magnetic shield, tank walls and other conducting parts. Stray losses may also be caused by circulating currents in parallel windings or strands. Load losses are measured by applying a short circuit across either the high voltage winding or the low voltage winding and applying sufficient voltage across the other winding to cause a specified current to flow in the windings. The power loss within the transformer under these conditions equals the load lo sses of the transformer at the temperature of test for the specified load current. The impedance voltage of a transformer between a pair of windings is the voltage required to circulate rated current through one of two specified windings when the other winding is short circuited, with the windings connected as for rated voltage operation. Impedance voltage is usually expressed in per unit or percent of the rated voltage of the winding across which the voltage is applied and measured. The impedance voltage is measured during the load loss test by measuring the voltage required to circulate test current in the windings. The measured voltage is the impedance voltage at the test frequency and the power loss dissipated within the transformer is equal to the load losses at the temperature of test and at rated load. The impedance voltage is corrected to the rated frequency and the load losses are corrected to a reference temperature using the formulas specified in this standard. 4.4.1.1 FACTORS AFFECTING THE VALUES OF LOAD LOSSES AND IMPEDANCE VOLTAGE The magnitude of the load losses and the impedance voltage will vary depending on the positions of tap changers, if any in various windings. These changes are due to the changes in the magnitudes of load currents and associated leakage-flux linkages as well as being due to changes in stray flux and accompanying stray losses. (a) Temperature Load losses are also a function of temperature. The I2 R component of the load losses increases with temperature, while the stray loss component decreases with temperature. Procedures for correcting the load losses to the standard reference temperature are described in 3.4.5. (b) Instrument Error at Low Power Factor At low power factors, such as those encountered while measuring the load losses and impedance voltage of power transformers, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks and accessories affect the load loss test results. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8.


176

4.4.2 Methods for Measuring Load Losses and Impedance Voltage 4.4.2.1 TEST CONDITIONS To determine the load losses and impedance voltage with sufficient accuracy, the following conditions shall be met : •

The temperature of the insulating liquid has stabilized and the difference between top and bottom oil temperatures does not exceed 5 0 C.

•

The temperature of the windings shall be taken immediately either before or after the load losses and impedance voltage test in a manner similar to that described in 4.1.2. The average shall be taken as the winding temperature for computation of losses.

•

The conductors used to short -circuit the low voltage, high current winding of a transformer shall have a cross-sectional area equal to or greater than the corresponding transformer winding leads.

•

The test current shall be at least 50% of the rated current of the winding across which the voltage is applied.

•

The measurement of losses shall be done at the earliest after excitation of the transformer to the test current to avoid heating of the winding resulting in increase in resistance.

4.4.2.2 W ATTMETER-VOLTMETER-AMMETER M ETHOD FOR LOAD LOSS AND IMPEDANCE VOLTAGE TEST For three-phase transformers, three-phase power measurement utilizing two wattmeter is possible but can result in very large errors at low power factors encountered in load loss tests of transformers. It is recommended that the two-wattmeter method should not be used for loss tests on three-phase transformers. 4.4.2.3 M EASUREMENT WITH POWER ANALYSER Now-a-days, digital power analysers or power meters are available for determination of load losses. Selection of these power analysers shall be based on the desired accuracy at low power factors. The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltage, current, power, frequency and temperature. 4.4.3 Calculation of Load Losses and Impedance Voltage Load loss measurements vary with temperature and in general must be corrected to a reference temperature. In addition, load loss measurement values must be corrected for metering phase angle error. Impedance voltage measurement to vary with frequency and the values must be corrected for rated frequency. The correction for temperature shall be as per Annexure E of IEC 60076-1-1993.


177

4.4.4 Calculation for Impedance The impedance shall be measured at rated frequency by applying an approximately sinusoidal supply to one winding, with the terminal of other winding short circuited, and with possible other winding open circuited. The supplied current should be equal to the relevant rated current. However, in case of limitation in the rating of supply source the current should not be less than the 50% of the rated current. Due to fluctuation in load the supply frequency may not be always be the rated frequency. Then frequency correction should be applied to calculate the actual impedance at rated frequency as following. The formula for calculating the percentage impedance with current and frequency correction is:

Z (%) =

V test I f × rated × r × 100 V rated I test ft

where Vtest =Test voltage Vrated =Rated voltage Itest = Test current Irated = Rated current ft

= Test frequency

fr

= Rated frequency

4.5

Measurement of Insulation Resistance

Insulation resistance tests are made to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance in such tests is commonly measured in mega-ohms, or may be calculated from measurements of applied voltage and leakage current. Note (a)

The insulation resistance of electrical apparatus is subjected to wide variation in design, temperature, dryness, and cleanliness of the parts. When the insulation resistance falls below prescribed values, it can, in most cases of good design and where no defect exists, be brought up to that required standard by cleaning and drying the apparatus. The insulation resistance, therefore, may offer a useful indication as to whether the apparatus is in suitable condition for application of dielectric tests.

(b)

Under no conditions, test should be made while the transformer is under vacuum.


178 4.5.1 Instrumentation

Insulation resistance may be measured using the following equipment: (a)

A variable-voltage DC power supply with means to measure voltage and current (generally in micro-amperes or milli-amperes)

(b)

A mega-ohmmeter

Mega-ohmmeters are commonly available with nominal voltages of 500 V, 1000 V, 2500 V, and 5000 V; DC or in multiples of 1000 V upto 10,000 V. 4.5.2 Voltage to be Applied The DC voltage applied for measuring insulation resistance to ground shall not exceed a value equal to the half of the applied power test voltage of the winding or 5 kV whichever is lower. 4.5.3 Procedure Insulation resistance tests shall be made with all circuits of equal voltage above ground connected together. Circuits or groups of circuits of different voltages above ground shall be tested separately. All external insulating parts of the transformer shall be cleaned thoroughly to remove dust, moisture etc. before the test. Examples (a)

High voltage to low voltage and ground, low voltage to high voltage and ground.

(b)

Voltage should be increased in increments of usually one kilovolt and held for one minute while the current is read.

(c)

The test should be disconnected immediately in the event the current begin to increase without stabilizing.

(d)

After the test has been completed, all terminals should be grounded for a period of time sufficient to allow any trapped charges to decay to a negligible value.

4.5.4 Polarisation Index (PI) The purpose of polarisation index test is to determine if equipment is suitable for operation or even for an overvoltage test. The polarisation index is a ratio of insulation resistance value at the end of 10 min test to that at the end of 1 min test at a constant voltage. The total current that is developed when applying a steady state dc voltage is composed of three components: •

Charging current due to the capacitance of the insulation being measured. This current falls off from maximum to zero very rapidly.

•

Absorption current due to molecular charge shifting in the insulation. The transient current decays to zero more slowly.


•

179

Leakage current which is the true conduction current of the insulation. It has a component due to the surface leakage because of the surface contamination.

The advantage of PI is that all of the variables that can affect a single IR reading, such as temperature and humidity, are essentially the same for both the 1 min and 10 min readings. Since leakage current increases at a faster rate with moisture present than does absorption current, the IR readings will not increase as fast with insulation in poor condition as with insulation in good condition. After 10 min the leakage current becomes constant and effects of charging current and absorption current die down. It is recommended that PI value for power transformer shall be better than 1.3. 4.5.5 Interpretation of Results Insulation resistance varies with applied voltage and reduces with increase in temperature. Any comparison must be made with measurements at the same voltage and temperature. The significance of values of insulation resistance tests generally requires some interpretation, depending on the design and the dryness and cleanliness of the insulation involved. When a user decides to make insulation resistance test, it is recommended that insulation resistance values be measured periodically (during maintenance shutdown) and that these periodic values be plotted. Substantial variations in the plotted values of insulation resistance should be investigated for cause. 4.6

Tests on On-Load Tap-Changers

4.6.1 Operation Test With the tap-changer fully assembled on the transformer the following sequence of operations shall be performed without failure: (a)

With the transformer de-energised, eight complete cycles of operations (a cycle of operation goes from one end of the tapping range to the other, and back again).

(b)

With the transformer de-energised, and with the auxiliary voltage reduced to 85% of its rated value, one complete cycle of operation.

(c)

With the transformer energized at rated voltage and frequency at no load, one complete cycle of operation

(d)

With one winding short circuited and, as far as practicable, rated current according to IEC 60076-1 in the two windings, 10 tap-change operations across the range of two steps on each side from where a coarse or reversing changeover selector operates, or otherwise from the middle tapping.


180

4.6.2 Auxiliary Circuits Insulation Test After the tap changer is assembled on the transformer, a power frequency tests according to IEC 60076-1 shall be applied to the auxiliary circuits as specified in IEC 60076-3. 4.7

Dielectric Tests

The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the specified insulation levels. The insulation requirements for the transformers and the corresponding dielectric tests are given in IS 2026 Part-3 and IEC Publication 60076-3 with reference to specific windings and their terminals. For oil immersed transformers, the requirements apply to the internal insulation only. The dielectric tests shall generally be made at the test tab of manufacturer with the transformers preferably at ambient temperature. Transformers, including bushings and terminal compartments when necessary to verify air clearances, shall be assembled prior to making dielectric tests, but assembly of items, such as radiators and cabinets, which do not affect dielectric tests is not necessary. Bushing shall, unless otherwise authorised by the purchaser, be those to be supplied with the transformer. If a transformer fails to meet its test requirements and the fault is in a bushing, it is permissible to replace this bushing temporarily with another bushing and continue the tests on the transformer to completion without delay. A particular case arises for tests with partial discharge measurements, where certain types of commonly used high-voltage bushings create difficulty because of their relatively high level of partial discharge. When such bushings are specified for the transformer, it is permitted to exchange them for bushings of a partial discharge free type during the testing of transformer. Test levels and other test parameters shall be as per IEC Publication 60076-3 and the corresponding IS 2026 Part-3. It is recommended to measure voltage at the high voltage terminal of its transformer. The measuring system shall be in accordance with IEC Publication 60060-2. In case of nonavailability of suitable high voltage divider, it is recommended to establish the ratio of HV to applied LV voltage upto 50% of the HV test level and then LV voltage shall be maintained to indicate the proper test voltage.

181


4.7.1 Rules for Some Particular Transformers In transformers where uniformly insulated windings having different Um values are connected together within the transformer, the separate source AC withstand test voltages shall be determined by the insulation of the common neutral and its assigned Um. In transformers which have one or more non uniformly insulated windings, the test voltages for the induced withstand voltage test, and for the switching impulse test, are determined by the winding with highest Um value, and the windings with lower Um values may not receive their appropriate test voltages. During switching impulse tests, the voltages developed across different windings are approximately proportional to the ratio of turns. Rated switching impulse withstand voltages shall only be assigned to the winding with the highest Um. Test stresses in other windings are also proportional to the ratio of numbers of turns and are adjusted by selecting appropriate tappings to come as close as possible to the assigned value. 4.7.2 Insulation Requirements and Dielectric Tests The basic rules for insulation requirements and dielectric tests for different categories of windings are described in Table 1(Refer IEC Publication 60076-3). Table 1 Category of winding

Highest voltage for equipment Um kV

Uniform

Um

Uniform and non-uniform insulation

72.5

Tests Lightning impulse

Switching impulse

(LI)

(SI)

Long Short Separate duration duration source AC AC(ACLD) AC(ACSD)

Type

Not applicable

Not applicable

Routine

Routine

72.5 < Um 170

Routine

Not applicable

Special

Routine

Routine

170 < Um < 300

Routine

Routine*

Routine

Special*

Routine

Um

Routine

Routine

Routine

Special

Routine

300

* If ACSD test is specified, the SI test is not required.

The standard dielectric requirements are verified by dielectric tests. They shall, where applicable and not otherwise agreed upon, be performed in the sequence as given below : Switching impulse test (SI) for the line terminal Lightning impulse test (LI) for the line terminals Lightning impulse test (LI) for neutral terminal


182

Separate source AC withstand voltage test (applied potential test) Short-duration induced AC withstand voltage test (ACSD) Long-duration induced AC voltage test (ACLD) 4.7.3 Switching Impulse Withstand Voltage Test, Transformer Winding Um > 170 kV This test is intended to verify the switching impulse withstand strength of the line terminals and its connected windings to earth and other windings, the withstand strength between phases and along the winding under test. The impulses are applied either directly from the impulse voltage source to a line terminal of the winding under test, or to a lower voltage winding so that the test voltage is inductively transferred to the winding under test. The detailed test procedures and specific test requirements are addressed in IEC Publication 60076-3. 4.7.3.1 SWITCHING IMPULSE W AVES (a)

Polarity

The polarity of test voltage shall be negative because this reduces the risk of erratic external flashovers in the test circuit. (b)

Wave Shape

The voltage impulse shall have a virtual front time of at least 100 ìs, a time above 90% of the specified amplitude of at least 200 ìs, and a total duration from the virtual origin to the first zero passage of at least 500 ìs but preferably 1000 ìs. (c)

Test Sequence and Records

The test sequence shall consist of one impulse of a voltage between 50% and 75 % of the full test voltage and three subsequent impulses of full voltage. If the oscillographic or digital recording should fail, that application shall be disregarded and a further application made. Oscillographic or digital records shall be obtained of at least the impulse wave-shape on the line terminal under test and preferably the neutral current. (d)

Test Connections

During the test the transformer shall be in a no-load condition. Windings not used for the test shall be solidly earthed at one-point but not short -circuited. For a single phase transformer, the neutral terminal of the tested winding shall be solidly earthed. A three-phase winding shall be tested phase by phase with the neutral terminal earthed and with the transformer so connected that a voltage of opposite polarity and about half amplitude appears on the two remaining line terminals which may be connected together.


183

To limit the voltage of opposite polarity to approximately 50% of the applied level, it is recommended to connect high ohmic damping resistors (10 k to 20 k ) to earth at the non tested phase terminals. (e)

Failure Detection

The test is successful if there is no sudden collapse of voltage or discontinuity of the neutral current if recorded on the oscillographic or digital records. Additional observation during the test (abnormal sound effect etc.) may be used to confirm the oscillographic records, but they do not constitute evidence in themselves. 4.7.4 Lightning Impulse Withstand Voltage Test This test is intended to verify the impulse withstand strength of the transformer under test. This test shall only be made on windings that have terminals brought out through the transformer tank or cover. When non-linear elements or surge diverters are installed for the limitation of transferred over-voltage transients, the evaluation of test records may be different compared to the normal impulse test. These non-linear protective devices connected across the windings may cause difference between the reduced full wave and the full-wave impulse oscillograms. To prove that these differences are indeed caused by operation of these devices, this should be demonstrated by making two or more reduced full-wave tests at different voltage levels to show the trend in their operation. The detailed test procedure and specific test requirements are addressed in IEC 60076-3. 4.7.4.1 IMPULSE W AVE The test impulse shall be a full standard lightning impulse: 1.2 µs ± 30% / 50 µs ± 20%. But in some cases this standard impulse shape cannot reasonably be obtained, because of low winding inductance or high capacitance to earth. In such cases wider tolerance may be accepted by the agreement between purchaser and customer. It is recommended to use IEC Publication 60722 as a guide for non-standard wave shapes. 4.7.4.2 TEST SEQUENCE The test sequence shall consists one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made.


184 4.7.4.3 TEST CONNECTIONS (a)

During Test on Line Terminals

The impulse test sequence is applied to each of the line terminals of the tested winding in succession. In the case of a three-phase transformer, the other line terminals of the winding shall be earthed directly or through a low impedance, not exceeding the surge impedance of the connected line. If the winding has neutral terminal, it shall be earthed directly or through a low impedance such as a current measuring shunt. In the case of separate-winding transformer, terminals of windings not under test are earthed directly or through impedances, so that in all circumstances, the voltage appearing at the terminals is limited to not more than 75% of their rated lightning impulse withstand voltage for star connected windings, and 50% for delta- connected windings. In case of auto transformer, when testing the line terminal of the high voltage winding the non-tested line terminal shall be earthed through resistors not exceeding 400 Ω to get the impulse waveform as needed. (b)

Impulse Test on a Neutral Terminal

Impulse withstand capability of neutral may be verified by : (i)

Indirect application Test impulses are applied to any one of line terminals or to all three line terminals connected together. The neutral is connected to earth through an impedance or is left open. Then standard lightning impulse is applied to the line terminal which shall not exceed 75% of the rated LI withstand voltage of the line terminal.

(ii)

Direct application Test impulse corresponding to the rated withstand voltage of the neutral is applied directly to the neutral with all line terminals earthed. In this case, however a longer duration of front time is allowed, upto 13 µs.

4.7.4.4 RECORDS OF TEST The oscillographic or digital records obtained during calibrations and tests shall clearly show the applied voltage impulse shape (front time, time to half value and amplitude). The oscillograms of the current flowing to earth from the tested winding shall also be recorded. 4.7.4.5 TEST SEQUENCE The test sequence shall consist of one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made.


185

4.7.4.6 FAILURE DETECTION (i)

Grounded Current Oscillograms

In this method of failure detection, the impulse current in the grounded end of the winding tested is measured by means of an oscilloscope or by a suitable digital transient recorder connected across a suitable shunt inserted between the normally grounded end of the winding and ground. Any significant differences in the wave shape between the reduced full-wave and final full-wave detected by comparison of the two current oscillograms, may be indication of failure or deviations due to non-injurious causes. They should be fully investigated and explained by a new reduced wave and full-wave test. Examples of probable causes of different wave shapes are operation of protective devices, core saturation, conditions in the test circuit external to the transformer. The ground current method of detection is not suitable for use with chopped-wave tests. (ii)

Other Methods of Failure Detection

Voltage Oscillograms: Any unexplained difference between the reduced full-wave and final full-wave detected by comparison of the two voltage oscillograms, or any such differences observed by comparing the chopped-waves to each other and to the full-wave up to the time of flashover, are indications of failure. Noise : Unusual noise within the transformer at the instant of applying impulse is an indication of trouble. Such noise should be investigated. Measurement : Measurement of voltage and current induced in another winding may also be used for failure detection. 4.7.5 Separate Source Voltage Withstand Test 4.7.5.1 DURATION, F REQUENCY AND CONNECTIONS A normal power frequency, such as 50 Hz, shall be used and the duration of the test shall be one minute. The winding being tested shall have all its parts joined together and connected to the terminal of the testing transformer. All other terminals and parts (including core and tank) shall be connected to ground and to the other terminal of the testing transformer.

186


4.7.5.2 APPLICATION OF VOLTAGE FOR SEPARATE SOURCE W ITHSTAND TEST The test shall be commenced at a voltage not greater than one-third of the full value and be brought up gradually to full value in not more than 15 s. After being held for the specified time of 60 seconds, it should be reduced (in not more than 5s) to one third or less of the maximum value and the circuit opened. 4.7.5.3 FAILURE DETECTION The test is successful if no collapse of the voltage occurs. Careful attention should be given for evidence of possible failure that could include items, such as an indication of smoke and bubbles rising in the oil, an audible sound such as a thump, or a sudden increase in test circuit current. Any such indication should be carefully investigated by observation, by repeating the test, or by other test to determine if a failure has occurred. 4.7.6 Induced AC Voltage Withstand Tests ACSD test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings, the withstand strength between phases and along the winding(s) under test. As per IS 2026 Part 3-1981 and IEC Pub. 60076-3 of 1981, the test is normally performed with partial discharge measurement (Method 2) for transformers with highest voltage winding of 300 kV. For transformer with highest voltage winding of < 300 kV, the test is performed without partial discharge measurement (Method 1). However, with the latest revision of IEC 60076-3 in the year 2000, the methods for induced over-voltage withstand test are referred as AC short duration test (ACSD) and AC long duration test (ACLD). For Um < 72.5 kV, ACSD test is carried out without partial discharge measurement for 60 seconds. For Um >72.5 kV, the test is normally performed with partial discharge measurements to verify partial discharge free operation of the transformer under operating condition. However, the requirements for partial discharge measurement during the ACSD test may be omitted. This shall be clearly stated at the enquiry and order stages. ACLD test is always performed with the measurement of partial discharge during the whole application of test. This test is not a design proving test, but a quality control test and is intended to cover temporary over voltages and continuous service stress. It verifies PD free operation of the transformers under operating conditions. An alternating voltage shall be applied to the terminals of one winding of the transformer. The voltage shall be as nearly as possible sinusoidal and its frequency is sufficiently above the rated frequency to avoid excessive magnetizing current during the test. The test voltage is the peak value of voltage divided by 2 .The test time at full test voltage shall be 60 sec for test frequency up to and including twice the rated frequency. For frequency above twice the rated frequency the time duration of test shall be:

187


120 ×

Rated frequency , but not less than 15 sec Test frequency

Table 1 shows the different conditions of induced AC voltage test as defined in IEC publication 60076-3. The time duration for the application of test voltage with respect to earth is shown in Fig. 2. Table 1 Induced AC voltage test Type of test

Type of winding

Uniformly insulated

Highest voltage of equipment Um ≤ 72.5 kV > 72.5 kV

Test voltage level

As per Table2 of IEC 60076-3 U1 =from Table D.1 of IEC 60076-3 U2 = 1.3 Um/ 3

AC short duration (ACSD)

Test Duration (Refer Fig 12) 60 sec C= 120 x Rated Freq. Test freq.

Remarks

No PD measurement PD level should be 300 pC at level U2

D=5 min

Non-uniformly insulated

Delta conne cted HV Star AC long Uniformly conne duration and non- cted (ACLD) uniformly HV insulated

>72.5 kV

72.5 < Um < 245 kV

Phase to earth test

U1 =from Table D.2 of IEC 60076-3 U2 = 1.5Um/ 3 Phase to U1 =from phase test Table D.2 of IEC 60076-3 U2 = 1.3 Um/ 3 U1 = 1.7 Um/ 3 U2 = 1.5 Um/ 3

72.5< Um < 300 kV

U1 = 1.7 Um/ 3 U2 = 1.5 Um/ 3

300 kV

U1 = 1.7 Um/ 3 U2 = 1.5 Um/ 3

C= 120 x Rated Freq. Test freq.

PD level should be 500 pC at level U2

D=5 min PD level should be 300 pC at level U2

D=30 min C= 120 x Rated Freq. Test freq. D=30 min C= 120 x Rated Freq. Test freq. D=60 min C= 120 x Rated Freq. Test freq.

PD level should be 500 pC at level U2 PD level should be 500 pC at level U2


188

C B

D

A

Ustart

1.1 U m/ 3

E U2

U1

U2

1.1 Um/ 3

< Ustart

A = 5 min B = 5 min C = test time in seconds D = 5 min for ACSD and 30/60 min for ACLD E = 5 min Fig. 2 Time sequence for the application of test voltage with respect to earth

Here Um=Highest voltage for equipment U1 = Test voltage U2 = Partial discharge evaluation level The detailed procedure and specific test requirements are addressed in IEC-60076-3 5.0

TYPE TESTS

5.1

Lightning Impulse Voltage Withstand Test, Transformer Winding Um

72.5 kV

Refer to the clause no 4.7.4 of this section. 5.2

Temperature Rise Test

Temperature rise test is performed to prove that temperature rise comply to limits specified in standards and to derive thermal characteristics for the transformer. The test is carried out by supplying losses (sum of maximum of copper loss and iron loss at related voltage) for sufficient time to ensure that the temperature rise of the winding and oil reach steady state values. The transformer shall be assembled completely with its cooling equipment. It is desirable to put the specified conservator with the transformer, if available. Alternatively, temporary conservator of suitable capacity can be used for the purpose of the test. The top oil temperature is measured by a temperature sensor in a pocket (filled with oil) at the top of the transformer tank, and this is used to verify that steady conditions have been reached. Final winding temperatures cannot be measured directly.


189

The transformers shall be tested in the combination of connections and taps that give the highest winding temperature rises as determined by the manufacturer and reviewed by the purchaser’s representative when available. This will generally involve those connections and taps resulting in the highest losses. All temperature rise test shall be made under normal (or equivalent to normal) conditions of the means of cooling. If it is not possible to feed the total losses due to test plant limitations, the test can be carried out at reduced loss as per IEC 60076-2. The temperature–rise test shall be made in a room that is free from drafts as practicable and equipped with its protective device. 5.2.1 Cooling Air Temperature Precautions should be taken to minimise variations of cooling air temperature specially when the steady state is approached. Rapid variation of reading should be prevented by providing at least three sensors, and average of their readings shall be used for evaluation. The sensors shall be distributed around the tank 1 m to 2 m away from the tank or cooling surface and protected from direct radiation. The sensors shall be placed at a level about half way up the cooling surfaces. 5.2.2 Cooling Water Temperature The temperature is measured at the intake of the cooler. Readings of temperature and rate of water flow should be taken at regular interval. 5.2.3 Test Method 5.2.3.1 SHORT CIRCUIT M ETHOD During this test the transformer is subjected to the calculated total losses, previously obtained by two separate determination of losses, namely load loss at reference temperature and no load loss. The purpose of this test is : •

To establish the top oil temperature rise in steady-state condition with dissipation of total losses

•

To establish the average winding temperature rise at rated current and with the top oil temperature rise as determined above.

This is achieved in two steps: (a)

Total Loss Injection

First the top oil and average oil temperature rises are established when the transformer is subjected to a test voltage such that the measured power is equal to at least 80% of the total


190

losses of the transformer. The test current will be above rated current to the extent necessary for producing an additional amount of loss equal to the no-load losses, and winding temperature rise will be correspondingly elevated. The oil temperature and cooling medium temperature are monitored, and the test is continued until a steady- state oil temperature rise is established. The test may be terminated when the rate of change of top oil temperature rise has fallen below 1°C per hour and has remained there for a period of 3 hour. For earlier truncation of the test, refer Annex C of IEC 60076-2. (b)

Rated Current Injection

When the top oil temperature rise has been established, the test shall immediately be continued with the test current reduced to 50% the rated current for the winding combination connected. This condition is maintained for 1 h, with continuous observation of oil and cooling medium temperatures. At the end of one hour, the resistance of windings are measured with suitable method. During the hour with rated current the oil temperature falls. The measured values of winding temperature shall therefore be raised by the same amount as the average oil temperature rise has fallen from the correct value. The corrected winding temperature value minus the cooling medium temperature at the end of the total losses injection period is the average temperature rise. By the agreement, the two steps of the test may be combined in one single application of the power at a level between load loss and the total loss. The temperature- rise figures for the top oil and for the windings shall then be determined using the correction rules. The power injected during the test shall however be at least 80% of the total loss figure. 5.2.4 Determination of Average Winding Temperature The average winding temperature is determined via measurement of winding resistance. A reference measurement (R1 ,θ1 ) of all winding resistances is made with the transformer at ambient temperature, in a steady condition. When the resistance R2 at different temperature (θ2) is measured this yields the temperature value Copper: : θ2 = R2 (235+ θ1 ) - 235 R1 Aluminium : θ2 = R2 ( 225+ θ1 ) - 225 R1

The external cooling medium temperature at the time of shutdown is θa The winding temp. rise is then, finally : ∆ θw =θ2 -θa


191

5.2.5 Determination of Winding Temperature Before Shutdown Immediately after disconnection of test power supply and removal of short circuit connection the resistance of winding is measured with a suitable measuring circuit. The winding has large electrical time constant therefore accurate reading obtained only after a certain time delay. The resistance of the winding varies with time as the winding cools down. It shall be measured for a sufficient time to permit the extrapolation back to instant of shutdown. The detailed procedure to determine the resistance at the instance of shutdown is accordance with IEC-60076-2. 5.2.6 Corrections If the specified values of power or current have not been obtained during the test, the result shall be corrected according to the following relation. They are valid within a range of ±20% from target value of power and ±10% from target value of current. The oil temperature rise above ambient during the test is multiplied by :  Total losses    Test losses 

X

X = 0.8 for distribution transformers X = 0.9 for larger transformers with ON cooling X = 1.0 for transformers with OF or OD cooling The average winding temperature rise above average oil temperature during the test is multiplied by:  Rated Current     Test Current 

Y

Y=1.6 for ON and OF cooled transformers Y= 2.0 for OD cooled transformers 6.0

SPECIAL TESTS

6.1

Lightning Impulse Test on Neutral Terminal Refer Clause 4.7, of this section.

6.2

Long-duration induced AC voltage test (ACLD), transformer winding 72.5
The test procedure is same as clause 4.7.6 of this section.


192 6.3

Short Circuit Withstand Test

6.3.1 General This test identifies the requirement for power transformer to sustain without damage the effects of over current originated by external short-circuit. The test demonstrates the thermal ability and dynamic effects of power transformer to withstand the rated short-circuit forces. The detailed procedures describing the magnitude of current, test duration, no. of tests and evaluation criteria shall be as per IEC 60076-5. All routine tests are to be repeated after short circuit test. 6.4

Measurement of Zero-phase-sequence Impedances on 3-phase Transformers

6.4.1 Zero-Phase-sequence Impedance Tests of Three–phase Transformers The zero-phase-sequence impedance characteristics of three–phase transformers depend upon the winding connections, and in some cases, upon the core construction. Zero-phasesequence impedance tests apply only to transformers having one or more windings with a physical neutral brought out for external connection. In all tests, one such winding shall be excited at rated frequency between the neutral and the three line terminals connected together. External connection of other windings shall be as described in succeeding paragraphs for various transformer connections. Transformers with connections other than as described in succeeding paragraphs shall be tested as determined by those responsible for design and application. The excitation voltage and current shall be established as follows: If no delta connection is present on the transformer, the applied voltage should not exceed 30% of the rated line–to–neutral voltage of the winding being energized, nor should the current through the neutral excced the rated phase current. If a delta connection is present, the applied voltage should be such that the rated phase current of any delta winding is not exceeded. The percent excitation voltage at which the tests are made shall be shown on the test report. The time duration of the test shall be such that the thermal limits of any of the transformer parts are not exceeded. Single–phase measurements of excitation voltage, total current, and power shall be similar to those described in for load loss measurements. The zero–sequence impedance in percent on kVA base of excited winding for the test connection is:

 E Ir  Z (%) = 300  .   Er I  where E = measured excitation voltage Er = rated phase–to–neutral voltage of excited winding


193

I = measured total input current flowing in the three parallel–connected phases Ir = rated current per phase of the excited winding Zero sequence impedance in percentages shall be determined for the same winding pair combinations considered for positive sequence impedance measurement during load loss test. The current through neutral shall not exceed rated current. The acceptance norms for % ZO shall be either as per customer specification or mutually agreed upon. A single-phase voltage shall be applied between the three shorted line terminal and neutral. The external terminals of all other windings may be open–circuited or shorted and grounded. The zero-sequence impedance is dependent upon the physical disposition of the windings and the magnetic parts and measurement of different windings may not therefore agree. 6.5

Measurement of Acoustic Sound Level

This test shall be done in accordance with the clauses given in NEMA TR1 and IEC-6007610. The detailed test procedure is given in the ANSI/IEEE standard, which has been approved by NEMA. Audible sound from transformer originates principally in the transformer core and is transmitted, either through the dielectric fluid or the structural support, to other solid surfaces from which it is radiated as airborne sound. The audible sound also contains the noise emitted by any dielectric fluid mechanical cooling system. Measurement should be made in an environment having an ambient sound pressure level at least five decibels below the combined sound pressure level of the transformer and the ambient sound pressure level. The transformer shall be located so that no acoustically reflecting surface is within 3 m of the measuring microphone, other than the floor or ground. The transformer shall be connected and energised at rated voltage and rated frequency, and shall be at no load with the tap changer on principal tap. Pumps and fans shall be operated as appropriate for the rating being tested. Sound measurements shall begin after the transformer being tested is energised and steady- state sound level conditions are established. Measurements may be made immediately on the transformers that have been in continuous operation. The rated voltage shall be measured line-line for connected windings and line-neutral for Y connected windings. The voltage shall be measured with a voltmeter responsive to the average value of the voltage but scaled to read the rms value of a sinusoidal wave having the same average value. The voltmeter should be connected between the terminals of the energized windings.


194

The reference sound-producing surface is a vertical surface that follows the contour of a taut string stretched around periphery of the transformer or integral enclosure (Fig. 3). The contour shall include radiators, coolers, tubes, switch compartments, and terminal chambers, but exclude bushing and minor extensions. The measurement shall be done with the microphone, which shall be calibrated as recommended by the sound level meter manufacturer before and after measurement. The first microphone locations shall coincide with the main drain valve. The number of microphone position is not less than 4. The microphone shall be located on the measurement surface spaced 0.3 m from the reference sound- producing surface. When fans are in operation, the microphone shall be located 2 m from any portion of radiators and coolers. For transformers having an overall tank or enclosure height of les than 2.4 m, measurements shall be made at half height. For transformers having an overall tank height of 2.4 m or more, measurements shall be made at one-third and at two-thirds height. The sound level of the transformer is determined using the following steps: (a)

Measure ambient sound pressure level

(b)

Measure combined ambient and transformer sound pressure levels

(c)

Compute ambient corrected sound pressure levels

(d)

Compute average sound pressure levels

The detailed calculation is done in accordance with the ANSI/IEEEC57.12.90-1993. The average sound pressure level of transformers should not exceed the values given in table 0-2 through 0-4 of NEMA TR1 when measured at the factory in accordance with the conditions outlined in ANSI/IEEEC57.12.90-1993. Microphone Location Fan cooled surface 2m

TANK Box

Drain Plug

Reference sound producing source

Measurement surface 1 m

0.3 m

2/3 Height 1/3 Height

Height

Fig. 3 Microphone location for measuring audible sound from transformers


6.6

195

Measurement of the Harmonics of the No-Load Current

The test voltage for the measurement of the harmonics shall be the rated rms voltage of the winding. The harmonics of the no–load current in all the phases are measured by means of harmonic analyzer and the magnitude of the harmonics is expressed as a percentage of the fundamental component. 6.7

Measurement of Power Taken by the Fans and Oil Pump Motors

The measurement shall be done by suitable instruments at rated voltage. 6.8

Test With Lightning Impulse Chopped on Tail

This test is a special test and should be used for special applications on the line terminals of a winding. When this test is performed it shall be combined with the full lightning impulse test. The peak value of the chopped impulse shall be 1.1 times the amplitude of the full impulse as per IEC 60076-3, 2000. Usually the same settings of the impulse generator and measuring equipment are used, and only the chopping gap instrument is added. The standard chopped lightning impulse shall have a time to chopping between 2 µs and 6 µs. The triggered type chopping gap should be used with adjustable timing, although a plain rodrod gap is allowed. The chopping circuit should be so arranged that the amount of over swing to opposite polarity of the recorded impulse will be limited to not more than 30% of the amplitude of the chopped impulse. For this purpose, it is permitted to put a resistance in service with the chopping gap. The test is combined with the full impulse test in a single sequence. The order of application is : one reduced level impulse one full level impulse one or more reduced level chopped impulse(s) two full level chopped impulses two full level impulses The same type of measuring channels and oscillographic or digital records are specified as for the full-wave impulse test. The detection of faults during chopped impulse test depends essentially on a comparison of the oscillographic or digital records of full level and reduced level chopped impulses. The neutral current record presents a superposition of transient phenomena due to the front of the


196

original impulse and from the chopping. Account should therefore be taken of the possible variations, of the chopping time delay. The recordings of successive full impulse tests at full level constitute a supplementary criterion of a fault, but they do not constitute in themselves a quality criterion for the chopped impulse test. 6.9

Determination of Capacitance and Dissipation Factor between Winding to Earth and between Windings

Capacitance and tan delta are usually determined for winding to earth and between windings by bridge measuring technique, such as Schering Bridge. The test specimen shall have the following requirements: All windings immersed in insulating liquid. All winding short-circuited. All bushings are in place. The applied voltage for measuring capacitance and tan delta shall not exceed half of the low frequency test voltage, for any part of the winding or 10 kV whichever is lower. This test may be performed with or without guard for the circuit combination as shown below : Method I Test without guard Two winding HV to LV and ground LV to HV and ground HV and LV to ground Three winding HV to IV, LV and ground IV to HV, LV and ground LV to IV, HV and ground HV and IV to LV and ground HV and LV to IV and ground IV and LV to HV and ground HV, IV and LV to ground

Method II Test with guard Two winding HV to LV and ground LV to HV and ground HV and LV to ground LV to ground, Guard on HV Three winding HV to LV and ground, guard on IV HV to ground, guard on LV & IV LV to IV & ground, guard on HV LV to ground, guard on HV & IV IV to HV & ground, guard on LV IV to ground, guard on HV & LV HV & LV to IV & ground HV & IV to LV & ground

6.10 Determination of Transient Voltage Transfer Characteristics When the low-voltage winding cannot be subjected to lightning over voltage from the low voltage system, this winding may, by agreement between supplier and purchaser, be impulse tested with surges transferred from high voltage winding.


197

This method is also used when the design is such that an impulse directly applied to the low voltage winding could result in unrealistic stressing of higher voltage windings, particularly when there is a large tapping winding physically adjacent to the low voltage winding. With the transferred surge method, the tests on the low voltage winding are carried out by applying the impulse to the adjacent high voltage winding. The line terminals of the low voltage winding is connected to earth through resistance of such value that the amplitude of the transferred impulse voltage between line terminals and earth, or between different line terminal or across a phase winding, will be as high as possible but not exceeding the rated impulse withstand voltage. The magnitude of the applied impulses shall not exceed the impulse level of the winding to which the impulses are applied. The details of the procedure shall be same as the lightning impulse test on line terminal of HV winding. 6.11 Temperature Correction Factors Reference of ANSI/IEEE C.57.90 is considered. Insulation temperature may be considered to be that of the average liquid temperature. When insulation power factor is measured at a relatively high temperature and the corrected values are unusually high, the transformer should be allowed to cool and the measurements should be repeated at or near room temperature. (for more details refer 8.11 of this section and clause 5 of section K-1) 7.0

ADDITIONAL TESTS

7.1

Magnetic Circuit (Isolation) Test

This test is done to detect the presence of inadvertent ground if exists. This test is done with help of megger or by AC supply. During this test other terminals should be in open circuit position. This test is done by applying the AC voltage between the core lamination to end frame, core lamination to tank and between end frame to tank (if end frame is isolated from tank). The value of test voltage shall be 2 kV unless otherwise specified . The duration of test voltage application is 60 seconds. Alternatively the test is performed with the help of megger. In which the value of insulation resistance is measured between two terminals. This test shall be conducted in accordance with IS-2026 Part 1. The tests will be successful if the terminals withstand the required AC voltage for test duration. The values of the insulation shall be at least 1 MΩ or as recommended by the manufacturer. Service aged equipment

: > 1 MΩ

Insulation deterioration

: < 1 MΩ

Destructive circulating current

: < 100 KΩ


198 7.2

Capacitance and Tan Delta Measurement on Bushings

It should be as per the IEC 60137-1995 along with capacitance and tan delta measurement of windings. Increasing trend in tan delta during periodical checking in field should be investigated. 7.3

Magnetic Balance Test on 3-phase Transformers

This test is conducted only in three-phase transformers to check the imbalance in the magnetic circuit. In this test, no winding terminal should be grounded; otherwise results would be erratic and confusing. The test shall be performed before winding resistance measurement. The test voltage shall be limited to maximum power supply voltage available at site. 7.3.1 Evaluation Criteria The voltage induced in the center phase is generally 50% to 90% of the applied voltage on the outer phases. However, when the center phase is excited then the voltage induced in the outer phases is generally 30 to 70% of the applied voltage. Zero voltage or very negligible voltage with higher excitation current induced in the other two windings should be investigated. 7.4

Dissolved Gas Analysis (DGA) of Oil Filled in the Transformer

7.4.1 Introduction For many years the method of analyzing gases dissolved in the oil has been used as a tool in transformer diagnostics in order to detect incipient faults, to supervise suspect transformers, to test a hypothesis or explanation for the probable reasons of failures or disturbances which have already occurred and to ensure that new transformers are healthy. The evaluation criteria with dissolve gas analysis is based on the fact that during its lifetime the transformer generates decomposition gases-essential from the organic insulation – under the influence of various stresses– both normal and abnormal. The gasses that are of interest for the DGA analysis are the following; •

H2 Hydrogen

•

CH4 Methane

•

C2 H4 Ethylene

•

C2 H6 Ethane

•

C2 H2 Acetylene

•

(C3 H6 Propene)– not always measure


•

(C3 H8 Propane) – not always measure

•

CO Carbon Monoxide

•

CO2 Carbon Dioxide

•

O2 Oxygen

•

N2 Nitrogen

•

TCG total combustible gas content (= H2 + CH4 + C2 H4 + C2 H6 + C2 H2 + CO)

199

All these gases except oxygen and nitrogen may be formed during the degradation of the insulation. The amount and the relative distribution of these depend on the type and severity of the degradation and stress. Around the world and during the years several different schemes have been proposed as evaluation scheme for the DGA. DGA is recommended for transformers of rating above 100 MVA and shall be carried out before and after temperature – rise test. 7.4.2 Procedure The DGA procedure consists of essential four steps: •

Sampling of oil from the transformer

•

Extraction of these gases from oil

•

Analysis of the extracted gas mixture in a gas chromatography, GC.

•

Interpretation of the analysis according to an evaluation scheme.

Sampling, extraction and analysis procedures are given in IEC publication 60599 7.4.3 Interpretation There are several different approaches how to explain and interpret the analyzed gas composition and to diagnose the condition of the transformer. The well known DGA analysis techniques are : •

Identification of the key gas, The key gas identifies a particular problem, e.g., H2 indicates a PD

•

Determination of ratios between gases, normally between gas levels.

•

Determination of rates of increase (“production rates”), in ppm / day or ml gas/day

200


The most common known schemes are the one proposed by Rogers forming the basis for the ANSI method and the scheme laid down in IEC Publication 60599. Both these methods are using ratios gas concentrations. This scheme can also be used to understand the evaluation – scheme based on ratios. For instance, the IEC method uses 3 ratios, C2 H2 / C2 H4 , CH4 / H2 , C2 H4 / C2 H6 CH4 /H2 is used to discriminate between a thermal fault and an electric fault. C2 H2 /C2 H4 indicates the presence of a strong discharge of very severe electric problem and C2 H4 / C2 H6 is an indication of the oil temperature. (For further detail refer para 2.0 of section K1 on condition monitoring) 7.5

Frequency Response Analysis (FRA)

Frequency response analysis (FRA) test is conducted on transformers and reactors to determine the frequency response of windings. The reference frequency responses obtained during laboratory testing serve as ‘fingerprints’ to monitor the condition of the transformer or reactor during service. The frequency response of an electrical winding is obtained by application of sweep frequency (sinusoidal). The winding will have a characteristic frequency response for the applied signal at different frequencies. The response is uniquely determined by the winding arrangement involved and any winding movement or other fault will modify the frequency response due to changes in inductances and capacitances. The sweep frequency voltage is applied through network analyzers. The frequency response of the winding is determined between the frequency ranges of 10 Hz to 2 MHz. The FRA test is performed on one winding of the electrical equipment at a time. The transformer / reactor shall be electrically isolated from any other electrical connections or systems, including earth connections during FRA test. The two end terminals of each winding shall be made available for measuring the frequency response across the winding. •

For star connected winding, the response shall be measured across the terminal and neutral.

•

For delta connected winding, the response shall be measured across two line terminals and in case of open-delta, across individual winding.

•

For auto connected winding, the response of series and common windings shall be measured separately.

For a transformer, it is normal practice to earth one end of every winding that is not being tested, leave the other open end. Alternatively, all other windings may be left unconnected from each other and from earth. In every case, the termination of each winding for each test should be recorded. The frequency response of the winding is determined by plotting the ratio of the output from the winding to the input in the frequency range of 10 Hz to 2 MHz.


201

Alternatively frequency ranges specified by the customer can be selected. The test is normally conducted at maximum, mean and minimum taps, in case of windings having tapping. While making measurements at mean tap, care should taken to move the tap from higher voltage taps, for proper comparison of FRA results of different phases of same transformer or different transformers. The FRA results are analyzed for : •

Changes in response of the winding

•

Significant difference between the FRA records of different phases of the same transformer.

•

Significant difference between same phase of identical transformers.

FRA test is primarily a condition assessment test and can be used in conjunction with other diagnostic tests for detailed analysis and interpretation of the transformer. (For other details refer para 6 of section K-1) 7.6

Measurement of Magnetization Current at Low Voltage

For 3-phase transformers, the test shall be conducted either with 415 V, 3-phase (neutral grounded) or 230 V, 1 phase (preferred). For single phase transformers, the test shall be conducted with 230 V. This test is performed to locate defect in magnetic core structure, shifting of windings, failures in turn insulation or problem in tap changers. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase, in case of three phase transformers. An agreement to within 25% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50% higher than the value measured during pre-commissioning checks, then the winding needs further analysis. 7.7

Functional Tests on Auxiliary Equipments

7.7.1 Acceptance Test for Oil (OTI) and Winding (WTI) Temperature Indicator (A)

Routine Test

1.

OTI (Range 20º-140º C)

(i)

Each completely assembled instrument shall be tested for accuracy over the complete range, i.e., at 40º, 60º, 80º, 100º & 120 º C by keeping the bulb in the hot oil bath continuously stirred. The accuracy of indication shall be ±1.5 % full scale deflection (FSD).


202 2.

WTI (Range 30º - 150º C) (i)

Each completely assemb led instrument shall be tested by injecting the current to its heater coil. Oil bath shall be maintained at 60 0 C, Total temperature and temperature rise shall be recorded for 0, 2, 3 and 4 amperes current. The accuracy of the indication, i.e., oil bath temperature measured by standard. Thermometer plus rise in temperature due to injection of current in heating coil, i.e., total temperature indicated by WTI shall be within ± 1.5 % FSD. Error allowed shall be (1.5 / 100) x 150 = ± 2.25 0 C Max.

(ii)

In case of repeater, both WTI and repeater shall be tested together by injecting the current as mentioned in 2(I). Accuracy of the repeater readings shall be within ± 1.5 % FSD, i.e., ± 2.25 0 C considering WTI readings as the reference temperature.

7.7.2 High Voltage Test on Insulation test of Auxiliary Wiring Unless otherwise specified the wiring for auxiliary power and control circuitry shall be subjected to a one minute power frequency withstand test of 2 kV rms to earth. Motors and other apparatus for auxiliary equipment shall fulfill insulation requirements according to the relevant IEC standard (which are generally lower than the value specified for the wiring alone, and which may sometimes make it necessary to disconnect them in order to test the circuits). Micro switches and other electronic items shall be disconnected during the test. (B)

Type Tests (one instrument of each lot / batch)

Switch setting and operations: Switches shall be able to set between 50–140 0 C and their operation shall be within ± 2.5% of pointer indication unless otherwise specified in purchase order. Switch setting will be done as below: OTI: Alarm (S1 )

–

90 0 C

Trip (S2 )

–

95 0 C

WTI: Alarm (S1 )

–

110 0 C

Trip (S2 )

–

120 0 C

Fan start (S3 )

–

80 0 C

Pump start (S4 )

–

90 0 C

Switch differential Each switch shall have adjustable differential (difference between make and break temperature) of 6 0 C to 90 0 C and will be set for 6 ± 1 0 C differential. Switch Rating 5 Ampere, continuous 250 V, AC or DC for make or break.

203


7.8

Tests on Oil Filled in Transformer

Following test on the oil filled in the transformer shall be necessary performed before conducting electrical test to ensure proper oil impregnation of the insulation system. 7.8.1 Dielectric Strength The voltage at which the oil breaks down when subjected to an AC electric field with continuously increasing voltage contained in the specified apparatus is called dielectric strength. The voltage is expressed in kV. The dielectric strength of oil is determined by the two methods. First method utilizes spherical capped electrode in the test cell, which is recommended primarily for filtered, degassed and dehydrated oil prior to and during filling of electrical power equipment rated above 230 kV and above. The second method utilizes flat electrodes and recommended for all other apparatus. The detailed test procedure is in accordance with IS 6792. The acceptance value of oil for the different test voltage of transformer in general is recommended as per the table given below. 7.8.2 Water Content The recommended value of water content are given in table below : System voltage of transformer kV Above Upto and including

BDV kV

Water content ppm, max.

Tan delta at 900 C

72.5 245 420

60 65 70 75

20 15 10 10

< < < <

72.5 245 420 800

0.05 0.02 0.01 0.01

High water content accelerates the chemical deterioration of the insulating paper and is indicative of the undesirable operating conditions or maintenance requiring correction. 7.8.3 Dielectric Dissipation Factor (Tan delta at 900 C) This test covers the determination of the power factor of new and service aged oil. This test is used to indicate the dielectric losses in the oil when used in an alternating electric field and of the energy dissipated as heat. A low power factor indicates low dielectric losses. It is useful as a means to ensure that sample integrity is maintained, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling. This test is satisfactorily performed in the field, as well as in a laboratory environment. The detailed test procedure and test equipment shall be in accordance with IS 6262.


204

Acceptable limit for the dielectric dissipation factor largely depend upon the type of apparatus and application. The power factor limits given for oil are based upon the understanding that this is an indicator test for contamination by excessive water or polar or ionic materials in the oil. High level of dissipation factor (0.5 % at 25º C) is because of contaminants may collect in the areas of high electrical stress and concentrate in the winding. Very high dissipation factor (>1.0%) in oil may be caused by the presence of free water which could be hazardous to the operation of a transformer. 7.8.4 Resistivity The resistivity (specific resistance) in ohm-centimetres of a liquid is the ratio of the dc potential gradient in volts per centimetre paralleling the current flow within the specimen, to the current density in amperes per square centimetre at a given instant of time and under prescribed conditions. This is numerically equal to the resistance between opposite faces of a centimetre cube of liquid. Resistivity measurements are made at many different temperatures. But for acceptance test, it is generally done at a temperature of 90º C, while for routine testing, it is usually made at room temperature or 90º C. The average electrical stress to which specimen is subjected to shall not be less than 200 V/mm nor more than 1200 V/mm the upper limit is set with the purpose of avoiding possible ionization if higher stresses are permitted. The detailed test procedure is in accordance with IS 6103. Useful information can be obtained by measuring resistivity at both ambient and at higher temperature such as 90º C. A satisfactory result at 90º C coupled with an unsatisfactory value at lower temperature is an indication of the presence of water or degradation products precipitated. (For other details on oil, specifications, testing etc. refer para 2.5 of section K) 7.9

Oil pressure Test on Completely Assembled Transformer

This test is done after completion of all electrical and temperature rise test. Transformer with cooling bank, bushing and other accessories shall be tested for any oil leakage at high pressure (normal pressure plus 35 kN per sq m measured at the base of tank) and at room temperature as specified by customer. The procedure for conducting this test is as follows: •

Conservator along with the Buchholtz relay shall be disconnected.

•

Calibrated pressure gauge shall be mounted at the bottom of the tank.

•

Bushings will remain mounted.

•

In welded cover type construction cooler bank, bushings shall be removed but all turrets and cover pipe work shall remain.


205

•

Fill the oil completely and release all trapped air.

•

The specified pressure shall be maintained for the specified test duration as specified in the test schedule or quality plan.

•

The test duration should be at least one hour unless otherwise specified.

7.9.1 Criteria for Oil Pressure Test During the pressure test, there shall not be any leakage. If there is pressure drop during the test either because of some trapped air inside the transformer or due to ambient temperature variation, the pressure shall be raised to the specified level. The unit will be considered to pass the test only if there is no visual oil leakage. Pressure drop shall be considered as failure of the unit in the test. 8.0

FIELD TESTS

8.1

General

Transformer is important and vital equipment between generation station and the utility and therefore necessary to ensure its proper performance through out its service life. During transportation, installation and service operation, the transformer may be exposed to conditions, which adversely affect its reliability and useful life. Field-testing and condition monitoring are the techniques to ensure good operating health of power transformers. Interpretations are also included to provide additional information on the particular test and to provide guidance on acceptable criteria. There is not necessary any direct relationship between field tests and factory tests. Interpretation of measured results is usually based on a comparison with data obtained previously on the same unit under similar condition. It should be noted that some times the results of several types of tests should be interpreted together to diagnose a problem. Manufactures acceptance criteria shall also be referred to. 8.2

Dew Point Measurement for Large Transformers Filled with Dry Air or Nitrogen Filled

Large rating transformers are transported to site from manufacturing works, without oil and filled with dry air or nitrogen due to weight limitations. Positive gas pressure is generally maintained at 0.175 kg/m2 during transportation and storage. As the insulation of transformer is hygroscopic, it absorbs moisture from atmosphere if positive pressure of gas is not maintained. After arrival of transformer at site it is necessary to check the gas pressure and if it is not positive there is every possibility that moisture must have gone inside the transformer during transportation. To ascertain this factor and to check the dryness of the insulation, dew point measurement is carried out at site. Dew point is the temperature at which the water vapours present in the gas filled in the transformer begin to condense. The procedure and acceptance limits are given in section K of this manual.


206 8.3

Winding Resistance Measurement

Transformer winding resistances are measured at site in order to check for abnormalities due to loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings. The resistance is measured by two methods : (a)

Voltmeter Ammeter method

(b)

Bridge method

(c)

Digital meters

The detailed test procedure of above methods is same as factory testing and is covered in clause 4.1 of this section. Precautions shall be taken during field testing as given below. The test shall be conducted at all taps of the transformer winding and the measured value shall be converted to 75 0 C. The acceptance criterion is usually agreement to within 5% of resistance measurements made separately on different phases, under field condition. But, for large transformers it is recommended to compare the resistance values with original data measured in the factory and in case of large variation, connection tightness to be checked. The current used for these measurements should not exceed 15% of the rated current in order to avoid heating the winding thereby changing its resistance. However, the current should not be too small, which may not be sufficient to avoid inductive effect, due to core magnetization The winding resistance shall preferably be measured when the difference in the top and bottom temperature of the oil (temperature of oil in steady-state condition) is equal to or less than 5°C. Winding resistance measurement shall be done only after measurement of magnetic balance and magnetization current (excitation current). The polarity of the core magnetization shall be kept constant during all resistance measurement. A reversal in magnetization of the core can change the time constant and result in erroneous readings.

207


8.4

Vector Group and Polarity

To determine the phase relationship and polarity of transformers The procedure to find out vector group shall in general be same as defined in section A3.2.2.2. Typical test connections and check measurements are shown below : (a)

Connect neutral point of star connected wiring with earth.

(b)

Join 1U of HV and 2U of LV.

(c)

Apply 415V, 3 phase supply to HV terminals

Example 1 For HV-Delta/LV-Star Transformer Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-1W, 1V-2V, 1V-2W, 1U-2W VOLTAGE RELATION

1U 2U

1U-1W=1U-2W+2W-1W

2W

1V

2V

1V-2V<1V-2W 1W-2V=1W-2W

1W Dy-I

Fig. 4 Connection diagram for HV-Delta / LV-Star Transformer

Example 2 For HV-Star/LV-Delta Transformer

Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V,

1U+2U 2V

2W-N, 1U-N, 2W-1V VOLTAGE RELATION 1U-N=1U-2W+2W-N 1W-2W = 1V-2W 1W-2V > 1V-2V

2W

N

1W

Yd11

1V

Fig. 5 Connection diagram for HV-Star / LV-Delta Transformer


208

The test shall be conducted with three-phase supply and voltmeters. By the measured voltage data it should ensure that the desired conditions of vector group and polarity are fulfilled. Ensure the isolation of transformer from high voltage and low voltage side with physical inspection of open condition of the concerned isolators. In case tertiary is also connected, ensure the isolation of the same prior to commencement of testing. 8.5

Voltage Ratio Test

To determine the turns ratio of transformers during commissioning and periodic interval decided by the customer for preventive maintenance The procedure shall be following : (a)

Keep the terminals of IV and LV open.

(b)

Apply 3-phase or single phase supply according to the transformer type on HV terminals.

(c)

Measure the voltage ratio of HV and IV as well as HV and LV.

(d)

Repeat the steps for each tap position separately.

The test shall be conducted with three-phase supply and voltmeters. The voltage ratio shall be measured between all pairs of windings of a transformer with externally available terminals. Results of the transformation turns/voltage ratio are absolute, and may be compared with the specified values measured during factory testing. The acceptance criteria should be that the measured values are within 0.5% of the specified values for all windings. One should also consider the trend of voltage ratio values with reference to the ratio values measured during the commissioning tests. The voltage should be applied only in the high voltage winding in order to avoid unsafe voltage. 8.6

Measurement of Magnetizing Current

This test is performed to locate defect in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish flux in the core. The procedure is as follows: (a)

First of all keep the tap position in the lowest position and IV and LV terminals open.


209

(b)

Apply 3, phase 415 V supply on the line terminal for 3-phase transformers and 1phase, 230 V supply on single phase transformer.

(c)

Measure the voltage applied on each phase (Phase-Phase) on line terminals and current in each phase of the line terminal.

(d)

After completion of the above steps keep the tap at normal position and repeat the steps a to c.

(e)

After completion of the above steps keep the tap positioning at Highest position and repeat the steps a to c.

(f)

Repeat the test with tap position in normal position.

The test shall be conducted with single-phase or three-phase supply according to test requirement, voltmeter and multimeter. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase, in case of three phase transformers. An agreement to within 30% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory tests then there is likelihood of a fault in the winding which needs further analysis. Care should be taken during exciting current measurement to avoid the effect of residual magnetism in the transformer core. The residual magnetism results in the measurement of higher than normal exciting current. 8.7

Magnetic Balance Test on 3-phase Transformer

This test is conducted only on three-phase transformer to check the imbalance in the magnetic circuit. The procedure for conducting test is as follows : (a)

Keep the tap in nominal tap position

(b)

Disconnect the transformer neutral from ground

(c)

Apply single phase 230 V across one of the HV winding terminal and neutral then measure voltage in other two HV terminals across neutral. Repeat the test for each of the three-phases.

(d)

Repeat the above test for IV winding also

The test shall be conducted with 230 V single phase supply and voltmeter.

210


The voltage induced in the center phase may be 50% to 90% of the applied voltage. However, when the center phase is excited then the voltage induced in the outer phases may be 30% to 70% of the applied voltage. Zero voltage or very negligible voltage induced in the other two windings should be investigated. Disconnect transformer neutral from ground and no winding terminal should be grounded, otherwise results would be erratic and confusing. 8.8

Magnetic Circuit (Isolation) Test

This test should be performed prior to a unit being placed in-service or modifications to the transformer which could affect the integrity of its core insulation. Refer clause 3.21 part A of this manual for test procedures and acceptance criterion. 8.9

Measurement of Short Circuit Impedance at Low Voltage

To find out the short circuit impedance of transformer. The measurement is performed in single phase mode. This test is performed for the combination of two winding. The one of the winding is short circuited and voltage is applied to other winding. The voltage and current reading are noted. The test shall be conducted with variac of 0-280 V, 10 A, precision RMS voltmeter and ammeter. The acceptable criteria should be the measured impedance voltage having agreement to within 3 percent of impedance specified in rating and diagram nameplate of the transformer. Variation in impedance voltage of more than 3% should be considered significant and further investigated. The conductors used for short -circuiting one of the transformer windings should have low impedance (less than 1m-ohm) and short length. The contacts should be clean and tight. 8.10 Insulation Resistance Measurement Insulation resistance tests are made to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance in such tests is commonly measured in mega-ohms, or may be calculated from measurements of applied voltage and leakage current. The test is conducted with the help of mega-ohmmeter. IR is proportional to the leakage current through/over the insulation after capacitive charging and absorption currents become negligible on application of DC voltage. Insulation resistance shall be measured after the intervals of 15 sec, 60 sec and 600 sec. The polarization index (PI) is defined as the ratio of IR values measured at the intervals of 600 and 60 seconds respectively. Whereas, the dielectric absorption is the ratio of IR values measured after 60 sec and 15 sec. IR is normally measured at 5 kV DC or lower test voltage, but the test voltage should not exc eed half the rated power-frequency test voltage of transformer windings.


211

Polarization index (PI) is useful parameters for logistic interpretation of IR test results. This ratio is independent of temperature and gives more reliable data for large power transformers. A PI of more than 1.3 and dielectric absorption factor of more than 1.25 are considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI of less than 1 calls for immediate corrective action. For bushings, an IR value of above 10000 M-ohms is considered satisfactory. The IR value of transformer is dependent on various factors such as configuration of winding insulation structures, transformer oil, atmosphere condition etc. therefore, present trend is to monitor oil characteristics for judging the condition of dryness of the transformer and not to rely solely on absolute values of IR. It may be note that no national/international standards specify minimum insulation resistance values of transformers. The value of IR may be very low under heavy fog or humid conditions. During IR measurement, we must ensure following conditions : Transformer is disconnected from other associated equipment Bushings are cleaned and free of moisture Transformer tank and core are properly grounded Both ends of winding under test are short-circuited. 8.11 Measurement of Capacitance and Dissipation Factor The insulation dissipation factor (tan delta) is the ratio of the resistance current to the capacitor current flowing through the insulation on application of sinusoidal voltage under prescribed condition. The capacitance values are relatively independent of temperature and prevailing atmospheric conditions. The voltage to be applied shall not exceed half of the power frequency test voltage or 10 kV whichever is lower. Detailed test procedure is given under clause no. 6.9 of this section and clause no. 5 of section K-1. The test is conducted with high voltage supply and Schering bridge Low dissipation factor is indicative of problem in insulation structure and predictive ageing of insulation. But, the comparative values of tests taken at periodic intervals are useful in identifying potential problems rather than an absolute value of tan delta. The initial reference can be drawn from the tan delta values measured during factory testing. However, the tan delta values measured in the field are not very accurate and deviation up to 30% from the values measured at the works should be accepted. The acceptance criterion to assess the probable condition of the insulation of the transformer is no substantial variation in the measured values of tan delta (dissipation factor) at periodic interval when compared with previous references. For bushings, the tan delta value shall not exceed 0.7%. The main capacitance of the bushing, i.e., the capacitance between high voltage terminal and test tap is not affected by the surrounding conditions and the accepted deviation from the

212


values measured at factory tests should be less than 10%. The capacitance between bushing test tap and ground is largely influence by the stray capacitances to ground parts in the transformer and hence large deviation in the measured value shall be accepted when compared with the factory test value. Environmental factors like variation in temperature, relative humidity, surrounding charged objects etc. have great influence on measurement of dissipation factor. Care shall be taken to control the above factors during measurements. 8.12 Dissolved Gas Analysis (DGA) Dissolved gas analysis is a well-established condition-monitoring tool for power transformers. The method is based on analyzing the types of gases dissolved in the transformer oil and their production rates. The relative concentration of these gases depends upon the type and severity (energy density) of fault. The test procedure to conduct DGA test is accordance with the IS 9434. The main gases formed as result of thermal and electrical faults in a transformer are Hydrogen (H2 ), Methane (CH4 ), Ethane (C2 H6 ), Ethylene (C2 H4 ), Acetylene (C2 H2 ), Carbon Monoxide (CO) and Carbon dioxide (CO2 ). Whereas Acetylene is mainly associated with arcing at very high temperature ranges, Ethylene is related with hot spots and medium temperature ranges. Hydrogen is mainly result of ‘cold’ gas plasma of corona discharges. Carbon monoxide and carbon dioxide are the result of thermal decomposition of cellulose material. For proper interpretation of DGA results, it is essential to collect data at periodic intervals during the service life of the transformer and the additional information regarding age of the transformer, past history of failures, loading pattern, history of filtration etc. 8.13 Tests on Oil Filled in Transformer as per IS 1866 Refer clause 2.5 of Section K of this Manual 9.0

CONDITION MONITORING

In addition to the well established low voltage routine tests, additional checks have been developed in order to increase reliability of the transformer by condition monitoring. These tools or checks conducted first time, determine the initial condition of the transformer at the beginning of its service life. These initial references are considered as 'benchmark' or 'fingerprints' to demonstrate subsequent changes in the dielectric, mechanical and thermal properties of the transformer, which are indicative for reliability. These tools provide useful data for predictive life and maintenance management of transformer, better known today as condition-based management (CBM). The detailed procedure and interpretation of the test results is dealt under sections K & K-1 of this Manual.


213

Appendix 1 METHOD OF DECLARING EFFICIENCY 1.0 The efficiency to be declared is the ratio of the output in kW to the sum of the output in kW and the following losses : (a)

No load loss, which is considered to be constant at all loads; and

(b)

Load loss, which varies with load.

The total loss, on load is the sum of (a) & (b). CALCULATION OF INHERENT VOLTAGE REGULATION 2.0

TWO WINDING TRANSFORMERS

2.1 The inherent voltage regulation from no-load to a load of any assumed value and power factor may be computed from the impedance voltage and corresponding load loss measured with rated current in the winding as follows: Let I = rated current in winding excited; E = rated voltage of winding excited; ISC = current measured in winding excited; EZsc = voltage measured across winding excited (impedance voltage); PSC = watts measured across winding excited; EXSC = reactance voltage= √[ E2 ZSC-(PSC/ISC)2 ] P=PSC corrected to 75 deg. C, and from current ISC to I; EX=EXSC X (I/ISC) Er= P ;

I

EX%=100 EX/E; Er%=100 Er/E; = Ia/I; and Ia =

current in winding in excited during the short circuit test corresponding to that obtained when loading at the assumed load on the output side and with rated voltage on the input side.


214

2.2 For rated load at unity power factor, the percentage regulation is approximately equal to: Er% + (Ex%)2 200 2.3 For rated load at any power factor cos φ, the percentage regulation is approximately equal to: Er% cos φ+ Ex% sin φ + (Ex% cos φ- Er % sin φ) 2 200 2.4 For any assumed load other than rated load and unity power factor, the percentage regulation is approximately equal to: n. Er% + (n. Ex%)2 200 2.5 For any assumed load other than rated load and at any power factor cos φ, the percentage regulation is approximately equal to: (n . Er% cos φ + n. Ex% sin φ) + (n . Ex% cos φ - n . Er% sin φ)2 200 2.6 The above formula are sufficiently accurate for transformers covered by this specification. 3.0

THREE WINDING TRANSFORMERS

3.1 The formula given in 2.1 for two-winding transformers can be applied to threewinding transformers and their regulation calculated with an accuracy comparable to that of the data available by assuming that the currents in the windings remain constant both in magnitude and phase angle even though the output terminal voltage change, due to the regulation, from their no-load values. On a three-winding transformer the no-load voltage of a winding will change with current in the other windings (even though it remains itself unloaded). Therefore, the voltages regulation of a winding on a three-winding transformer is expressed with reference to its no-load open-circuit terminal voltage when only one of the other windings is supplied and the third winding is on no-load, that is the basic voltage for each winding and any combination of loading is the line no-load voltage obtained from its turns ratio. For the frequent case of two output winding (W2 and W3 ) and one input winding (W1 ) the voltage regulation is usually required for three loading conditions, namely: Only W2 loaded,


215

Only W3 loaded, and Both W2 and W3 loaded. For each condition two separate figures should be quoted, that is the regulation of each output winding W2 and W3 (whether carrying current or not) for constant voltage supplied to the winding W1 . Note : The regulation between W2 and W3 relative to each other for this simple and frequent case is implicit in the values (W1 to W2 ) and (W1 to W3 ) and nothing is gained by expressing it separately.

3.2 The data required are the impedance voltage and load losses derived by testing the three windings in pairs and expressing the results on a basic kVA, which can conveniently be the rated kVA of the smallest winding. It should be determined from the transformer as built. From the data, an equivalent circuit 3 winding transformer is derived as shown below:

3.3

The equivalent circuit is derived as follows:

Let a12 and b 12 be respectively the percentage resistance and reactance voltages referred to the base kVA and obtained on a test, short-circuiting either winding W1 or W2 and supplying the other, with the third winding W3 on open-circuit: a23 and b23 similarly apply to a test on the windings pair W2 and W3 (with W1 on opencircuit);


216

a31 and b 31 similarly apply to a test on the windings pair W3 and W1 (with W2 on opencircuit); d = the sum (a 12 + a23 + a31 ); and f = the sum (b 12 + b 23 + b 31 ) Then the mathematical values to be inserted in the equivalent circuit are: Arm W 1 a1 = d/2 - a23 , and b 1 = f/2 - b 23 Arm W 2 a2 = d/2 - a31 , and b 2 = f/2 - b 31 Arm W 3 a3 = d/2 - a12 , and b 3 = f/2 - b 12 It is to be noted that some of these mathematical values may be negative or may even be zero (depending on the actual physical relative arrangement of the windings on the core). For the desired loading conditions the kVA operative in each arm of the equivalent circuit is determined and the regulation of each arm is calculated separately. The regulation with respect to the terminals of any pair of windings is the algebraic sum of the regulations of the corresponding two arms of the equivalent circuit. 3.4 The detailed procedure to be followed in the case of two output windings and one supply winding is as follows; (a)

Determine the kVA in each winding corresponding to the loading being

considered.

(b)

For the output windings W2 and W3 , this is the specified loading under consideration; deduce n2 and n 3 , where n is the ratio of the actual loading to the base kVA used in the equivalent circuit.

(c)

For the input winding W1 kVA should be taken as the vectorial sum of the outputs from W2 and W 3 windings, and the corresponding power factor and quadrature factor (sin φ) deduced from the in-phase and quadrature components.

3.5 When greater accuracy is required, an addition should made to the above vectorial sum of the outputs as follows: Add to the quadrature component, to obtain the effective input kVA to windings W1 ; (output kVA from winding W2 ) x b2 x n2 100 + (output kVA from winding W3 ) x W x b3 100

xn

n for each arm being the ratio of the magnitude of the actual kVA loading of the winding to the base kVA employed in determining the network.


217

A more accurate solution is obtained by adding the corresponding quantities (a x n x output kVA) to the in-phase component of the vectorial sums of the outputs, but the difference is rarely appreciable. 3.6 Apply the final formula of 2.1.5 separately to each arm of the network, taking separate values of n for each arm as defined in 2.2.5. 3.7 To obtain the regulation between the input winding and either of the loaded windings, add the separate regulations determined under 2.2.6 for the corresponding two arms, noting that one of these may be negative. (Note that the summation is algebraic, but not vectorial). Note : A positive value for the sum determined indicates a voltage drop from no-load to the loading considered while a negative value for the sum indicates a voltage rise.

3.8

Repeat the operation described in 2.2.7 for the other loaded winding.

3.8.1 The above procedure is applicable to auto-transformers if the equivalent circuit is based on the effective impedances measured at the terminal of the auto-transformer. 3.8.2 In the case of input to two windings and load from one winding, the above procedure can be applied if the division of loading between the two supplies is known. 3.8.3 Example of Application to a Three-Winding Transformer. Assume that: W1 is a 66 kV primary winding, W2 is a 33 kV output winding loaded at 2000 kVA having a power factor cos φ of 0.8 lagging, and W3 is a 11 kV output winding loaded at 1000 kVA having a power factor cos φ of 0.6 lagging. The following information is available, having been calculated from test data and is related to a base kVA of 1000: a12 = 0.26

b 12 = 3.12

a23 = 0.33

b 23 = 1.59

a31 = 0.32

b 31 = 5.08

It can be deduced that d = 0.91 and f = 9.79 If follows, therefore, that for W1 , a1 = 0.125 and b 1 = +3.305


218 W2 , a2 = 0.135 and b 2 = -0.185 W3 , a3 = 0.195 and b 3 = +1.775

The effective full load input kVA to winding W1 is: (a)

With only output winding W2 loaded

= 2000 kVA at power factor 0.8 lagging.

(b)

With only output winding W3 loaded


(c)

With only output winding W2 and W3 loaded


Applying the formula of 2.1.5 separately to each arm of the network we have for the regulation of that arms alone: W1 under condition (a) where n 1 = 2.0 is + 4.30 per cent W1 under condition (b) where n 1 = 1.0 is + 2.74 per cent W1 under condition (c) where n 1 = 2.98 is + 7.15 per cent W2 under condition where n 2 = 2.0 is - 0.005 per cent W3 under condition where n 3 = 1.0 is +1.54 per cent Therefore, the total transformer regulation is: For condition (a) - with output winding W2 fully loaded and W3 unloaded: At terminals W2 = 4.30 - 0.005 = 4.295 per cent At terminals W3 = 4.30 + 0 = 4.30 per cent For condition (b) - with output winding W2 unloaded and W3 fully loaded: At terminals W2 = 2.74 + 0 = 2.74 per cent At terminals W3 = 2.74 + 1.54 = 4.28 per cent For condition (c) - with both output windings W2 and W3 loaded: At terminals W2 = 7.15 - 0.005 = 7.145 per cent At terminals W3 = 7.15 + 1.54 = 8.69 per cent

219


Section-A, Appendix 2 LIST OF TRANSFORMER ACCESSORIES AND TEST-CERTIFICATES REQUIRED FOR THEM Sl. No 1.

Accessory Condenser Bushing

Ref. Std.

IS 2099

2.

IS Bushings IS 2099

3.

OLTC

IS 8468

4.

Winding Temperature Indicator

5.

Oil Temperature Indicator

6.

Pressure Relief Valve

7.

Cooling Fan IS 2312

8.

Transformer Oil Pump IS 325 & IS 9137

Test -certificates required. 1. Appearance, construction and dimensional check. 2. Test for leakage of internal filling at a pressure of 1.0 kg/cm2 for 12h. 3. Insulation resistance measurement with 2000 V megger. 4. Dry power frequency voltage withstand test. 5. Dry power frequency voltage withstand test for test tap insulation. 6. Partial discharge measurement upto 1.5um /√3 kV. 7. Measurement of tangent delta and capacitance. 1. Appearance, construction and dimensional check. 2. Insulation resistance measurement with 2000 V megger. 3. Dry power frequency voltage withstand test. 1. Oil tightness test for the diverter switch oil chamber at an oil pressure of 0.5 kg/cm2 at 100 0 C for 1h. 2. Mechanical operation test. 3. Operation sequence measurement. 4. Insulation resistance measurement using 2000 V megger. 5. Power frequency voltage withstand test on diverter switch to earth and between even and odd contacts. 6. Power frequency voltage withstand test on tap selectorbetween stationary contacts, between max. and min. taps, between phases and supporting frames, between phases. 7. Operation test of complete tap changer. 8. Operation and dielectric test of driving mechanism. 1. Calibration test. 2. Dielectric test at 2 kV for one minute. 3. Accuracy test for indication and switch setting scales. 4. Test for adjustability of switch setting. 5. Test for switch rating. 6. Measurement of temperature rise with respect to the heater coil current. 1. Calibration test. 2. Dielectric test at 2 kV for one minute. 3. Accuracy test for indication and switch setting scales. 4. Test for adjustability of switch setting. 5. Test for switch rating. 1. Functional test with compressed air to check bursting pressure, indication flag operation and switch operation. 2. Dielectric test at 2 kV for one minute. 3. Switch contact testing at 5 A 240V AC. 1. Insulation resistance measurement. 2. Dielectric test at 2 kV between winding and body for one minute. 3. Operation check. 4. Appearance, construction and dimensional check. 1. Insulation resistance measurement. 2. Cold resistance measurement at ambient temperature. 3. Motor efficiency at full load. 4. No load voltage, current, power input, frequency and speed. 5. Locked-rotor readings of voltage, current and power input. 6. Water pressure test for pump casing at 5 kg/cm2 for 10 minutes at ambient temperature.


220

9.

Oil Flow Indicator/ Water Flow Indicator

10.

Buchholz Relay

IS 3637

11.

Oil Level Indicator

12.

Pressed Steel Radiators

7. Transformer oil pressure test for the pump set assembly at 2 kg/cm2 for 30 minutes at 80 0 C. 8. Measurement of head, discharge, current, power input to motor and overall efficiency of the pump set at rated voltage. 9. Appearance, construction and dimensional check. 1. Observation of flow with respect to requirement. 2. Switch contact test at 5 A 240V AC. 3. Dielectric test at 2 kV for one minute. 4. Appearance, construction and dimensional check. 1. Leak test with transformer oil at a pressure of 3 kg/cm2 for 30 minutes at ambient temperature for relay casing. 2. Insulation resistance measurement with 500 V megger. 3. Dielectric test at 2 kV for 1 minute. 4. Elements test at 1.75 kg/ cm2 for 15 minutes using transformer oil at ambient temperature. 5. Loss of oil and surge test. 6. Gas volume test. 7. Mechanical strength test. 8. Velocity calibration test. 9. Appearance, construction and dimensional check. 1. Test for oil levels. 2. Switch operation for low level alarm. 3. Switch contact test at 5A 240V AC. 4. Dielectric test at 2 kV for 1 minute. 5. Appearance, construction and dimensional check. 1. Air pressure test at 2 kg/cm2 under water for 15 minutes. 2. Appearance, construction and dimension check.

SECTION K

Erection, Commissioning and Maintenance

222



223

SECTION K

Erection, Commissioning and Maintenance As the continuity of supply is of paramount importance in modern power systems, it is necessary to take all possible precautions during erection and commissioning of the transformers’ followed by regular preventive maintenance. This section covers erection, commissioning and maintenance of Power and Distribution Transformers. 1.0

PACKING AND DISPATCH

1.1 After testing each transformer it shall be dispatched from the works ready for reassembly of external components which are dismantled for facilitating transportation. According to the transport facilities available and weight/ height restriction for the route, transformers are transported either by rail, road or sea depending on the size of transformer. Power transformers should be dispatched with external fittings dismantled and packed separately. However distribution transformers can be transported in fully assembled condition. 1.2 Transformer tank is filled with oil or pure dry nitrogen/ air depending upon the transport weight limitations. If nitrogen is used, the information shall be stenciled on the tank prominently. In case the tank is filled with oil, sufficient space is left above the oil to take care of the expansion of oil. This space is filled with pure dry air/nitrogen gas under atmospheric pressure. In case the tank is filled with inert gas/nitrogen a positive pressure according to the manufacturer’s standard practice shall be maintained. The temperature, pressure and dew point (-250 C maximum after filling) at the time of gas filling (reading taken after stabilization) shall be painted on the transformer tank. External gas cylinders should be provided to make up any gas leakage during transit for transformers having rating 50 MVA and above and voltage rating 132 kV and above. Transformer shall be accompanied by escort decided by the manufacturers or the customers as per the agreement. For large rating transformers of above 100 MVA the impact recorders/ indicators may be provided during shipment from transformer manufacturer’s works to site. Manufacturer shall specify the limiting values to the purchaser in advance. The impact recorder is required to be sent back to the manufacturer for their analysis. Manufacturer shall send the analysis report of impact recorder back to user / utility for their record. In case the impact recorder indicates some serious shock during shipment, the core and coil will have to be subjected to thorough inspection before erection. Note : Impact recorders are attached to the main body of the transformer during transportation to monitor the shock and impact, which the transformer may be subjected to during transportation. The impact recorder is a electronic or mechanical recording accelerometer whose main feature is its ability to record shock and impact from three directions axial, lateral and longitudinal.

For transformers with a gas pressure of 0.3 PSI, the acceptable limits of dew point shall be as under: (Source: Courtesy BHEL, Bhopal)


224 Temperature of insulation in °F

Temperature of insulation in °C

0

Maximum permissible dew point in °F -78

-17.77

Maximum permissible dew point in °C -61.11

5

-74

-15.0

-58.88

10

-70

-12.22

-56.66

15

-66

-9.44

-54.44

20

-62

-6.66

-52.22

25

-58

-3.33

-49.99

30

-53

-1.11

-47.22

35

-48

+1.66

-44.44

40

-44

+4.44

-42.22

45

-40

+7.44

-39.39

50

-35

+9.99

-37.22

55 60

-31 -27

12.77 15.55

-34.99 -32.77

65

-22

18.33

-29.99

70

-18

23.11

-27.77

75

-14

23.88

-25.55

80

-10

26.66

-23.33

85

-6

29.44

-21.11

90

-1

32.22

-18.33

95

+3

34.99

-16.11

100

+7

37.75

-13.88

110

+16

43.33

-8.88

120

+25

48.88

-3.88

130

+33

54.44

+0.55

140

+44

59.99

+5.55

1.3 All external fitting such as Conservator, Buchholtz relay, Dehydrating Breather, Turrets, Bushings, Explosion Vent/ Pressure Relief Devices (PRDs), OLTC – Driving Mechanism and Motor Operated Mechanism (MOM) Boxes, Marshalling Kiosks, Radiators, Rollers, Cooling Fans, Pumps, Gasket etc., which are liable to damage in transit, shall be removed and packed separately. All openings created on the tank by the removal of components are blanked with identifiable blanking plates & suitable gaskets. All openings on the fittings removed are also closed with blanking plates & suitable gaskets. 1.4 When any internal parts like tap changers, etc., are removed for transportation, they are dispatched in tanks filled with oil/inert gas or suitable measures taken so that they do not absorb moisture. 1.5 All fragile parts such as temperature indicators, oil level gauges, etc., shall be carefully packed to avoid breakage in transit.


225

All blanking plates, tank valve guards, etc., used exclusively for transport is to be preserved in safe custody by the purchaser for future use. 1.6

Rail Transport

1.6.1 Transformers may be transported by rail / road trailers depending on size of transformer, destination, delivery time and the route limitations. In case of transformers where the weight and dimensions of the main body exceed limits, special well wagons are employed. Detached parts are packed/crated and normally dispatched along with the main body of the unit, so that all the parts are received at the destination station with the unit. 1.6.2 Loading 1.6.2.1 Most of the transformer manufacturers have their own railway siding and transformers are loaded at their works using their cranes. In cases where separate siding facilities are not available, road tractor-trailers are used for the transport from works to loading railway stations. Mobile cranes and railway cranes are also used for loading the units into the wagons. In the absence of any crane facility, the transformers are unloaded from the trailers near the railway track into a platform of adequate height built up of wooden sleepers. Jacks and pull lifts chain pulley block of adequate capacity are used to slide the transformer over a pair of rails placed on the sleeper platform bridging the trailer width in full. To prevent the trailer from toppling when the transformer is moved to the platform the stage end of the trailer is supported by a sleeper pack. From the platform the transformer is slid onto the wagon taking care to have the rails for the full width of wagon. The rails are greased for; easy movement. The rollers of the transformer are removed before leaving the works. While providing a sleeper stage it should slightly be at a higher level to allow for the increase in height of the trailer while the load is released due to the springs getting relaxed. 1.6.3 Lifting and Jacking 16.3.1 Transformers should be lifted by jacking at the jacking pads provided for the purpose and simultaneous use should be made of all such lugs or lifting bollards in order to avoid any unbalance in lifting. Before lifting the complete transformer it should be ensured that all cover bolts are tightened. Apart from the main lifting points designed to take the total weight of the unit, the transformer has subsidiary lifting points suitable for particular components only. Care must be taken to distinguish between them. It is advisable to use a spreader between slings so that the lift on the hooks is in the vertical direction. The slinging angle is not to exceed 60°. Safe loads of wire ropes and the multiplying factors to be used corresponding to the lifting angles are furnished in Fig. 1. Gunnies are used on the slings to avoid metal contact and consequent damage to the slings.


226 Safe load of wire ropes Dia. of wire rope (mm)

Safe load (Kg)

Multiplying factor for different lifting angles Lifting angles

Multiplying factor

8

600

0

1

12

1,300

20

1.015

16

2,300

40

1.065

20

3,500

60

1.155

24

5,000

28

7,000

32

9,000

36

11,000

40

14,000

44

17,000

56

24,500

64

33,500

70

40,000

Fig. 1 Correct method of slinging

1.6.3.2 Where it is necessary to use jacks for lifting, only the Jacking pads provided for the purpose of jacking should be used. Jacks should never be placed under valves or cooling tubes or stiffeners. Not more than two jacks should be operated at the same time. When two jacks are being operated the opposite side of the transformer should be firmly supported by sleepers. Jacks are also not to be left in position with load for a long time. The transformer should always be handled in the normal upright position. During the handling operation care must be taken to prevent overturning or even tilting.


227

1.6.3.3 Loading on the railway wagons is done as centrally as possible to distribute the weight equally on all axles and wheels. Manufacturer’s drawing in this regard is to be referred to. It is desired that the transformer be loaded longitudinally on the wagon. To prevent damage to the transformer base, the unit is loaded on a row of sleepers or wooden planks placed on the wagon floor. 1.6.4 Lashing The transformer is lashed on all four sides by wire ropes or chain of adequate size and tightened using turn buckles with locking facility. Wooden props are also used. After the movement of the wagon for a short distance the tightness of the lashing is to be checked. When enroute transhipment is involved, lashing is to be checked again. 1.7

Road Transport

1.7.1 Transformers may be shipped by road, where well developed roads exist & the route conditions permit. Multi-axle tractor driven low-platform trailers are used for road transport. The tractors are to have adequate hauling capacity and the trailers loading capacity. 1.7.2 Route Survey 1.7.2.1 The road system is examined in detail on the following points: (i)

Width of the road

Normally not less than 5 m

(ii)

Bridges and culverts

To have sufficient strength to take the moving load; consultation with the Highways Department is necessary (Refer Fig. 2 for assessing the axle load.)

(iii)

Encumbrances enroute

Like telephone, telegraph, traction and electric utility wires, avenue trees, cross beams of bridges, subways, aqueducts, etc., across the roadway.

(iv)

Sharp bends

Steep gradients up or down with respect to the maneuverability of the tractor-trailer.

(v)

Road worthiness

Of the route like sandy stretches, waterlogged areas, crowded localities like market places, schools and other public places.


228 (vi)

Operational

Constraints of the tractor-trailer to be used. To clear up any doubts as to the feasibility of the route a rehearsal drive of the tractor-trailer unit is performed.

All turnings clearance from local authorities, PWD, Motor Highways Department should be obtained before movement. All transport diversions should be thoroughly checked.

Fig. 2

WTa WTi WP Ra Rb

= = = = =

Self weight of tractor Self weight of trailer Pay load 50 tonnes Load in the front axle of tractor Load in all the 3 axles of trailer (Divide by 3 for axle load) Rc = Load in all the 2 axles of this trailer (Divide by 2 for axle load) Take the momentum at the points at Ra, Rb and Rc and solve the equations, to get Ra, Rb and Rc. In this case if we assume tractor and trailer of equal weight (i.e.) WTa = WTi = 10 tonnes and WP = 50 tonnes. Ra = 5 tonnes axle load in front axle Rb = 35 tonnes axle load in 2nd axle Rc = 30 tonnes axle load in 3rd axle Load per rear axle of the tractor = 35/3 = 11.66 tonnes Load per rear axle of semi-trailer = 30/2 = 15 tonnes To limit the load per axle to 10 tonnes at the rear axle of trailer, the payload should not exceed 30 tonnes Otherwise the number of axles for the trailer should be increased.


229

1.7.2.2 Movement of the transformer shall be through the route surveyed for the purpose and no deviation of the route shall be followed unless it is surveyed again. 1.7.3 Strengthening the Route 1.7.3.1 Such of those bridges and culverts which require strengthening are strengthened by adopting suitable measures like propping up using timber and steel in consultation with the Highways Department. For uniform loading, chequer plates, M.S. plates, etc., are used. For details refer Figs 3, 4 and 5. In some cases an alternative route by-passing such bridges and culverts may be cheaper than propping up. 1.7.4 Loading, Lifting, Jacking and Lashing Refer clauses 1.2.2, 1.2.3 and 1.2.4 (Rounded of thicknesses)

Fig. 3 Spreading gravel cushion and placing steel plates above

Fig. 4 Dry stone packed with lose earth in -between

Fig. 5 Wooden sleeper stage platform built on river bed with the plates underneath


230 1.7.5 Movement

1.7.5.1 A pilot vehicle with all tools and tackles, jacks, sleepers, chequer plates, crowbars, etc., and sufficient trained staff should run in front of the vehicle. Red flags and danger lamps should be exhibited at prominent places to warn traffic on the route. 1.7.5.2 The branches of avenue trees that are likely to foul the equipment should be cleared while the load is moved. Electric utility power lines likely to foul should be switched off and lifted temporarily / dismantled while the load is moved. 1.7.5.3 After moving the load for a short distance tightness of the lashing should be checked. 1.7.5.4 In the case of night halt or stoppage of the loaded trailer for a fairly long duration the trailer should be supported either by sleepers or providing supporting jacks on all sides thus releasing the load from the tyres. Danger lights should be displayed in the front and rear of the vehicle. 1.7.5.5 For the normal running, it is desirable not to run the vehicle over 15-20 km per hour with no load and 10-15 km with loads on good surfaced roads. For bad roads it is desirable to run the vehicle at much lower speeds. 1.7.5.6 The brake system on the tractor-trailer has to be carefully operated whenever the vehicle is running with load. While running over any bridge or culvert the vehicle should be run only at a very slow speed. Long before the approach to the bridge the speed should be brought down and the vehicle allowed to proceed over the bridge without creating any impact, which is sometimes caused by applying brakes when running at high speeds. Till all the wheels of the tractor-trailer are clear of the bridge, the speed should not be increased. Transportation should be avoided during heavy rains. 1.7.5.7 In case of the vehicle getting locked up in a slipping soil, the safest procedure would be to detach the tractor after the trailer had been anchored suitably by sleepers. The tractor may be moved forward and anchored suitably with sufficient sleepers. The trailer can be pulled up by winches in the rear of the tractor. 1.8

Water Transport

Water transport is the cheapest mode of transport. While ocean going ships are used for the high seas, barges are used for inland navigation routes. Special care is to be taken for prevention of rusting of parts and ingress of moisture like use of anti-corrosive paints, silica gel packing, sealing using polythene covers etc. Packages should not be left on wharves for more than 2 weeks. Storing of heavy packing is to be done only in consultation with port authorities so that the safety of the wharf is not endangered. 1.8.1 Loading Usually the wharf cranes may not have sufficient capacity for handling very heavy packages. Special floating cranes or cranes of ships are used for loading the packages from the wharf to ships or barges. In the case of barges, special care is required to prevent overturning of the barge at the time of loading. Packages are to be placed inside the hold. Decks of ships are not to be used for keeping the packing during transport.


1.9

231

Unloading at Site

In cases where the substations are having adequate crane facility, the transformer is unloaded by crane. Alternatively, mobile cranes are used. Where no crane facility is available a trench is dug to a depth equal to height of the trailer platform and the transformer is slid to position. If this also is not possible the transformer is unloaded into a sleeper platform and gradually lowered to plinth level (Fig. 6 for guidance). The sleeper platform level is to be at a slightly higher level to allow for the increase in height of the trailer while the load is released due to the springs getting relaxed. Winches are to be used for putting the transformer into position.

Fig. 6

1.10 Inspection and Storage A thorough external examination shall be made immediately on arrival of the transformer at site. If any damage is suspected, open delivery is to be taken and a claim made against the carriers in accordance with the terms of contract. The manufacturers and under-writers are also to be informed about the details of inspection done jointly in carriers. 1.10.1 Unpacking and Inspection 1.10.1.1 Packages are to be opened carefully so that the tools used for opening do not cause damage to the contents. 1.10.1.2 When the transformers are dispatched gas filled, pressure shall be checked and it should be ascertained whether there was any leakage of atmospheric air into the tanks. Dryness of insulation should be checked by dewpoint measurement (clause 1.1.2) after filling

232


with dry air/nitrogen and after a stabilization period of 12-24 hours. If insulation is wet, the transformer has to be dried. If the pressure is positive and varies with temperature of the surrounding air the seal can be taken to be effective. 1.10.1.3 When the transformer is dispatched filled with oil, oil level in main tank at the time of receipt is to be verified by comparing the oil level before dispatch. Any shortage is to be duly recorded in the documentation and intimated promptly to the supplier. A sample of oil shall be taken from the bottom of the tank and tested to IS 1866. If the sample of oil does not meet the requirements, the matter should be reported to the supplier along with insulation resistance values of the various windings to earth. 1.10.1.4 Drums containing transformer oil which have been dispatched separately shall be examined carefully for leaks. All drums are dispatched filled up to their capacity and any shortage should be reported. 1.10.1.5 In case of bushings, oil level shall be checked. The porcelain portion is to be checked for any crack or chipping. The terminals should be checked for any bends. 1.10.1.6 Fragile instruments like oil level gauge, temperature indicator, etc. are to be inspected for breakages or other damages. 1.10.1.7 Any damaged or missing components should be reported to the supplier within the insurance liability period, so that the same can be investigated and shortage made up as per the terms of contract. 1.10.1.8 Any paint damages are to be touched up after proper cleaning with wire brush, emery and applying a coat of primer. Generally, manufacturers supply adequate quantity of finish paint for such purposes. 1.10.9 Storage 1.10.9.1 After arrival at site, it is desirable to erect and commission the transformer with minimum delay. In case this is not possible the transformer should be erected at its permanent location with conservator and breather fitted, the transformer should be vacuum dried and processed oil should be filled in the tank and oil condition to be monitored during long storage. If this is also not possible, then, it is preferable that the tank containing core and coil be placed under shed. The tank shall preferably be filled with processed oil up to core & winding level with dry air/nitrogen filling above the oil. Gas pressure is to be monitored periodically. 1.10.9.2 The transformer shall never be stored in polluted areas likely to cause corrosion. 1.10.9.3 Equipment meant for indoor use, such as control panels should be stored indoor. Fragile components are to be stored carefully.


233

1.10.9.4 Bushings, if not mounted on the transformer, should be stored in the cases under the shed. 1.10.9.5 All packing shall be kept above ground by the use of supports so as to allow free airflow underneath. It is preferable to store all loose parts under cover. 1.10.9.6 Transformer oil when received in drums shall be stored under cover. Drums should not stand on end but are to be placed on their sides with the bung at 45° downward. (3-9 O’ clock position) 1.10.9.7 If the accessories are to be stored for a long period they can be repacked. It is advisable to store the accessories; especially the electrically operated ones’ in a rain protected area and packing list should be retained so that the contents of cases or crates can be checked at the end of the storage period. Heaters for marshalling kiosks, etc., shall be kept energized. 1.10.9.8 While in storage, the gas pressure, oil samples, breather condition are to be checked frequently. 2.0

INSTALLATION

2.1

Erection Schedule

When the erection work is to be carried out by the supplier under the contract agreement, the supplier and the user shall prepare a schedule of the works to be carried out with specific period for each item of work involved. All the assembly and erection drawings should be available at site. 2.2

Precautions

2.2.1 As far as possible no work shall be done during rainy season to avoid moisture absorption. 2.2.2 Extreme care must be taken to prevent any foreign material from being dropped into the transformer. Workmen having access to the interior of a transformer should empty their pockets of all loose materials. Any spanners or other tools used shall be securely tied so that they can be recovered easily if accidentally dropped. 2.2.3 Fibrous cleaning materials shall not be used. The presence of loose fibres in suspension in transformer oil can reduce its insulating properties. 2.2.4 All components dispatched separately shall be cleaned inside and outside before being fitted. 2.2.5 If any internal temporary transportation braces are provided they are to be removed without disturbing any permanent internal arrangements. Such parts should be clearly marked on the transportation drawings.


234

2.2.6 The transformer shall be erected on a level foundation. 2.3

Site Preparation

2.3.1 Since all electrical installation shall comply with the requirements of the Indian Electricity Act and Rules made thereunder, it is essential that they are complied with. The provisions of the Factories Act and Rules are also to be complied with to the extent applicable. 2.3.1.1 All tools, tackles and other equipments required for the erection work may be arranged at the site before the work is started. Generally the following items will be required. (i)

Lifting Equipment : Depending upon the transformer, crane of sufficient capacity will be required. In the absence of crane facility, a derrick is erected and work carried out using a chain and pulley block. Wire rope slings, D-shackles, etc, are also required.

(ii)

Oil Purifier : A vacuum oil purifier of sufficient capacity provided with thermostatically controlled heating and filtering facility is required. The following table may be used as a guide for selecting the oil purifier capacity. Oil purifier capacity litres/ hr

Quantity of oil to be purified litres

2000 4000 5000 and above

Up to 20,000 More than 20,000 but Less than 50,000 More than 50,000

A higher vacuum filter with a vacuum of 0.1 torr or lower is to be used for a voltage class of 245 kV and above. (iii)

Vacuum Pump: A vacuum pump with a vacuum hose and other fittings capable of producing a vacuum up to 759 mm mercury.

(iv)

Oil Storage Tank: One or more oil tanks of sufficient capacity to store the entire quantity of the transformer oil will be useful for filling from drums and also when oil is received at site in tankers for transformers dispatched gas-filled. This will reduce the time of filling.

(v)

Pressure Vacuum Gauge: To read – 0.5 to + 0.5 kg /cm2 for checking inert gas pressure.

(vi)

Oil Testing Apparatus: Conforming to IS: 335

(vii)

5000/ 2500/ 1000 V motorized megger


235

(viii) Voltmeters, 0 to 500 V, 0 to 100 V range and 0 to 5 V range, milli-ammeter, low power factor watt meter. (ix)

Set of spanners suitable for metric sizes and B.S. sizes.

(x)

Set of drum opener, crowbar, pipes, hammer etc.

(xi)

Set of screwdrivers, cutting pliers, screw spanners and pipe wrench.

(xii)

Clean cotton cloth and cotton waste.

(xiii) Electric hand lamp. (xiv) 12 mm vinyl hose of approximate 10 m length for being used as an oil level indicator during erection. (xv)

Brushes & spray gun with compressor for painting.

(xvi) PVC wires for connecting meters during testing. (xvii) Set of tarpaulins of suitable size. (xviii) Earth discharge rods. The above is only a general list of tools, and measuring instruments etc., used for erection of transformers. The capacity, sizes, etc., of each tool would depend on the type and size of transformer. 2.3.2 The installation site shall have easy accessibility for inspection and routine maintenance etc. Foundation for the transformer should have level floor strong enough to support the weight, vibrations and prevent accumulation of water. The transformer foundation should be provided with adequate oil soak pits and drains. Typical designs of such pits are shown in Fig. 7.


236

10~15 cm volume

Fig. 7

2.3.3 In case of large and costly transformers, fire protection walls may be necessary on either side of the transformer for isolating each transformer from the rest. 2.3.4 The transformer installation position shall be such that the breather, thermometers, oil level indicators, diagram plates, etc., can be safely examined with the transformer energized. It should also be possible to have access to the operating mechanism of on load / off circuit tap changing equipment, marshalling box, etc, Sampling valve, drain valve, etc., shall be at accessible and convenient locations, if need be through construction of raised platforms. 2.3.5 For outdoor transformers where rollers are not fitted, level concrete plinth with bearing plates of sufficient size and strength can be adopted. The plinth shall be above the maximum flood or storm water level of the site and of the correct size to accommodate the transformer in such a way that no unauthorized person may step on the plinth. 2.3.6 Where rollers are fitted, suitable rail tracks shall be provided and when the transformer is in the final position, the wheels shall be locked to prevent accidental movement of the transformer. 2.4

Unit Erection

2.4.1 Positioning of the Unit 2.4.1.1 The transformer tank containing the core and coil-assembly should be first placed in the position selected for assembly. 2.4.1.2 In case the rollers are to be fitted, it should be ensured that the wheel shafts are well greased and the wheels rotate freely. While fixing the rollers, the flanges should come on the inner side of the rails. The transformer shall then be jacked up and the roller fitted to the bottom frame. In some of the transformers, skid is provided in place of rollers.


237

2.4.1.3 All parts, gaskets, bushings, radiators and coolers etc., should be readily available in good condition. The transformer oil required for filling must also be readily available. 2.4.1.4 In case special foundation bolts are supplied, these are to be used for arresting the movement of the transformer, including anti-earthquake devices. 2.5

Oil

When oil is dispatched to site separately, it may be done in sealed steel / HDPE drums or epicoated road tankers. At the time of filling the drums, it is ensured that the oil is filtered, clean and dry. 2.5.1 Oil Specification 2.5.1.1 The oil as received at site for filling and topping up in the transformer must comply with IS: 335 (latest revision) for acceptance criteria. Specification for uninhibited mineral insulating oil-new/unused before filling in transformer Sl. No. 1.

Characteristics / property Appearance

(a) (i) (ii) (b) (c) (d) 14.

Density at 29.5 ° C, Max. Kinematic viscosity at 27 ° C, Max at 40 ° C, Max Interfacial Tension (IFT) at 27° C, min. Flash point, pensky martin (closed), min. Pour point, max. Neutralization value Total acidity, max. Inorganic acidity/alkalinity Corrosive sulphur Electric strength (breakdown voltage) min New unfiltered oil After filtration Dielectric dissipation factor ( Tan δ) DDF at 90 °C, Max. Specific resistance (resistivity) At 90 ° C , min. At 27 ° C , min. Oxidation stability Neutralization value after oxidation, max. Total sludge after oxidation, max. Ageing characteristics after accelerated ageing (Open breaker method with copper catalyst) Specific resistance (resistivity) At 27 ° C , min. At 90 ° C , min. DDF at 90 ° C , max Total acidity, max. Total sludge value, max. Presence of oxidation inhibitor

15.

Water content-new unfiltered oil

2. 3. 4. 5. 6. 7. a. b. 8. 9. a. b. 10. 11. (a) (b) 12. (a) (b) 13.

IS 335 Clear and transparent free from suspended matter or sediments 0.89 g / cm3 27 cst under consideration 0.04 N/m 140 ° C -6 °C 0.03 mg kOH / g NIL Non - Corrosive 30 kV, rms 60 kV, rms 0.002 35*10 12 Ω-cm 1500*10 12 Ω-cm 0.40 mg kOH / gm 0.10 % by weight

2.5*10 12 Ω-cm 0.2*10 12 Ω-cm 0.2 0.05 mg kOH/g 0.05 % by weight The oil shall not contain any anti-oxidants additives 50 ppm


238

2.5.1.2 At manufacturer’s works oil sample shall be drawn before and after heat run test and shall be tested for the following: • Dissolved Gas Analysis: Samples for DGA, wherever specified, shall be taken from sampling device, which is fitted to transformer tank during testing, prior to commencement of temperature rise test and immediately after this test. The acceptance criteria / norms for various gas generation rates during the temperature rise shall be as per IS: 10593 (based on IEC 60599). • Frequency Response Analysis may be carried out after routine test before dispatch to obtain initial signature, which shall be passed on to customers for future reference. 2.5.1.3 The oil sample from the transformer tank, after filling (new/unused) in tank before commissioning should meet the following specifications as per IS: 1866. Sl. No. 1.

Property

2.

Density at 29.5ºC gm/cm3 – max Viscosity at 27ºC-max – cSt Flash point ºC – min Pour point ºC – max Neutralization value mg kOH/gm of oil – max Water content –(moisture in ppm) max Interfacial Tension (Mn/M) –min Dielectric dissipation factor (Tan ä) at 90 ºC and 4060Hz -max Resisitivity at 90 ºC-min (x 10 12 ohm-cm) Breakdown voltage (BDV) (kV rms)-min Oxidation stability of uninhibited oil (i) Neutralization value mg kOH/gm of oil –max (ii) Sludge percent by mass – max Oxidation stability for inhibited oil

3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

Appearance

Highest voltage of equipment <72.5 kV 72.5 – 170 kV >170 kV Clear, free from sediment and suspended matter 0.89

0.89

0.89

27

27

27

140 -6 0.03

140 -6 0.03

140 -6 0.03

20

15

10

35

35

35

0.015

0.015

0.01

6

6

6

40

50

60

0.4 0.1

0.4 0.1

0.4 0.1

Similar values as before filling

Notes : 1. These properties are very sensitive to storage and processing, i.e., the temperature and vacuum of filtration, the cleanliness of the processing system including filter machine, pipes, valves, cleanliness of transformer and its cooling system, etc., Extreme care should hence be taken in these areas to achieve the values indicated above. 2. When unused oil has been filled in the tank then tests given against Sl. nos. 7, 9, 10 & 11 in the above table are important. Other parameters only form base data for future comparison. 3. Used oil shall meet the BDV and moisture content requirement given in the Table whereas other parameters may lie in between the above values and the limiting values given under cl. 2.5.1.4.


239

2.5.1.4 The recommended limits for mineral insulating oil filled in power transformers in service (as per IS: 1866) are as follows: Sl. No. 1. 2. 3.

Characteristics / property Appearance Interfacial tension (IFT) min. Flash point

4.

Neutralization value (total acidity) max.

5.

Di-electric strength (breakdown voltage) BDV min

6.

Dielectric dissipation factor (Tan δ) DDF at 90 ° C and 40-60Hz, max.

7.

Specific resistance (resistivity) (i) At 90 ° C, min. (ii) At 20 ° C, min. Water content, max

IS 1866-2000 Clear and transparent and without visible sediments 0.015 N / m, min Max. decrease of 15° C from initial value. However the absolute value should not be less than 125 ° C 0.3 mg kOH / g Increase frequency of testing if more than 0.2 mg kOH / g Above 170 kV – 50 kV 72.5 KV- 170 kV -40 kV Below 72.5 kV – 30 kV Above 170 kV - 0.2 Max. Below 170 kV - 1.0 max.

8.

9.

Sediment and sludge

0.1*10 12 Ω-cm 1.0 * 10 12 Ω-cm Above 170 kV- 20 ppm max 72.5 kV - 170 kV - 40 ppm max. Below 72.5 kV – No free moisture at room temperature No sediment or precipitable sludge (below 0.02 % by mass)

Note : Recondition/replace the oil if one or more of the above parameters are beyond the specified limits.

2.5.2 Precautions 2.5.2.1 Oil is easily contaminated. When sampling the oil and filling the tank, it is very important to keep the oil free from contamination. 2.5.2.2 All equipment used in handling the oil must be clean and should be washed with clean transformer oil before use. (The oil used for washing must be discarded). Particular attention shall be paid to the cleanliness of bungs, valves and other points where the dirt or moisture tends to collect. 2.5.2.3 Sampling and type of container for DGA, wherever specified, shall be as per IS: 9434 or IEC: 600567.

240


2.5.2.4 Flexible steel hose is recommended for handling insulation oil. Some kinds of synthetic rubber hose are also suitable but only those known to be satisfactory should be used. Ordinary rubber hose should not be used for this purpose as oil dissolves the sulphur from the rubber and thereby gets contaminated. Hose used for handling oil should be clean and free from loose rust or scale. 2.5.2.5 Transformers must always be disconnected from the electricity supply system before the oil level in the tank is lowered. 2.5.2.6 Oil must not be emptied near naked lights heater/fire, as vapour released is inflammable. 2.5.2.7 Minute quantities of moisture (particularly in the presence of fibres or dust) lower the dielectric strength of the oil. Therefore, to reduce the risk of condensation of the moisture entering the oil, containers taken into a warm room shall not be opened until the entire body has attained the same temperature as the room temperature. It is preferable not to mix oils from different suppliers. However, if the oil-required to be mixed meet the requirements of IS: 335 and if these are made from the same feed stock, these can be mixed. 2.5.3 Oil Sampling Oil takes up moisture readily and its condition should always be checked before use. Oil of a muddy colour is certain to be wet. Water and water-saturated oil are both heavier than dry oil and sink to the bottom of any container. Samples shall, therefore, be taken from the bottom. Samples should not be taken unless the oil has been allowed to settle for 24 hours, if from a drum or two / three days if from a large transformer. 2.5.4 Samples from Tank Dirt from the draw-off valve or plug should be removed. To ensure that the valve is clean, some quantity of oil should be allowed to flow into a separate container before collecting samples for testing. Samples shall be collected either in glass bottle (refer IEC 60567) or in stainless steel bottle. Detailed sampling procedure for steel bottle is given in Annexure-III. It is recommended that: Oil sample for any of the major tests (BDV, PPM, Resistivity, Tan Delta, DGA) must be taken from both top and bottom sampling valves and while drawing the sample the corresponding top oil temperature must be furnished. The lesser of the values obtained from the 2 samples shall be considered for decisions regarding BDV and resistivity, while the higher values shall be reckoned for PPM, Tan Delta. 2.5.5 Sample from Oil Drum The drum should first be allowed to stand with the bung vertically upwards for at least 24 hours. The area around the bung should be cleaned. A clean glass or brass tube long enough to reach to within 10 mm of the lowermost part of the drum should be inserted, keeping the uppermost end of the tube sealed with the thumb whilst doing so. Remove the thumb, thereby allowing oil to enter the bottom of the tube. Reseal the tube and withdraw an oil


241

sample. The first two samples should be discarded. Thereafter, the samples should be released into a suitable receptacle. 2.6

Oil Filling

2.6.1 Before filling oil in the tank, it should be tested to meet the requirements as per IS: 335 (with its latest amendments). In case the oil does not meet the requirement, it should be processed and shall only be used when it meets the requirements. 2.6.2 For transformers dispatched gas filled, the filling of oil inside the tank is done under vacuum. Transformers of high voltage rating (132 kV and above) have their tanks designed to withstand full vacuum. Below 132 kV manufacturer’s instructions should be followed regarding the creation of full or partial vacuum during filling the oil in the tank. 2.6.3 When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank and that a filter press or other reliable oil drying and cleaning device should be interposed between the pump and the tank. 2.6.4 Some radiators may be suitable only for partial vacuum. No higher vacuum than as could be withstood by radiators should be applied to such radiators type tanks even if the radiator valves are closed. It should also be ensured that the bushings, tap changer board, relief vent diaphragm, Buchholtz relay, conservator, etc. are not subjected to full vacuum as these may not designed for the same. (Specific guidelines from manufacturer may be followed) Note : Now a days pressed steel radiators, bushings – condenser as well as porcelain, buchholz relay, conservator etc. are designed for full vacuum and are readily available.

2.6.5 In case the transformer is provided with an on load tap changer of in-tank type, while evacuating the main transformer tank, the diverter switch compartment must also be evacuated simultaneously so that no undue pressure is allowed on the tap changer chamber. While releasing vacuum, the tap chamber vacuum should also be released simultaneously. For this one pressure equalizer pipe should be connected between main tank and tap changer. 2.7

Drying of Transformers

Before the drying out is started all fittings except coolers and associated accessories shall be fitted. The coolers, etc., can be conveniently fitted after the successful dry-out of windings and insulation. The process of drying out a transformer is one requiring care and good judgement. If the drying out process is carelessly or improperly performed, great damage may result to the transformer insulation through overheating etc. In no case shall a transformer be left unattended during any part of the dry-out period. Transformer should be carefully watched throughout the dry out process and also observations shall be carefully recorded.


242

When the transformer is dried out it is necessary to ensure that the fire fighting equipment is available near the transformer. The dry out of transformer is necessary in the following cases: (a)

On first commissioning

(b)

After prolonged storage at site without nitrogen

(c)

After detection of free moisture/ high moisture content in oil

(d)

Due to exposure of core and coil assembly for 48 hours or more in case of inspection at site

Various methods can be adopted for drying out a transformer depending upon the facilities available at site. Some of them are described below: 2.7.1 Drying out a Transformer using Filter Machine 2.7.1.1 The most practical method of drying out is by circulation of hot oil through a high vacuum filter machine incorporating oil heater and vacuum chamber (or other oil cleaning and moisture removing device). The vacuum pump of the filter machine should have the capacity of creating vacuum as high as possible but not less than 710 mm of mercury. Where possible, a vacuum pump can be connected to the tank top cover to keep the oil in tank under vacuum consistent with tank suitability. This may speed up the drying out process. It is preferable to lag or blanket the transformer tank to prevent loss of heat. Oil is drawn from the bottom and let into the transformer at the top. This will remove any settled moisture/ impurities. After about 8-12 hours circulation in this manner, the cycle is reversed and oil is drawn from the top and fed at the bottom. 2.7.1.2 The oil temperature as measured by the oil temperature indicator should be of the order of 55°C. It should be seen that the oil temperature at the filter machine in no case exceeds 65°C. The circulation is continued till the insulation resistance and oil samples tests are satisfactory. 2.7.1.3 Plot IR values taken at regular intervals against temperature readings. It will be observed that in the beginning IR values drop down as the temperature goes up. The IR values will be low till moisture is coming out of the insulation and start rising before steadying. A typical dry out curve is shown in Fig. 8.


243

Fig. 8 Typical dry-out curve

2.7.1.4 The heat can also be provided by short circuit heating method as mentioned as clause 2.7.3. 2.7.2 Hot Air Circulation 2.7.2.1 Hot air from external heaters is blown into the transformer after draining the oil, through an opening in the bottom of the tank and is allowed to escape through an opening in the tank top cover (Fig. 9). The speed of the air must not exceed 600 m per minute. The temperature of the air is raised gradually to 55°C in the first 8 hours, 65°C in the second 8 hours, and 75°C-80°C in the third 8 hours. In this method also, hourly readings of temperature and insulation resistances are taken till steady values are obtained.

Fig. 9


244 2.7.3 Short Circuit Heating

2.7.3.1 The transformer can also be heated up by short-circuiting the low voltage winding and supplying at the high voltage terminals a reduced voltage. The supply voltage should be maintained in such a way that the current in the windings does not exceed 70 per cent of normal full load current and the oil temperature about 75°C. In this case, temperature of oil should be measured at the bottom of the tank also. Constant watch is to be kept to ensure that the temperature limits are not exceeded. The temperature of the windings which can be measured by the following formula should in no case exceed 90°C : T2 = R2 / R1 (235+T1 )-235 where, T2 = Final average temperature of copper T1 = Initial average temperature of the copper R2 = Final resistance of the windings. R1 = Initial resistance of the windings. This method should be used in conjunction with streamline filter as described in clause 2.7.1. 2.7.4 Drying by Vacuum Pulling, N2 Filling and Heating 2.7.4.1 This is the most effective method and the quickest too but cannot be applied in case of transformers not designed for withstanding the vacuum pressure. The vacuum should be drawn from the top of the tank connecting suitable pump to any valve fitted at the top of the tank. Vacuum is applied after draining the oil. The transformer core and coil are heated externally by the use of heater. Temperature at the tank wall to be of the order of 60-70 °C. Another method of heating active part is to do hot oil circulation for 2-3 volumes and then drain oil immediately. 2.7.4.2 The leakage rate has to be less than 40mbar-lit/sec- so as to get better vacuum during the drying out process. After ascertaining that there is no leakage in the transformer from gaskets/ valves etc., pull out vacuum and keep the transformer under near absolute vacuum (1 torr or less) for about 96 hours running the vacuum pump continuously. The duration of vacuum can vary between 48 to 96 hrs. depending upon the dew point being achieved. Keep Vacuum machine ON and collect condensate for measurement. 2.7.4.3 Then the vacuum is broken with dry nitrogen. The dew point of the inlet of nitrogen is to be measured and will be of the order of - 50 °C or below. When the nitrogen comes to the positive pressure of 0.15 kg/cm2 , it is stopped and kept for 48 hours. Then the nitrogen pressure is released and the outlet nitrogen dew point is measured. If the dew point is about 30 °C or below then the dryness of transformer is achieved. If not again the transformer is taken for vacuum treatment and then nitrogen is admitted as mentioned above and tested. The cycle to be continued till dew point of -30°C or below is achieved. 2.7.4.4 Duration of vacuum cycle may vary between 48-96 hrs. Initially one or two nitrogen cycles may be kept for 24 hrs. After that it may be kept for 48 hrs depending upon dew point being achieved. 2.7.4.5 When condensate collection rate is less than 40 ml/hr for 24 hrs and Dew point of Nitrogen is about –30 °C at outlet.


245

2.7.4.6 After completion of the drying of the transformer the following parameters are to be checked. (a)

PI (Ratio of R600 and R60 ) may be 1.5 or more

(b)

BDV and moisture content of the oil is as per clause 2.5.6.1 (IS 1866).

(c)

Power factor of winding – Less than 0.75%.

2.8

Circulation of Oil in Coolers and Tap Changers

2.8.1 Coolers and tap changers are filled with clean dry oil. Oil samples are taken out from them and tested. Further circulation of oil is carried out till the oil results are satisfactory and meet the requirements as per IS 1866. It is advisable to carry out the circulation in the main tank and selector switch/ diverter switch tank simultaneously to remove moisture from the tap changer terminal board/diverter switch cylinder provided on the tank. 2.9 Important Fittings and Accessories 2.9.1 Gaskets 2.9.1.1 Whenever blanking plates are removed to fix detached parts such as bushing turrets, etc., a new gasket shall be used while fixing the same. A set of new unused gaskets of correct size and thickness is supplied with every transformer for this purpose. 2.9.1.2 Gaskets shall be stored in hermetically sealed containers in a cool place. They must be protected from damp, oil and grease. 2.9.1.3 To make a gasketed joint, first clean the metal surfaces ensuring that they are free from oil, rust scale, etc. Then a film of the special gasket adhesive if supplied by the manufacturer may be applied to one of the surfaces. The gasket may be then stuck to the surface after the lapse of a few minutes. The other metal surface may also be given a film of adhesive and placed over the gasket. Both may then be tightened according to the special instructions of the manufacturers. Some type of gasketed joints is shown in Fig. 10. (a)

Metal Fit Type: In this case the flanges are tightened uniformly till the two metals touch each other.

(b)

Ordinary Type: In this case the gasket should be uniformly compressed such that its thickness comes down to above 60 per cent of its original thickness.

(c)

Distance Piece Type: In this system the flanges are tightened uniformly till the upper flange touches the metal piece welded to the lower flange.

Joints in gaskets should be scarfed or dove tailed as shown in Fig. 11. 2.9.2 Bushings 2.9.2.1 Normally three types of bushings are used: (i)

Plain porcelain type

(ii)

Plain oil filled

(iii)

Condenser type

•

Plain porcelain type bushings are used up to rated voltage 36 kV.

246 •


Plain oil-filled type bushings can either have its oil in communication with the main tank or separately sealed. These bushings will be bulkier in appearance and used up to 110 kV.

Fig. 10

Fig. 11

2.9.2.2 Condenser type bushings are also of three categories, one is the SRBP type, resin impregnated paper type (RIP) and the other is the oil impregnated paper condenser (OIP) type. The last one is distinguishable by the presence of porcelain shell below the flange level. 2.9.2.3 The bushings shall be checked for any damage at the oil end as well as the porcelain before fixing and shall be cleaned thoroughly. The bushings shall be lifted by using the lifting eyes and soft manila ropes. Steel Wire ropes or slings shall not be used (Fig. 12). The line lead of H.V. winding if coiled inside the transformer is drawn through the


247

bushing using a string when the bushing is lowered into position. The cable ferrule is fixed in position at the top of the bushing brass tube. The lower end of the bushing shall be inspected through the inspection cover for proper sealing. The line connection should be tight and should not strain the terminals. Sufficient flexibility in the connection leads should also be provided to avoid mechanical stress on the bushing. The shield barriers, if any shall be inspected through the inspection cover for proper seating. The line connection should be tight and should not strain the terminals. The arcing horns, if any, shall be in proper position as shown by the supplier in general arrangement drawing.

Fig. 12

248


2.9.3 Tap Changers 2.9.3.1 Change over switch or link arrangement provided in a multi-ratio transformer has to be checked for proper ratio. 2.9.3.2 Off-Circuit Tap Changer: The off-circuit tap changer forms an integral part of the transformer. Since the operation is to be carried out from outside, the operating handle may at times be dispatched separately. This has to be fitted as per manufacturer’s instructions. Care has to be taken to have correct alignment of the handle. The actual position of tap changer is confirmed when the ratio tests are done. Before changing taps, isolate the transformer from supply on all windings. In no case should the tap switch handle be left half way and unlocked to prevent damage due to inadvertent operation. 2.9.3.3 On-Load Tap Changers: If the tap changer is dispatched separately from works, it is to be fitted on the tank. Before mounting on the tank, the insulation resistance value of each tap changer lead to earth should be measured and in case of low value, the cause should be investigated. The leads from the tap changer are then connected to their respective position on the terminal board provided on the tank. The tightness of all connections on the selector switch and terminal board is to be ensured. The tap changer is then to be filled with clean oil and drying out is to be carried out. Oil filling and drying out is carried out simultaneously along with the transformer as explained earlier. 2.9.4 Cooling Equipments 2.9.4.1 The cooling equipments and associated pipe work and fittings are to be thoroughly cleaned and flushed with clean dry transformer oil before assembly. The pressure gauge, differential pressure gauges, etc., if any are fitted in position. The cooler and associated pipe work is then filled with clean dry oil keeping all the cooler circuit open, except the transformer inlet and outlet valves. Air is released from all the pipe work during filling. The oil is circulated through a filter press using the filter valves provided in the cooler inlet and outlet branches. The cooler control circuit is to be checked for correct operation in all positions of the selector switch. Test push buttons are provided for checking of the working of motors individually. The cooler system is then connected to the main tank by opening the tank inlet and outlet valves. 2.9.4.2 Cooling Fans: Cooling fans are mounted as per manufacturer’s instructions. The fans are tested for insulation value and normal running before they are mounted.


249

2.9.4.3 S EPARATE COOLERS (i)

Forced Oil Cooled Transformers

In the case of forced oil cooled transformer, oil pumps are provided for circulating the oil. The pumps are dispatched separately after blanking both suction and delivery sides. The pump is connected at the proper position as per the general arrangement drawing. New gaskets should be used at the joints and the bolts tightened. The pipe work at the pump connection is done as per the matching marks on the flanges to avoid undue stress on the flanges of the pump when the bolts are tightened. In some pumps an air release plug is provided on the body. This plug should be checked for tightness. Oil flow meters are provided on the pipeline connecting the pump. The flow meter being a delicate instrument is packed separately and sent. The flow meter should be taken out carefully and mounted on the flange provided on the pipe connection. The mounting position should be as per the outline general arrangement drawing. In large transformers the radiators are sometimes separately mounted. In such cases there will be a header each at top and bottom, which are supported on frames. Flanges are provided on these headers for fixing the radiators. Radiator valves are fitted to the headers and dispatched. The end frames are to be erected first. The frames should be positioned correctly with respect to the transformer. The distances between center lines of transformer and cooler should be strictly as per the general arrangement drawings as otherwise the connecting pipe work will not match. After erecting the end frames the top and bottom headers are mounted. The headers will have to be properly leveled so that the connecting pipe work can be easily fixed. Then radiators are fixed. If the conservator is to be provided on the cooler the same may be mounted on it and all fittings for the same attached. The interconnecting pipe work may be done taking care to connect correct pieces at the correct location. Usually expansion joints are provided in the pipeline connecting the transformer tank to the cooler. Special care should be taken to see that these are installed correctly. (ii)

Forced Water Cooled Transformers

In the case of forced water-cooled transformers the oil to water shell tube heat exchangers are dispatched separately and properly blanked. On receipt at site, it shall be checked whether blanking is all right. If the blanking is found to be defective, the matter has to be referred to the manufacturer. In such a case, moisture/rain water might have entered the heat exchanger oil circuit and there might be rusting. It may be necessary to take out the different parts of heat exchangers separately and clean them thoroughly and put them back.

250


The brackets for mounting the heat exchanger may be attached to the transformer first taking care of the matching marks. The heat exchanger may then be mounted on the support in the correct position after referring to the general arrangement drawing. The oil pump, oil flow meter and the connecting pipes may be fixed after this, in the correct position. In the water circuit the necessary water pipes may be connected. It is also to be made sure that on the outlet side water is allowed to discharge freely without any obstructions. Usually a water flow meter is placed on the outlet pipe to indicate that there is a positive water flow. It is to be made sure that there is no restriction in the water outlet pipe as any obstruction in this pipe will increase the pressure in the water circuit and may result in the water pressure exceeding the oil pressure and creating leakage of water into oil circuit, which is detrimental to the transformer. The heat exchanger oil circuit is sealed from the water circuit with special seals and the circuit is pressure tested at the supplier’s works to make it absolutely sure that there is no leakage. The sealing should not be tempered in any manner, as it is detrimental to the transformer. If there is any doubt about this sealing, the matter should be intimated to the manufacturer. 2.9.5 Conservator 2.9.5.1 Conservator, where fitted, should be assembled with its pipe work, etc., making sure that all gasketed joints are oil-tight and the pipe work is clean and free from moisture. The mechanism of the float type oil gauge inside the conservator might be locked to prevent damage during transit. After placing the conservator in position, it should be released by turning the locking belt in the direction indicated on the plate. 2.9.5.2 While topping up oil in the transformer, it should be ensured that oil is filled to a level indicated by the oil gauge on the conservator in commensurate with the filling oil temperature.


251

2.9.5.3 Procedure for mounting air cell and oil filling inside the conservator. Oil conservator with air cell

Fig. 13 General arrangement for oil conservator

•

Set up the air cell inside the conservator. Care should be taken to see that the hooks on air cell are properly engaged in the brackets provided in side the conservator.

•

Inflate the air cell at a pressure as shown in the instruction plate (DO NOT APPLY EXCESS PRESSURE AS IT MAY DAMAGE THE AIR CELL) through the breather connection pipe. Follow the instructions given in the Instruction Plate fixed on the transformer.

•

Ensure that there is no leakage.

•

The conservator with Air Cell is pressure tested and dispatched from the factory at a slightly positive pressure. Confirm that there is no oil leakage.

•

Fix three numbers air release valves on the conservator.

•

Keep air release valves open. Fix air filling adapter on breather pipe and inflate the air cell at an air pressure indicated on the INSTRUCTION PLATE affixed on the transformer and hold air pressure.

•

Open the air release valves and start oil filling from the bottom filter valve of the transformer.


252 •

Observe the air release valves and as soon as oil starts overflowing, close the air release valves one by one. Stop oil filling when all air release valves are closed.

•

Remove the air-filling adapter.

•

Continue oil filling and observe the Magnetic Oil Level Gauge (MOG)

•

Stop the filling when the needle of MOG shows the level corresponding to the ambient temperature at the time of filling.

•

Fix silica gel breather.

Caution: •

Do not open any of the air release valves after completion of oil filling. If air release valve is opened, air will enter and oil level will drop.

•

The plain oil level gauge on the end cover of the conservator should indicate full oil level always. If air enters the conservator, it can be seen by a fall in the oil level in plain oil level gauge.

•

The plain oil level gauge should be monitored on regular basis.

2.9.6 Buchholtz Relay 2.9.6.1 The Buchholtz is checked for correct functioning of the mercury switches by injecting air through the test petcock when full of oil. When mounting on the pipe work, the correct direction is maintained with the help of arrow provided. The angle of inclination is also to be checked and should be between 3 to 7°. The gas release pipe, if provided is to be connected to the top petcock. In service the top petcock should be open and gas release pipe should be full of oil. When the gas is to be collected through the gas release pipe, initially the oil will flow out and then the gas can be collected. 2.9.7 Dehydrating Breather 2.9.7.1 The breather pipe work shall be properly cleaned. The oil level in the oil seal at the bottom should be filled to the correct level with transformer oil. Any oil that might have over flown should be wiped off. It is to be ensured that the breathing hole at the bottom of the seal is not blocked by dirt, etc. and silica gel to be filled into the breather is dry. 2.9.8

Pressure Relief Device (PRD)/ Explosion Vent

2.9.8.1 PRD: Mount PRD as per manufacturer’s leaflet and also the G.A. drawing of transformer. Check operation of alarm/ trip contacts. 2.9.8.2 Explosion Vent: The temporary cover, which is provided over the explosion vent flange on the tank cover, should be removed and the explosion vent fitted with suitable gaskets. Care being taken to ensure that the top diaphragm with its gaskets makes an airtight joint. As the top diaphragm is sent blanked from works, the blanking plate shall not be


253

removed till the oil level inside the transformer comes above the tank cover. The fixing can be done after vacuum application. 2.9.9 Temperature Indicators 2.9.9.1 Before installing, the accuracy of the instrument shall be checked by hot oil or water bath. The switches are adjusted to make contact at the desired temperature depending upon the site conditions, i.e., ambient temperature, loading conditions, etc. The check is done through hot oil bath. 2.9.9.2 The capillary tube is protected adequately to withstand all normal handling. It should not, however, be bent sharply or repeatedly and should be supported by clips to prevent sagging. On no account it must be cut. 2.9.9.3 The thermometer pocket should be filled with transformer oil 2.9.9.4 The connection of the winding temperature indicator C.T. is made to the thermometer pocket as per instructions given on the WTI Terminal Board. 2.9.10 Bushing Current Transformers 2.9.10.1 It is not advisable to remove the bushing CTs unless situation warrants for the same. In such cases great care shall be taken in handling current transformers. Current transformer should be kept flat at all time. If it is not handled properly, it will deform in shape resulting in an increase in excitation current. All C.T. Secondary terminals should be short circuited or loaded before energizing the transformer. This will prevent excessive voltage developing across. C.T. secondary, which can damage the C.T and be a hazard if touched. 2.11 Completion of Erection Work Final topping up is now done up to a level in conservator commensurate with filled oil temperature. Any other work such as wiring of various alarm/trip contacts, fan motors, pump motors and other apparatus, earthing of neutral and tank is als o to be completed. The interposing valves between the radiators and the tank are opened. Tank surface is retouched with paint wherever required and transformer is made ready for the commissioning tests. 3.0

TESTING AND COMMISSIONING

If the foregoing instructions have been carefully followed, the transformer can now be safely put into service after pre-commissioning tests. 3.1

Tests

The following pre-commissioning tests shall be carried out.


254

3.1.1 Checking of Ratio, Polarity and Phase Relationship 3.1.1.1 The ratio shall be checked on all taps and between all the windings and the results should tally with the manufacturer’s factory test results. Preferably Transformer Turns Ratio Meter (TTR) should be used. In case of OLTC, continuity during tap change is to be ensured. This is best done by applying single phase voltage on each HV winding in turn and observing variations of LV voltage at the instance of diverter operation by using analogue voltmeter. Break will be indicated by any major deflection of the analogue meter towards zero. 3.1.1.2 Polarity and interphase connections shall also be checked 3.1.2 Resistance Measurement of Windings 3.1.2.1 Kelvin Bridge meter or automatic winding resistance measurement kit (ohm meter, preferably 25 A kit) should be used for the measurement of resistance. Tapped winding resistance shall be measured at all tap positions. Pre-commissioning values are to be compared with factory values after applying temperature correction factors. 3.1.3 Insulation Resistance 3.1.3.1 IR values between windings and between windings to earth are checked; while checking these values no external lines, lightning arrestors, etc., should be in circuit. Bushings are thoroughly cleaned before taking IR values. A 5000 / 2500 /1000 volts megger preferably motor operated should be used for measuring IR values. One minute and 10 minute IR values can be taken to find out the polarization index also. (PI = R600 /R60 ) Care should be taken that the lead wires of the megger do not have joints. Note : Now-a-days digital meters are also available in the market which can read Polarisation Index (PI) directly and compensate for magnetic interference.

3.1.4 Magnetizing Current 3.1.4.1 Magnetising current may be measured using single-phase 230 volts supply for each phase individually and compare the results with manufacturer’s factory test results. If the test results at the factory are available with 3 phase supply then magnetizing current at site may also be measured using 3 phase 415 volts supply. Note : Measurement of magnetizing current is a standard feature available in most of the available Automatic C & Tan Delta Test kits.

3.1.5 C and Tan Delta / Power Factor Measurement of Transformer Windings and Bushings (a)

For Transformer windings, measurements shall be done after opening the jumpers and isolating the transformer from other equipment and the ground.


255

(b)

The test kit should be suitable to work in charged switchyard environment i.e. induction suppression unit should be provided.

(c)

Test modes shall be selected as below : Bushings UST mode between HV and test tap. Windings (i)

Between two windings – UST mode

(ii)

Between winding to Earth-GSTg mode with other winding (s) guarded.

Notes •

While carrying out the test, all 3 phases of the same winding are to be shorted to compensate/nullify the effect of winding reactance.

•

The bushing porcelain and test tap are to be properly cleaned before the commencement of test.

•

Pre-commissioning values are to be compared with factory values after applying temperature correction factors.

•

Tan Delta/ Power Factor values should be more frequently monitored if faster deterioration trend is observed.

Apart from above, the following measurements could also be considered during precommissioning depending upon the availability of suitable instrument for EHV class transformers: Frequency Response Analysis (FRA) - measures mechanical movement of windings and core during transit or in operation. Recovery Voltage Measurement (RVM) - directly measures moisture content in solid insulation. Partial Discharge (PD) measurement – detects and locates partial discharges with-in transformers. 3.1.6 Tap Changer 3.1.6.1 The sequence of operation of the tap changers shall be checked. Check should be made for : (a)

Manual Operation.

(b)

Local Electrical Operation.

(c)

Remote Electrical Operation.

(d)

Group Operation, if applicable.


256 3.1.7 Buchholtz Relay Test

3.1.7.1 Check whether the Buchholtz relay is mounted at an angle by placing a spirit level on the top of the relay. Confirm that the relay does not operate when pumps are switched on in forced oil cooled transformers. Buchholtz relay operation for alarm and trip are checked by injecting air through the test petcock (For transformers with ATMOSEAL conservators relay may be tested either by putting in test position or by draining oil from relay after closing valves from either sides). 3.1.8 Magnetic Oil Level Gauge 3.1.8.1 The float level of the oil level indicator is moved up and down between the end positions to check that the mechanis m does not stick at any point. The low oil level alarm of the oil gauge shall be checked. This can only be checked before installation. 3.1.9 Temperature Indicators 3.1.9.1 The contacts of WTI and OTI for alarm and trip are checked and set at required temperatures depending upon ambient temperatures and loading conditions. 3.1.10 Fans and Pumps 3.1.10.1 It shall be checked that the specified number of fans are mounted on radiators as per general arrangement drawing. IR values and settings for operation of fan motors and oil pumps are checked. Check also that the direction of rotation of fans and pumps is correct. 3.1.11 Marshalling Box 3.1.11.1 The wiring from various accessories to marshalling box shall be checked. 3.1.12 Oil 3.1.12.1 Oil samples from top and bottom of main tank are tested as per IS: 1866 (table given in clause 2.5.1.3). DGA tests are to be done to obtain benchmark before charging, one month after charging, three months after charging, and thereafter every year. 3.1.12.2 Oil of diverter switch should be checked for BDV at the time of commissioning and subsequently yearly or 5000 operations, whichever is earlier. 3.1.13 General Checks (a)

All oil valves are in correct positions, closed or opened as required.

(b)

All air pockets are cleared.

(c)

Thermometer pockets are filled with oil.


257

(d)

Oil is at correct level in the bushings, conservator, diverter switch and tank etc.

(e)

Earthing connections are done.

(f)

The colour of silica gel and oil in the breather cup.

(g)

Arcing horn gaps on bushings (where provided) are properly adjusted.

(h)

Heaters in cubicles, conservator, etc., where provided should be checked.

(i)

To check alarm/trip contacts of all accessories, instruments flow meters, differential pressure gauges etc.

(j)

In the case of water cooled transformers, the pressure gauge readings on both water and oil sides to confirm that the water pressure is less than the oil pressure. The oil and water flow should not be less than that specified.

If all the above tests/ checks are found satisfactory, allow a settling time of at least 24 hours for oil and release air from all venting points. Now the transformer can be energized after setting the protective relays to the minimum extent possible. Wherever possible, the voltage should be built up in steps. Any abnormality during commissioning such as vibration of radiator parts, hum etc., should be observed. After a few hours of energisation at no load, the transformer shall be switched off. The Buchholtz relay should be checked for collection of air/ gas. Abnormalities noticed should be corrected. All protective relays should be reset to normal values. Transformer can now be re-energized and loaded gradually. After commissioning, the following details should be furnished to the manufacturer: (i)

Details of transformer including its serial number.

(ii)

Date of commissioning, with test results.

(iii)

Substation/generating station where commissioned.

(iv)

Protection given to the transformer such as lightning arrestor, differential protection, circuit breaker on H.V/L.V etc.

(v)

Loading details with complete temperature log.

4.0

MAINTENANCE

4.1

General

4.1.1 If a transformer is to give long and trouble-free service it should receive a reasonable amount of attention and maintenance. Following are the causes of breakdown of transformers: (i)

Faulty design and construction.

(ii)

Incorrect erection, operation and maintenance.


258 (iii)

Wear and tear, ageing and other deterioration.

(iv)

Accidents.

4.1.2 A rigid system of inspection and preventive maintenance ensures long life, trouble free service and low maintenance cost. Maintenance consists of regular inspection, testing and reconditioning where necessary. 4.1.3 Records must be kept giving details of any unusual occurrence and also if any test results taken. 4.1.4 The principal objective of maintenance is to maintain the insulation in good condition. Moisture, dirt and excessive heat are the main causes of insulation deterioration and avoidance of these will in general keep the insulation in good condition. 4.1.5 No work should be done on any transformer unless it is disconnected and isolated from all external / energized circuits, and all windings have been solidly earthed. 4.2

Factors Affecting the Life of a Transformer

4.2.1 Transformer oil readily absorbs moisture from the air. This reduces the dielectric strength of the oil. It is also reduced by solid impurities present in the oil. Care should be taken that moisture does not penetrate inside the transformer. 4.2.2 Much of the mechanical strength of paper and pressboard comes from the long chain cellulose polymer. Although temperature is a major factor, Oxygen and water clearly have a significant effect on the degradation of cellulosic material (Kraft paper). It is seen that moisture is formed in service–aged transformers due to thermal ageing, which results in lower degree of polymerization (DP) indicating weakening of mechanical strength of paper. 4.3

Maintenance Procedure

The maintenance procedure listed in subsequent clauses is to be attended to at the intervals of time noted against each item in Annexure I. 4.3.1 Oil 4.3.1.1 For maintenance of oil reference may be made to “IS 1866: Code of Practice for Electrical Maintenance and Supervision of Mineral Insulating Oil in Equipment” which gives recommendations in detail for the maintenance of insulation oil. A few short notes on the subject are given below: The oil level should be checked at frequent intervals and any excessive leakage of oil investigated. There may be slight loss of oil by evaporation; this need cause no concern if the tank is topped up at regular intervals.


259

All leaks should be repaired as quickly as possible so as to avoid possible trouble caused by low oil level. Oil for topping up should comply with IS 335: New insulating oils and should preferably be from the same source as the original oil because the oil refined from different crudes may not be completely miscible and may separate into layers. Furthermore, there may be a greater tendency to form acidity or sludge in a mixture than in oil from a single source of supply. Used oil shall not be mixed. New oil may be added as make up only, not exceeding about 10 per cent. Samples of the oil should be taken at regular intervals preferably yearly and tested for DGA and oil parameters. It may be mentioned that the dielectric strength does not give a true indication of the deteriorated condition of the oil. Even oil, which is highly deteriorated, may give a high dielectric strength, if dry. Normal methods of oil purification only maintain the dielectric strength, but do not improve the deteriorated condition of the oil. It is, therefore, advisable not to rely solely on the dielectric strength of the oil by periodic tests. In addition to chemical tests other tests as given in Clause 2.5.1.4 should also be carried out. If the dielectric strength is below, the recommended limit, the oil should be reconditioned by passing it through either a centrifugal separator or a filter. After reconditioning, the breakdown voltage should be more than 50 kV r.m.s. across a standard gap (2.5+0.05 mm apart) and other parameters to be tested as per clause 2.5.1.4 (IS 1866). Other Oil parameters play important role for healthy operation of the transformer. In case any of the parameters like resistivity , IFT reaches the limiting value given in clause 2.5.1.4 (IS 1866), oil should be monitored more frequently and in case the values continue to deteriorate, a decision regarding change of oil is required to be taken. It may be noted that reconditioning by vacuum filtration only improves BDV, moisture content and remo ves dust, dirt suspended material etc. from oil. This process does not improve any other parameters of oil and will tend to retard the process of deterioration of oil. Other methods may have to be followed to improve resistivity, acidity etc. In such a case it is better to change the oil and the old oil may be sent to an oil refinery for reclaiming it. 4.3.2 Rollers 4.3.2.1 After a transformer has been in service for a long period, rollers should be examined carefully. They should be greased and rotated to see that they turn freely. Rollers should also be inspected for overheating when moved on tracks, during initial erection. 4.3.3 Transformer Body 4.3.3.1 The transformer tank and other parts should be inspected periodically for any rust or and oil leak. Rusted portions, if any, should be cleaned thoroughly and repainted with proper paints. Transformer should be completely painted at proper intervals. If any leak is found, it

260


should be investigated. If it is due to defective welding, the same should be rectified after consulting the manufacturer. Leaking joints can be rectified by tightening the bolts to the correct pressure or by replacing the gaskets. 4.3.4 Core and Winding 4.3.4.1 It is recommended that the core and winding be removed from the tank for visual inspection as per time schedule given in inspection table. The windings should be examined to ensure that no sludge has been deposited blocking the oil ducts. Any loose nuts and bolts should be tightened. 4.3.4.2 Before lifting the core and winding from the tank, it is usually necessary to disconnect the windings from the bushings or cable boxes inside the tank to disconnect the off-circuit tap switch handle or leads of the on-load tap changer and to remove any earthing strips between the core clamps and the tank. 4.3.4.3 The core and winding must be removed with great care. It should be placed under cover and in a dry place. 4.3.5 Bushings 4.3.5.1 The bushings should be inspected for any cracks or chippings of the porcelain at regular intervals and kept free from dust and dirt. In location where special and abnormal conditions prevail, such as sand storm, salt deposits, cement dust, oil fumes etc., bushings should be cleaned at more frequent intervals. 4.3.5.2 Oil level in oil filled bushings should be checked periodically. 4.3.6 Cable Boxes 4.3.6.1 The sealing arrangements for filling holes should be checked each year. When screwed plugs are sealed with a bituminous compound, the compound should be examined for cracks. If the compound has cracked it should be replaced as the cracks may lead to an accumulation of water around the plug. Gasketed joints should be examined and tightened whenever required. 4.3.7 External Connections 4.3.7.1 All external connections should be tight. If they appear to be blackened or corroded, the same can be cleaned or should be replaced, if required. 4.3.8 Conservator and Magnetic Oil Gauge 4.3.8.1 Conservators are so arranged that the lower part acts as a sump in which any impurities entering the conservator will collect. A valve/plug is fitted at the lowest point of the conservator for draining and sampling. The inside of the conservator should be cleaned every two to three years. A removable end is generally provided for this purpose.


261

4.3.8.2 The oil level indicator should be kept clean. Generally the oil level is visible through a transparent material. In case of breakage immediate replacement is essential. When conservator is stripped for cleaning, the mechanism of the oil gauge should be cleaned. 4.3.9 Dehydrating Breather 4.3.9.1 Breathers should be examined to ascertain if the silicagel requires changing. The frequency of inspection depends upon local climatic and operating conditions. More frequent inspections are needed when the climate is humid and when the transformer is subjected to fluctuating loads. So long as the silicagel is in active stage its colour is blue but as it becomes saturated with moisture its colour gradually changes to pale pink. The gel should then be replaced or reactivated. The saturated gel can be regenerated by heating up to 110-130 0 C for 8 to 10 hours or 150-200 0 C for two to three hours and can be used again. 4.3.9.2 The level in the oil seal must be maintained at the level marked in the cup. 4.3.10 Buchholtz Relay 4.3.10.1 Routine operation and mechanical inspection tests should be carried out at one and two yearly intervals respectively. 4.3.10.2 During operation if gas is found to be collecting and giving alarm, the gas should be tested and analysed to find out the nature of fault. Sometimes, it may be noticed that the gas collected is only air. The reasons for this may be that trapped air if any is getting released or due to leakage of the suction side of the pumps. The trapped air is released in initial stages only when vacuum is applied during filling of oil. The internal faults can be identified to a great extent by chemical analysis of collected gas. 4.3.10.3 Buchholtz relay may also give alarm/trip signal due to the oil level falling below the required level. 4.3.11 Explosion Vent 4.3.11.1 The diaphragm, which is fitted at the open end of the vent should be inspected at frequent intervals and replaced, if damaged. Failure to replace the diaphragm quickly may allow the ingress of moisture which will contaminate the oil. If the diaphragm has broken because of a fault in the transformer an inspection must be carried out to determine the nature and cause of the fault. 4.3.12 Gaskets 4.3.12.1 Gaskets sometimes shrink during service. It is, therefore, necessary to check the tightness of all bolts fastening gasketed joints. The bolts should be tightened evenly round the joints to avoid uneven pressure. Leaking gaskets should be replaced as soon as the circumstances permit.


262 4.3.13 Small Pipe Work

4.3.13.1 The pipe work should be inspected at least once a year. Leaks may be due to slack unions, which should be tightened, or to badly seated joints requiring the pipes to be aligned and joints remade. 4.3.14 Temperature Indicators 4.3.14.1 At each yearly maintenance inspection, the level of oil in the pockets holding thermometer bulbs should be checked and the oil replenished, if required. The capillary tubing should be fastened down again if it has become loose. Dial glasses should be kept clear and if broken, replaced as soon as possible to prevent damage to the instrument. Temperature indicators should be calibrated with standard thermometer immersed in hot oil bath if found to be reading incorrectly. 4.3.15 Coolers and Cooling Fans 4.3.15.1 There are variety of coolers. For radiator type coolers, maintenance primarily consists of replacing damaged elements, cleaning the outer surface to remove settled dust, repainting etc. 4.3.15.2 Fan blades are cleaned to remove dust; bearings of the fan motors should be lubricated occasionally. Greases should not be added while the motor is running. For other coolers, manufacturer’s instructions should be followed. 4.3.16 On-load Tap Changer 4.3.16.1 Since all on-load tap changers are not of the same design and construction, special instructions of manufacturer’s should be followed. However, a few points are enumerated. (a)

Diverter Switch: The maintenance primarily consists of servicing of diverter switch contacts, checking the oil level in the diverter switch chamber, and replacement of diverter switch oil when the same becomes unsuitable for further service.

(b)

Motor Driving Mechanism (i)

Do not allow dirt to accumulate between contact rings of notching controller.

(ii)

Do not use oil/grease on contacts rings on notching controller.

(iii)

Check the operation of anti-condensation heater.

(iv)

If the contacts of contactors are silver faced, no touching up be ever done, if any is worn out, it should be replaced. Copper contacts may be lightly touched up with a file when they become rough. The pole faces of electromagnet must be kept clean.


(v)

(c)

263

Do not oil/grease the contact surface of radial multi-contact switches, unless a special contact lubricant is used. The space between the rings should be cleaned occasionally. If necessary, a few drops of Benzene be used.

Selector Switch: The contacts do not make/break current. As such, the wear is only due to mechanical movement of moving contacts. These may be inspected once in 2/3 years.

4.3.17 Spares 4.3.17.1 It is a healthy practice to have essential spares like one number of each type of bushings, one thermometer, one cooling fan, pump, buchholz etc., for each group of similar transformer. Suppliers’ recommendations may also be considered in this connection. 4.3.18 Inspection and Maintenance Schedule 4.3.18.1 The frequency of inspections should be determined by the size of the transformer. Local climatic and atmospheric conditions will also influence the inspection schedule. Use Annexure I as a guide for determining the inspection schedule. 4.3.19 Transformer Preservation in De-energised Condition for Long Storage 4.3.19.1 Transformers fitted with conservator oil preservation system (COPS)/ diaphragms are recommended for preservation of Transformers in de-energised condition. During deenergised condition, valves between main tank and radiator should be closed. This is to reduce area of contact of outside air directly with oil / winding insulation paper and outside air. Suitable cautions to be put in placing advising to open the valves before energizing the transformer. Heaters in MB should be kept ON. Oil level in the conservator should be monitored. (Low oil level could be indicative of rupture in air cell of COPS (Conservator Oil Preservation System). 4.3.19.2 Following checks to be followed while the transformer is kept for long storage in de-energised condition : (A)

Quarterly

•

Open the valves between tank and radiator, and run the oil pumps for two hours.

•

Carry out oil tests for:

•

BDV, Moisture content and Resistivity.

•

Test for IR and Tan delta of windings if moisture content in oil is more than the value given in Table 2.5.1.4

(B)

Half yearly

•

IR and Tan delta of windings, bushings.


264 •

BDV of Diverter switch oil.

•

DGA for air ingress.

(C)

Yearly

•

Checks as per item B above.

•

Measurement of Tan delta of Bushings.

•

Operate Tap changer 2-3 times over full range.

•

DGA for O2 & N2 (contents to be constant for no air ingress).

•

Hot oil circulation, if moisture is more than the value given in Table 2.5.1.4 and IR / Tan delta indicates ingress of moisture in insulation.

(D)

Once in three years

•

All checks as per item C above.

•

Balance routine tests on the transformer.

•

Hot oil circulation to be carried out and ensure oil parameters as per clause 2.5.1.3

•

If possible charge the Transformer for 48 hrs.

•

DGA after soak test as above.


265

Annexure I TABLE 1 RECOMMENDED MAINTENANCE SCHEDULE FOR TRANSFORMERS OF CAPACITIES UP TO 10 MVA & 33 kV Sl. No

Inspection frequency

1.

Hourly

Items to be inspected

(i) Load (amperes) (ii) Temperature

Daily Daily

(iii) Voltage Position of tap switch air cell conservat or

3

Fortnightly

Dehydrating breather

4

Monthly

(i) Oil level in transformer.

2

(ii) Connections (iii) Explosion vent (iv) Diaphragm (Pressure relief device) 5

6

7

Quarterly

Half yearly

Yearly

Bushings

(i) Non-conservator transformer. (ii) Cable boxes, gasket joints, gauges and general paint work (i) Oil in transformer

(ii) Earth resistance (iii) Relays, alarms, their circuits, etc.

8

5 Yearly

Non-conservator transformers

9

10 Yearly

-

Inspection notes

Action required if inspection shows unsatisfactory conditions

Check against rated figures oil temperature (OTI) and winding temperature(WTI) and ambient temperature Oil level glass to indicate full. Check that air passages are clear. Check colour of active agent Check transformer oil level. Check tightness. Check for cracks/ damages. Check for any oil spillage. Examine for cracks and dirt deposits Check for external connections Check for moisture under cover. Inspect. Check for dielectric Strength and water content. Check for acidity and sludge. < 1 ohm Examine relay and alarm contacts, their operation, fuses, etc. Check relay accuracy, etc. Internal inspection above core. Overall inspection including lifting of core and coils.

Note : Inspection of core & coil to be done in consultation with manufacturer.

If level drops, check and re-commission air cell. If silicagel is pink, change by spare charge. The old charge may be reactivated for use again. If low, top up with dry oil. Examine transformer for leaks. If loose, tighten. Replace if cracked.

Clean or replace. Tighten, if required Improve ventilation, maintenance of breathers to be ensured. Take remedial measures. Take suitable action to restore quality of oil. Take suitable action if earth resistance is high. Clean the components and replace contacts and fuses if necessary, Change the setting, if necessary. Filter oil regardless of condition. Wash by hosing down with clean dry oil.


266

TABLE 2 MAINTENANCE SCHEDULE RECOMMENDED MAINTENANCE SCHEDULE FOR CAPACITIES OF 10 MVA AND ABOVE Sl. No 1.

2

3

Inspection frequency Hourly

Daily

Quarterly

Items to be inspected (i) Ambient temperature (ii) Winding temperature (iii) Oil temperature (iv) Load (amperes) (v) Voltage (vi) Tap position of tap changed. (i) Oil level in Main and OLTC conservator (ii) Oil colour and level in condenser bushings (iii) Leakage of water into cooler. (v) Dehydrating breather (vi) Cooler oil Pumps

(vii) N2 pressure, if applicable (i) Bushings (ii) Indoor transformers Checking of oil leaks

4

Yearly

Physical checking of oil level

Inspection notes

Check that temperature rise is reasonable. Check against rated figures. -

Check that air passages are free. Check colour of active agent. Positive pressure to be ensured Examine for cracks and dirt deposits. Check for clearance of arcing horns, if applicable Check ventilation. Check with dip stick method

Action required if inspection shows unsatisfactory conditions Shut down the transformer and investigate if either is persistently higher than normal. If low, top up with dry oil, examine transformer for leaks. Contact manufacturer in case of major changes If half of silica gel is pink, change by spare charge. The old charge may be reactivated for use again. Clean or replace. Tighten top terminals and terminal connectors

Top up, if required

Oil in transformer Check for dielectric strength (BDV) and water content (moisture) Dissolved gas Analysis of oil sample Test for all fault DGA (IS 10593/ IEC 60599/ Gases ANSI/ IEEE C57.104)

Oil parameters as per cl no. 2.5.14 (IS 1866/ IEC 60422)

Check for parameters

(iii) Cooler fan bearings, motors and operating mechanism.

Lubricate bearings Check gear box. Examine contacts. Check manual control and interlocks.

(iv) OLTC

oil

Check oil in OLTC driving mechanisms.

Take suitable action to restore quality of oil. Action to be taken as per DGA test results like increase in frequency of sampling to have trend and any specialized test if required. Filter or replace based on test results Replace burnt or worn contacts or other parts


267 Table 2 (contd.)

C& tan delta of bushings IR (windings and bushings) measurements Polarization Index (PI) Checking of WTI, OTI pockets Calibration of WTI, OTI Checking of operation of buchholz Relay (i) OLTC Oil

value should be less than 0.007, rate of rise of c& tan delta to be monitored Check presence of oil

Replace based on test result

Check for dielectric strength (BDV) and water content (moisture)

Filter or replace based on test results.

(ii) Disconnecting Chamber

Check BDV of oil

Replace oil in disconnecting chamber if BDV < 50 kV

(iii) Gasket Joints

Tighten the bolts evenly to avoid uneven pressure

Replace leaking.

(iv) Cable boxes.

Check for sealing arrangements for filling holes. Examine compound for cracks.

Cracked compound around screwed holes to be replaced.

(v) Surge diverter and gaps.

Examine for cracks and dirt deposits.

Clean or replace.

(vi) Relays, alarms, their circuits etc.

Examine relay and alarm contacts, their operation, fuses, etc. Check relay accuracy etc.

Clean the components and replace contacts and fuses, if necessary. Change the setting, if necessary.

(vii) Earth resistance

––

LV tests like ratio, winding resistance at all taps position, c & tan delta, magnetizing current and magnetic balance,

Test results to be compared with precommissioning test results

6.

SOS (In case of any protection tripping due to internal fault or after any major maintenance work) 10 Yearly

Take suitable action; if earth resistance is high. If major deviations found then specialized tests like FRA, RVM, PD tests to be carried out in consultation with manufacturer.

7.

20 Years

Life assessment test

Yearly (or earlier if), the transformer can conveniently be taken out for checking).

5.

Overall inspection including lifting of core and coils. Degree of Polymerization and Furan measurement

Top up if required

gasket,

if

Wash by hosing down with clean dry oil. Will help decision making for taking up any major refurbishment action on transformer.

Notes • • • •

Sl. No. 1 & 2 are in purview of Operation staff whereas the rest to be taken care of by maintenance staff. With respect to on-load tap changers, the manufacturer’s recommendation should be followed. The silica gel may be reactivated by heating it to 150 to 200 degree C. Every time the drying medium of breather is changed, oil seal should also be changed.


268 • •

No work should be done on any transformer unless it is disconnected from all external circuits and the tank and all windings have been solidly earthed. In case of anything abnormal occurring during service, maker’s advice should be obtained, giving him complete particulars as to the nature and the extent of occurrence, together with the name plate particulars in order to assist identification.

TABLE 3 TROUBLE SHOOTING CHART FOR ALL TRANSFORMERS Trouble Rise in temperature High temperature

Cause Over voltage Over current

High ambient temperature Insufficient cooling Lower liquid level Sludged oil Short circuited core Electrical troubles Winding failure

Lightning, short circuit, Overload, Oil of low dielectric strength, Foreign material Core insulation breakdown (core bolt, clamps, or between laminations)

Remedy Change the circuit voltage or transformer connections. to avoid over-excitation If possible, reduce load. Heating can often be reduced by improving power factor of load. Check parallel circuits for circulating currents which may be caused by improper ratios or impedances. See Electrical Troubles, below. Either improve ventilation or relocate transformer in lower ambient temperature. If unit is artificially cooled, make sure cooling is adequate. Fill to proper level. Use filter press to wash off core and coils. Filter oil to remove sludge. Test for exciting current and no load loss. If high inspect core and repair. See Electrical Troubles, below. Usually, when a transformer winding fails, the transformer is automatically disconnected from the power source by the opening of the supply breaker or fuse. Smoke or cooling liquid may be expelled from the case, accompanied by noise. When there is any such evidence or a winding failure, the transformer should not be reenergized at full rated voltage, because this might result in additional internal damage. Also it would introduce a fire hazard in transformers. After disconnection from both so urce and load, the following observations and tests are recommended : (a) External mechanical or electrical damage to bushings, leads, potheads, (b) Level of insulating liquid in all compartments. (c) Temperature of insulating liquid wherever it can be measured.

Core failure high excitation current

Short-circuited core

Open core joints

(d) Evidence of leakage of insulating liquid or sealing compound. Test core loss. If high, it is probably due to a short circuited core. Test core insulation. Repair if damaged. If laminations are welded together, refer to manufacturer. Core loss test will show no appreciable increase. Pound joints together and retighten clamping structure.


Incorrect voltage

Audible internal arcing and radio interference

Bushing flashover Mechanical troubles Leakage through screw joints

Leakage at gasket

Leakage in welds Pressure relief diaphragm cracked.

Pressure-relief diaphragm ruptured

Moisture condensation in open type transformers and air filled compartments Moisture condensation in sealed transformers.

Audio noise

Improper ratio Supply voltage abnormal Isolated metallic part

269

Change terminal board connection or ratio adjuster position to give correct voltage. Change tap connections or readjust supply voltage. The source should be immediately determined. Make certain that all normally grounded parts are grounded, such as the clamps and core.

Loose connections Low liquid level, exposing live parts Lightning Dirty bushings

Same as above. Tighten all connections. Maintain proper liquid level.

Foreign material in threads. Oval nipples Poor threads Improper filler Improper assembly Poor scarfed joints Insufficient or uneven compression Improper preparation of gaskets and gasket surfaces. Shipping strains, imperfect weld. Improper assembly. Mechanical damage.

Make tight screw joints and or gasket joints.

P rovide adequate lightning protection. Clean bushing porcelains, frequency depending on dirt accumulation.

Make tight screw joints or gasket joints.

Repair leaks in welds. Replace diaphragm. Inspect inside of pipe for evidence of rust or moisture. Be sure to dry out transformer if there is a chance that drops of water may have settled directly on windings or other vulnerable locations, as oil test may not always reveal presence of free water.

Internal fault In conservator type transformers-obstructed oil flow or breathing. In gas-seal transformerobstructed pressure relief valve. In sealed transformers – liquid level too high. Improper or insufficient Ventilators

Check to see that valve between conservator and tank is open and that ventilator on conservator is not blocked. Make certain that relief valve fun ctions and that valve discharge lines are open. Liquid level should be adjusted to that corresponding to liquid temperature to allow ample space for expansion of liquid. Make sure that all ventilator openings are free.

Cracked diaphragm

See remedies above for cracked and ruptured diaphragms. Filter oil Make certain all joints are tight. Tighten loose parts. In some cases parts may be stressed into resonant state. Releasing pressure and shimming will remedy this condition.

Moisture in oil Leaky gaskets and joints. Accessories and external transformer parts are set into resonant vibration giving off loud noise.

270 Rusting and deterioration of paint finish Fractured metal or porcelain parts of bushings


Abraded surfaces and weathering Unusual strains placed on terminal connections

Oil Troubles ( see also IS: 1866 - 2000) Low dielectric strength Condensation in open type transformers from improper ventilation

Bare metal of mechanical parts should be covered with grease. Cables and bus bars attached to transformer terminals should be adequately supported. In the case of heavy leads, flexible connections should be provided to remove strain on the terminal and bushing porcelain.

Make certain unobstructed.

that

ventilating

openings

are

Broken pressure relief diaphragm Leaks around cover accessories Leaky cooling coil

Replace diaphragm.

High moisture content

Ingress of moisture in oil/ winding

Badly dis-coloured oil

Contaminated by varnishes. Carbonized oil due to switching Winding or core failure Exposure to air

Filter and monitor ppm for three months. In case moisture content increases again, check points of moisture entry and take appropriate action. Retain oil if dielectric strength, resistivity and tan delta values of oil are satisfactory as per IS 1866. Cl 2.5.1.4 of this section

Oxidation (sludge or acidity)

High operating temperatures

Replace gasket, if necessary. Test cooling coil and repair.

‘Wash down’ core and coils and tank. Filter and reclaim or replace oil. Same as above. Either reduce load or improve cooling.

*Code of practice for maintenance and supervision of insulating oil in service (first revision). In any event, filter oil or dry transformer by heating, or both, to restore dielectric strength. Notes In addition to the above instructions given in this section reference may also be made to IS: 10028 Part 1,2 &3 – Code of Practice for Selection, Installation and Maintenance of Transformers Part 1-Selection, Part 2-Installation & Part 3-Maintenance In case of anything abnormal occurring during service, manufacturer’s advice should be obtained, giving them complete particulars as to the nature and the extent of occurrence, together with the nameplate particulars in order to assist identification. Special list of testing equipment required at site are included in Annexure –II.


271

Annexure II EQUIPMENT’S REQUIRED FOR PRE-COMMISSIONING AND MAINTENANCE TESTS Sl. No.

Test

Equipment Required

Ref.

1.

IR value of transformer winding. DC resistance of transformer winding. Ratio of transformer windings. Electric strength of transformer oil. Moisture content of transformer oil. Capacitance and Tan delta of transformer bushings and windings. Dissolved Gas Analysis of transformer oil. Vibration measurement. Moisture content of solid insulation. Frequency Response Analysis.

Battery and mains operated 5kV motorized Insulation Tester Transformer Ohmmeter (25 Amp.)

IS : 2026

Automatic ratio meter

IS : 2026

BDV test kit

IS : 6792

Karl Fisher Apparatus

IS : 2362

2. 3. 4. 5. 6. 7. 8. 9. 10.

Capacitance and Tan measurement kit (Automatic / manual). Portable DGA test set.

delta

Vibration cum noise level meter. Recovery Voltage Measuring kit On line Moisture measurement kit Sweep Frequency Response Analyzer

IS : 2026

IEC 60137 IEEE C 57 IS: 10569 IEEE C 57 No IS Std available, New diagnostic test No IS Std available, New diagnostic test

Note: HV lead of test kits like Insulation tester and tan delta kits should be with double screen and other leads should be at least single shielded.


272

Annexure III OIL SAMPLING PROCEDURES Title

:

Sampling of oil from oil filled electrical equipment.

Scope

:

This procedure describes the techniques for sampling oil from oil filled equipment such as power transformer and reactors using stainless steel sampling bottles fitted with valves on both sides.

Purpose :

Gases may be formed in oil filled electrical equipment due to normal ageing and also as a result of faults. Operation of the equipment with fault may seriously damage the equipment. It is valuable to detect the fault at an early stage of development. During the early stages of fault the gases formed will normally dissolve in the oil. By extracting dissolved gas from a sample of oil and determining the quantity of composition of gases the type and severity of fault can be inferred.

Responsibility : Maintenance engineer. Reference: I.E.C. 567 IS 9434 Apparatus: (i) Stainless steel sampling bottle of volume one litre as per IS 9434. (ii) Oil proof transparent plastic or transparent PVC tubing. (iii) A drilled flange in case sampling valve is not suitable for fixing a tube. Sampling Procedure:

(Refer Fig. 14) (i) Remove the blank flange or cover (2) of the sampling valve and clean the outlet with a lint free cloth to remove all visible dirt. (ii) If the sampling valve is not suitable for fitting a tube, it may be necessary to use a separate flange with a nozzle in the centre suitable to connect the transparent plastic / PVC tube. (iii) Connect a short oil proof plastic tube (around one meter long) at both end of the stainless steel sampling bottle (5) as shown in (Fig. 14)


Fig. 14

273

274


(iv) Open the valves (4) & (6) on the stainless steel bottle (5), allow 250 ml (approx.) of oil to flow into the bottle by opening valve (1). Close (4), (6) and (1). Disconnect tube from the flange and rinse by gently tilting the bottle upside down such that no air bubble is formed inside during rinsing. Expel this oil into the waste bucket (7) by opening valves (4) and (6). (v) Connect the tube (3) to the flange (2). Hold the bottle in vertical position as shown in Fig. (14). Slowly open the equipment sampling valve so that oil flows through the sampling bottle. (vi) After stainless steel sampling bottle (5) has been completely filled with oil, allow about one litre to two litres of oil to flow to waste bucket (7), till no air bubbles are seen from top outlet. (vii) Stop the oil flow by closing of first the valve (6) and then valve (4) and finally the sampling valve (1). (viii) Disconnect the sample bottle (5) and then disconnect the tubing from the main equipment and the sampling bottle. (ix) Label the sample. (Refer Annexure III-A) (x) Send the information as per Annexure-III-B along with the samples (xi) In case of critical samples furnish information as per Annexure-III-C also Precautions:

(i) When sampling oil, precaution should be taken to deal with any sudden release of oil. (ii) Sample should normally be drawn from the bottom sampling valve. (iii) Proper closing of both the valves (4) & (6) of the bottle should be ensured immediately after the collection of sample. (iv) Due care should be taken to avoid exposure of oil to air while sampling.


(v) Sampling should be done preferably in a dry weather condition. (vi) Sample should be taken when the equipment is in its normal operating condition. (vii) Care should be taken to hold the bottle in place inside the container when transporting. (viii) Testing should be carried out as early as possible.

275


276

Annexure III A LABELLING OF THE OIL SAMPLE BOTTLE (a)

Bottle Number

:

(b)

Manufacturer’s Name

:

(c)

Name of the site

:

(d)

Equipment Name or Serial No.

:

(e)

Sampling Date

:


277

Annexure III B DETAILS TO BE FURNISHED ALONG WITH THE SAMPLES 1.0 Sample Sent By Name / Designation Organisation Name and Address Tel No & Fax E-mail 2.0 Oil Sample Details Date & Time of Sampling

Bottle Numbers

Sampling Point Oil Temp degC

Weather Condition Winding Temp, degC

Load at the time of sample (MW/MVA/AMPS) Sample Remarks

Voltage at the Time of Sampling (kV)

3.0 Equipment Details Substation / Plant Name Make Capacity of Equipment (MVA)(1Ph/3Ph) Type of Cooling

Type of Oil Date of Installation Any Other Information

Trans Name / Feeder Name / Location ID Manufacturer Sl. No. Voltage Rating (In kV)

ONAN/OFAN /ONAF//OFAF OFWF Paraffinic / Nephthenic Inhibited / Uninhibited

Breather Arrangement

Quantity of oil in the Equipment, kL

Date of Last Filtration

Diaphragm / Air Cell/Conventional / Drycol


278

4.0 Tests to be done: (please tick desired standard and required tests) Reference Standard: Oil Parameters: IS-1866 (Before Charging) / IS-1866 (In-Service) / IS335 / IS-12463 / Dissolved Gas Analysis (IS-9434)

Water Content (IS-13567)

Specific Resistance at 90degC (IS6103)

Dielectric Dissipation Factor at 90degC (IS6262) Flash Point (IS-1448-P-21)

Inter Facial Tension at 27°C (IS6104) Kinematic Viscosity at 27°C (IS1448-P-25) Carbon Type Composition (IS13155) Oxidative Aging (IS-12177 (Method-A)) Furan Analysis (IEC-61198)

Dielectric Strength (IS6792) Total Acidity (IEC-62021 Vol-1 / IS-1448 P-1) Sludge Content (IS-1866)

Pour Point (IS-1448 P-10)

Density at 29.5°C (IS1448 P-16) Oxidation Stability SK Value (Annex-D of (Annex-C of IS-335) IS-335) Corrosive Sulphur (Annex- Inhibitor Content (ISB of IS-335) 13631)

In case of new transformer following additional information to be furnished : Date of commissioning

:

MVA rating

:

kV rating

:

Oil type (Parafinic / Naphthanic)

:

Cooling (ONAN/ONAF/OFAF/OFWF)

:

Type of oil preservation: (Air cell/diaphragm type/Direct breathing) Make

:


279

Annexure III C ADDITIONAL DATA INPUT FORMAT FOR CRITICAL EQUIPMENTS 1.

Voltage profile for last six months indicating maximum and minimum values and % of time voltage more than rated voltage.

2.

Loading Pattern (Monthwise) of the transformer for last six months Max. load

Current (A) ....

mw ...... mvar .......

Min. load

Current (A) ....

mw ...... mvar .......

Normal load

Current (A) ....

mw ...... mvar .......

3.

Date of last filtration carried out

4.

Type of oil preservation system: air cell in conservator/diaphragm in conservator/direct breathing

5.

Any Buchholz alarm/ trip operation in past:

YES/NO

6.

Any oil topping up done in the past:

YES/NO

7.

Whether complete oil was changed any time:

YES/NO

8.

Present BDV / Moisture content value:

9.

Color of silicagel

10.

Date of commissioning:

11.

Manufacturer’s serial number:


280

Annexure IV SPECIFICATIONS FOR TEST EQUIPMENTS I

5 kV BATTERY OPERATED INSULATION RESISTANCE TESTER

The equipment offered shall be for the measurement of insulation resistance of electrical equipment. Technical Requirements Rated Voltage selection 0.5/1.0/2.5/5kV (DC volts) Rated insulation resistance 0-10,000 Mega ohms Type Portable, compact and direct reading type of multi voltage and multi rated insulation resistance ranges. It shall be suitable for DC battery operation. Batteries shall be rechargeable with 230V, 50 HZ AC supply. The necessary accessories for this purpose shall be supplied by the supplier. The operating temperature shall be up to 50 deg C and humidity 85%. There should be provision for infinity adjustment. The instrument shall be supplied in a carrying case with 2 m long mains lead and 20 m long test leads with carrying case. It shall generally comply with the requirements of IS: 2992 and IS: 11994 or relevant internationally acceptable standards. As per requirement of ISO – 9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and transportationmovement from one place to another is unavoidable. Therefore equipments shall be robust in design so that it gives desired performance even in adverse site conditions. Though the instrument is capable of operating on battery and are provided with battery condition indicators, it would be advisable to conduct the tests on mains supply input power to the extent possible. Usage of battery must be resorted to sparingly. The supplier should have adequate “After sales service” in India. II

TRANSFORMER DC WINDING RESISTANCE MEASUREMENT TEST KIT

The instrument is used for measuring DC winding resistance of the transformer/reactor where large inductance is present.


281

The test kit shall be able to withstand inductive kicks from transformer winding. Variation in test current shall not result in loss of accuracy. The display of resistance should be through LED/LCD without requiring any balancing of decades to obtain stable readings. It should employ four wire method and no lead compensation shall be required for measurement. Built-in discharge circuit should be provided to discharge the specimen when test is completed when current lead accidentally disconnects or when instrument power supply is lost. Technical Parameters Test current Resolution Range Accuracy Open circuit voltage

25 Amp 1 milliohm 0 to 100 ohms +- 0.5% of full scale reading or better min. 30 Volts, DC

General Requirements The instrument shall contain all standard accessories including test leads of 20 meters with suitable clamps/connectors and carrying case. The kit should have been proven for repeatability of test results in charged switchyard conditions and documentary evidence should be furnished along with the bid. The kit should have been tested for EMI /EMC as per standards. Input supply of the kit shall be 230 Volts AC, 50 Hz, Variations + 15% and 5% in voltage and frequency respectively. As per requirement of ISO-9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. Battery / mains operated micro ohmmeters employing currents less than 5 amps are not recommended for transformers. The supplier should have adequate “after sales service” in India.


282 III

AUTOMATIC TURNS RATIO TESTER

The equipment offered shall be used for measurement of turns ratio of various transformers and bushing current transformers automatically displaying the ratio without requiring any manual balancing of decades. Technical Requirement Operation Voltage:

240 volts: 50Hz, single phase A.C

Test Voltage:

240 volts AC

Measuring range:

1 to 2000

Accuracy:

± 0.5 % of FSD

Measuring Principle It should display actual turns ratio of different vector groups in three phase transformers without conversion The kit should be supplied with 15 m of test lead. The kit should be tested for successful operation in charged 420 kV switch yard and be tested for EMI and EMC. The kit shall be capable of operating at a temperature of 500 C and at a humidity upto 85%. As per requirement of ISO 9001calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. IV

100 kV AUTOMATIC TEST SET

TRANSFORMER

OIL

BREAKDOWN

VOLTAGE

The equipment offered shall be suitable for determination of electrical strength (break down voltage) of insulating oil conforming to IS-335 and IS-1866 upto 100 kV when measured in accordance with IS:6792. The test cell shall be as per IS: 6792 and IEC-156 suitable for BDV upto 100 kV without external flash over. The unit shall be automatic type having control unit and high voltage transformer in a common cabinet with necessary partition. HV chamber interlocking and zero start interlocking shall be provided.


283

The unit shall have motorized drive to increase voltage linearly as per the rate specified in IS: 6792. Provision shall also be available for manual increase of voltage. The unit shall be complete with test cell stirrer, calibrator and necessary gauges for adjusting the gap. The equipment shall be suitable for operation at 240 volts, 50Hz, single phase AC supply. As per requirement of ISO-9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. V

MOISTURE CONTENT MEASUREMENT SET

The test kit shall make use of automatic Karl Fischer titrator capable of measuring water in oil upto 1 ppm. The measured moisture shall be displayed in microgram or PPM or percentage. The set should have following features : •

Should be compact and portable.

•

Should operate with Karl Fischer reagent which is available.

•

Should have error indicator for indicating any defect in vessel charge or generator solution and electrodes.

•

Should have facility for auto-display of ions in parts per million.

•

Should be free from effect of humidity, parasitic reactions and inherent drift of circuitry.

•

Should have 3 sets of syringes required for measurement and 6 sets of vessel charge bottles.

•

Should be suitable for 240 V 50Hz AC supply.

•

Should have back up indication for mains on, instrument error, stirrer moving and titration over.

Resolution range = 1 in ppm The speed of stirrer should preferably be controllable. The set should be accompanied with six sets of bottles of reagent.


282

As per requirement of ISO 9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. VI

TECHNICAL SPECIFICATIONS FOR CAPACITANCE AND TAN DELTA KIT

The kit shall be suitable to measure capacitance and tan delta of EHV class transformers (1/2/3 windings), Bushings, Windings of shunt reactors, Current transformers, Bus and line CVTs and grading capacitors of circuit breakers at site in a charged switchyard upto 420kV AC and 500 kV DC. The kit shall be capable of measuring capacitance and tan delta of each winding of the transformer in suitable mode so that capacitance of the windings does not affect the reading. The kit shall comply with the requirements laid down in internationally accepted standards. The kit shall be complete with measuring bridge, HV power supply unit of 10 kV, standard capacitor etc. The effects of induced voltage on instrument during testing for getting null point should be fully compensated. Technical Requirements Output voltage of the kit shall be from 0 to 10 kV in continuously adjustable range. The kit shall measure Tan delta ranging from 1 10-4 to 1 10-2 with an Accuracy of ±1% of the measured value. Resolution ±1 10-4 . x

x

x

The kit shall measure Capacitance ranging from 1.0 pF to 0.1 Micro farads with an accuracy of + 1% of the measured value. Resolution ± 1pF. General Requirements The instrument shall contain all standard accessories including testing lead of 20 meters with suitable clamps/connectors and carrying case. The instrument should have been proven for repeatability of test results in charged switchyard conditions. The kit shall be compatible for EMI/EMC environment. The kit shall be compatible in static and magnetic interferences as well as harmonics.


285

Input supply of the kit shall be A.C 230 Volts, 50 Hz, Variations ± 15.% and 5% on Voltage and frequency. As per requirement of ISO 9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. Correction chart for temperature if applicable shall also be supplied for arriving at the reference values of results. The test kits are generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. VII

SPECIFICATIONS FOR AUTOMATIC CAPACITANCE AND TAN DELTA KIT

The kit shall be suitable to automatically measure capacitance and tan delta of transformers (1/2/3 windings), windings of shunt reactors, Current transformers, Bus and Line CVTs and grading capacitors of Circuit Breakers at site automatically in a charged switchyard upto 420 kV AC and 500 kV DC. The kit shall be capable of measuring capacitance and tan delta of each winding of the transformer in suitable mode so that capacitance of other windings does not affect the reading. The kit shall comply with the requirements laid down in internationally accepted standards. The kit shall be complete with measuring bridge, HV power supply unit of 10 kV, standard capacitor etc. The effect of induced voltage on instrument during testing for getting null point should be fully compensated. The kit shall be capable of measuring excitation current of transformer winding at 10 kV. Technical Requirements Output voltage of the kit shall be from 0 to 10 kV in continuously adjustable range. The kit shall measure Tan delta ranging from 1 10-4 to 1 10-2 with an Accuracy of ±1% of the measured value. Resolution ± 1 10-4 . x

x

x

The kit shall measure capacitance ranging from 1.0pF to 0.1 Micro Farads wilth an Accuracy of ±1% of the measured value. Resolution ±1pF. General Requirements The instrument shall contain all standard accessories including testing lead of 20 metres with suitable Clamps/connectors and carrying case. Kit should be able to measure capacitance and tan delta/power factor automatically without balancing any decade and also interference suppression shall be automatic.

286


The instrument should have been proven for repeatability of test results in charged switchyard conditions. The kit should be capable of operating and storing data at temperature from 10 degree C to 50 degree C and humidity upto 90%. The kit shall be compatible for EMI/EMC environment. The kit shall be compatible in static and magnetic interferences as well as harmonics. Input supply of the kit shall be A.C 230 Volts 50 Hz, Variations ±15.% and 5% on voltage and frequency. As per requirement of ISO 9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. Supplier shall also provide correction chart for temperature if applicable for arriving at of results. The test kits are generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sale service” in India.


VIII

287

TECHNICAL SPECIFICATIONS OF RECOVERY VOLTAGE MEASURING (RVM) KIT

The recovery voltage measuring kit shall be supplied as per following specifications and complete with analysis software for direct display of the equivalent moisture content in the solid insulation of the transformer. Test voltage Measuring range Basic setting Max. deviation from the set value Max. output current

Electrometer Measuring range Error limits Current input Charging and discharging time range Charging to discharging time ratio (t C/td ) Resistance measuring range Temperature range

Power supply Display Interface

50 V ... 2000 V DC adjustable in 1 V steps 2000 V DC ± 0.2 % 5 mA (permanent) 200 mA (100 m) delayed short circuit protector -200 V ... +1000 V ±1% ≤ 1 pA tC, t d : 10 m ... 99 999 s 0.1 ... 10; basic setting: 2 1 MΩ ... 1000 GΩ error limits (up to 100 GΩ): ± 1.5 % Instrument operating temperature: 0°C ... 50°C Recommended test object temperature: ≥ 8°C 240 VAC at 50 Hz max. 40 VA 16 40 character back-lit black and white graphic LCD RS 232 / parallel port x


288 IX

SPECIFICATION OF F.R.A. TEST SET

The FRA test set consisting of various components like network analyzer, auxiliary equipment, test leads, connectors and suitable software (to operate the hardware, to store all tests data, to view and compare the data and for long term storage and archival of data) should be capable of doing frequency response analysis of inductor capacitor network of Transformers (up 315 MVA capacity autotransformer and converter transformer) and Reactors by adopting swept frequency method, to assess mechanical condition (winding movements and mechanical distortion, i.e., loss of mechanical integrity) of the transformers and reactors. Detailed specifications of various components of FRA test set are as below: (1) Network Analyzer (signal Generator and measuring device) Analyzer must be capable of measuring the amplitude change and phase shift over a frequency range of 10 Hz. to 10 MHz. in five frequency bands, mentioned as below: (i) (ii) (iii) (iv) (v)

10 Hz. to 2 KHz. 50 Hz. to 20 KHz. 500 Hz. to 200 KHz 5 KHz. to 2 MHz. 25 KHz. to 10 MHz.

Analyser should have capability for following: •

In above mentioned frequency range of 10 Hz. to 10 MHz, total measurements spaced logarithmically at interval of 1.2%.

•

Measurement band width adjustable to provide adequate signal to noise ratio.

•

A constant excitation level must be maintained for each frequency measurement.

•

“Auto-scale” - each frequency measurement to provide an overall range of 0 to 80 dB with an accuracy of ± 1 dB (the capability to provide constant excitation and autoscaling with Highest possible Resolution)

•

Measurements should have repeatability.

•

The analyzer should have 03 output channels with details as below : (a)

one output channel providing a swept frequency signal of output power range of at least 15 dB over entire frequency range.

(b)

Two measurement channels, capable of measuring amplitude down to at least – 85 dB, with an accuracy of ± 1 dB and constant input impedance (50 ohms), over the entire frequency range.


289

Controller : (a) (b) (c) (d) (e) (f) (g)

Processor Operating System Data storage Communication Key board Pointing drive Display

-

Pentium III 866 MHz/256 MB – SDRAM Microsoft windows –2000 15 GB Hard Drive, USB 3.5” Floppy drive 1 Parallel, 2 USB, 10 Base T Ethernet 87 Key Two button touch pad SVGA, color TFT ( > 10”)

Input supply

:

230 volts. ± 15%, 1 phase, 50 Hz.

Output Calibration

: :

10 volts Peak to peak at 50 ohms. Internal, while in field use.

Data Collection

:

(a) (b)

: :

SFR – Analysis 10 Hz. to 10 MHz.

I.F. bandwidth

:

10% of active frequency

Data Display

:

(a)

Scale

:

Log/linear

(b)

Frequency Range

:

User defined (within 10 Hz. to 10 MHz)

(c)

Plotting

:

Frequency Vs. Magnitude/phase

Test method Frequency Range

(2) Test-leads, Connectors, Earth-leads provided, should have following characteristics :(a) All leads must have same input impedance as the network analyzer (b) Test leads should be made from low loss RF coaxial cable shielded with shields of cable of being earthed at both ends. (c) Suitable leads and connectors are : (i)

50 ohm characteristic impedance cable type “RG 213/U UHF with type ‘N’ connector

(ii)

Cable type “RG 58/CU with type BNC – connectors, using N- series to BNC adapters

(d) Minimum length of set of test leads: 15 metres. (e) No. of test lead set = two sets.


290

(f) Connectors should be large enough to make good contact/connection when clamped directly on bushings (g) Earth leads: should have connectors capable of connection to the shields of test leads and forming a good earth connection with Transformer tank. (3)

Software To provide following functions suitable software should be provided:-

(a)

To operate hardware to measure the response (amplitude and phase) over the required frequency bands, at suitably spaced frequencies within each band.

(b)

To Store all test data and transformer nameplate and location data

(c)

Long term storage and archival of data.

(d)

View and compare the test data : - from tests carried out at different times on transformer. - between different phases and tap positions of a transformer. - from different transformers of same size and same make/same design.

(4)

Auxiliary Equipment

Preferably the test equipment, i.e., analyzer should have inbuilt front-panel adequate enough for operating it. If not so, suitable interface unit between analyzer and laptop computer should be provided to facilitate testing, along with laptop computer as a part of test set. (5)

Capability of Storage of Data

Equipment should have capability to store measurements in computer readable forms. (6)

Presentation of Results

(a)

For each frequency scan the final results should be in a computer file with records containing following:-

(b)

(i)

Frequency (in Hz.), Attenuation (in dB) and phase shift (in degrees)

(ii)

The results should be in such format that following is possible:-

Comparison to finger prints.

-

Comparison between different phases of same transformer.

-

Comparison with similar types of transformers.

The test set should be capable of performing initial diagnosis at site itself to ensure errors, if any, due to testing and to identify serious transformer defects.


291

(7)

Operating Temperature: 0 to 50o C

•

Operation Humidity : upto 85% non-condensing

•

Weight and portability : The analyzer should be designed as robust, fully portable and weight should be less than 15 kg.

(8)

Other Essential Requirements

•

Measurement should not be affected by electromagnetic /electrostatic noise of A.C./D.C - switchyards/sub-stations.

•

Test equipment should be field proven, i.e., should not fail when repeatedly used in field.

•

Technical support for analyzing the test results, (on continuous basis).

•

The analysis to be done by the experts in the field of F.R.A.

•

Provide service for continuous upgrade of hardware & software.

•

On site training of user.

SECTION K1

Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors

SECTION K1

Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 1.0

GENERAL

1.1 Condition monitoring may be defined as a predictive method making use of the fact that most equipment will have a useful life before maintenance is required. It includes the life mechanism of individual parts of equipment or the whole equipment, the application and development of special purpose equipment, the means of acquiring the data and the analysis of that data to predict the trends. 1.2 Initial stage of a condition monitoring programme consists of establishing the base line parameters and then recording the actual base line (or finger prints) values. The next stage is the establishment of routine testing of plants and equipment observing the running condition and assessing the parameters previously determined for the baseline. These readings are then compared with the fingerprint and the state of the present plant condition can be determined from the absolute figures. The rate of degradation and an assessment of the likely to failure can be estimated from the trend. 1.3

The benefits of condition monitoring can be summarized as below: Reduced maintenance costs Results provide a quality – control feature Limit the probability of destructive failures, leading to improvements in operator safety and quality of supply For assessing possibility & severity of any failure and consequential repair activities. Provides information on the plant operating life, enabling business decisions to be made either on plant refurbishment or replacement.

1.4

Condition Monitoring Techniques

The following condition-monitoring techniques are currently being used by Power Utilities world over to assess the health of transformer/ reactors in service: 1.4.1 Dissolved Gas Analysis (DGA) provides an early warning of various incipient faults in transformer winding or core. 1.4.2 Oil parameters Testing: Low BDV indicates moisture or particulate contaminants in the oil. High moisture content varying with temperature indicates wet winding. Acidity, resistivity and interfacial tension (IFT) indicate oil condition. (For other details refer clause 2.5 of section K)


296

1.4.3 Capitance & Tan δ measurement of bushing and winding to assess the condition of insulation, bushing and winding (For details refer clause 6.9 & 8.11 of section J) 1.4.4 Winding resistance measurement to detect problem of broken sub-conductors, winding contact joints and OLTC connections (For details refer clause 4.1 & 8.3 of section J) 1.4.5 Impedance measurements with precision instruments to check for dynamic movements of winding due to system short-circuit faults 1.4.6

Turn ratio test indicates problem in winding and verifies correct tap changer connections

1.4.7 Excitation/ magnetization current tests to locate faults in the magnetic core structure such as shorted laminates or core bolt insulation breakdown or shorted turns due to insulation failures, which have resulted in conducting paths between winding turns. 1.4.8 IR measurement to indicate the presence or absence of harmful contamination (dirt, moisture etc.) and stress degradation of insulation. 1.5 Some of the latest trends in condition monitoring techniques which are in development/ testing stage but can be a powerful tool in future for detecting transformer problems can be summarized as follows: 1.5.1 Frequency response analysis (FRA) to check for system resonance condition and dynamic movements and detection of winding mechanical distortion during transportation and through fault. 1.5.2

PD measurement and acoustic localization of faults

1.5.3 Furfuraldehyde (FFA) analysis in oil (HPLC chromatography) to detect ageing in cellulosic material without taking paper samples 1.5.4 On-line dielectric dissipation factor (DDF) monitoring of H. V. bushings: Signals from transducers connected to test taps of bushings are collected and transmitted to User Interface Module for processing the data by proprietary software and converted to dielectric loss angle/ dielectric dissipation factor (DDF) and leakage current values. 1.5.5 Polarization spectrum or recovery voltage measurement (RVM) giving general indication of moisture in insulation and possible paper ageing and oil condition 1.5.6

On-line Gas Monitors

(a)

On-Line hydrogen monitors to provide earliest possible detection of gas build up and alert the user to the need for more detailed analysis

(b)

On-line moisture content measurement to continuously monitor water content in oil, which indicates the status of solid insulation. The data is stored in monitor’s memory and can be down loaded remotely or locally which can be subsequently analyzed using window based control software. The software automatically gives trend analyses; generate graphs and reports for the user to take action based on the same.


297

1.5.7 On-line PD detection using either ceramic transducer attached to the tank or wave guide which transmit ultrasonic signals to a detector outside the tank 1.5.8 On-line temperature monitoring-direct measurement through fibre optic sensors for continuous monitoring of hot spot temperature, can be the basis for an on-line load management system which can be accessed by operating staff to control loading pattern and thermal ageing of the transformer. 1.5.9

Thermo-vision Scanning of Transformer

2.0

DISSOLVED GAS ANALYSIS (DGA)

2.1 The transformer undergoes electrical, chemical and thermal stresses during its service life which may result in slow evolving incipient faults inside the transformer. The gases generated under abnormal electrical or thermal stresses are hydrogen(H2), methane(CH4), ethane (C2 H6), ethylene(C 2H4 ), acetylene(C 2H 2), carbon monoxide(CO), carbon dioxide(CO 2), nitrogen(N2)and oxygen(O2) which get dissolved in oil. Collectively these gases are known as Fault Gases, which are routinely detected and quantified at extremely low level, typically in parts per million (ppm) in dissolved Gas Analysis (DGA). Most commonly used method to determine the content of these gases in oil is using a Vacuum Gas Extraction Apparatus and Gas Chromatograph. 2.2 DGA is a powerful diagnostic technique for detection of slow evolving faults inside the transformer by analyzing the gases generated during the fault which gets dissolved in the oil. For Dissolved Gas Analysis to be reliable, it is essential that sample taken for DGA should be representative of lot and no dissolved gas shall be lost during transportation and laboratory analysis. Suggested sampling procedure based on IEC 60567 is given in Annexure-III of Chapter K. Effective fault gas interpretation should basically tell us first of all, whether there is any incipient fault present in the transformer. If there is any problem, what kind of fault it is. Whether the fault is serious and the equipment needs to be taken out of service for further investigation. DGA can identify deteriorating insulation and oil, hot spots, partial discharge, and arcing. The health of oil is reflective of the health of the transformer itself. DGA analysis helps the user to identify the reason for gas formation and materials involved and indicate urgency of corrective action to be taken. 2.3 The evolution of individual gas concentrations and total dissolved combustible gas (TDCG) generation over time and the rate of change (based on IEC 60599 and IEEE C 57-104 standards) are the key indicators of a developing problem. Some of the recognised interpretation techniques are discussed below: 2.3.1

Individual Fault Gases Acceptable Limits

To ensure that a transformer (with no measured previous dissolved gas history) is behaving normal, the DGA results are compared with the gassing characteristics exhibited by the majority of similar transformers or normal population. As the transformer ages and gases are generated, the normal levels for 90% of a typical transformer population can be determined. From these


298

values and based on experience, acceptable limits or threshold levels have been determined as given in Table 1 below: Table 1 Ranges of 90% Typical Conc Values (all Types of Transformers) as per IEC 60599 /1999 Transformer Sub Type

FAULT GASES (in µ l/l) or ppm

No OLTC Communicating OLTC

H2 60-150

CH4 40-110

C2H6 50-90

C2H4 60-280

C2H2 3-50

CO 540-900

CO2 5100-13000

75-150

35-130

50-70

110-250

80-270

400-850

5300-12000

Note 1 - The values listed in this table were obtained from individual networks. Values on other networks may differ. Note 2 - “Communicating OLTC” means that some oil and /or gas communication is possible between the OLTC compartment and the main tank or between the respective conservators. These gases may contaminate the oil in the main tank and affect the normal values in these types of equipment. “NO OLTC” refers to transformers not equipped with an OLTC, or equipped with an OLTC not communicating with or leaking to the main tank. Note 3 - In some countries, typical values as low as 0.5µ l/ l for C2H 2 and 10 µ l / l for C2H4 have been reported.

However it is improper to apply threshold level concept without considering the rate of change of the gas concentration. When an abnormal situation is indicated by above table, a testing schedule is devised with increased sampling frequency. 2.3.2 Total Dissolved Combustible Gas (TDCG) Limits The severity of an incipient fault can be further evaluated by the total dissolved combustible gas (TDCG) present. Limits for TDCG are as given in Table 2. An increasing gas generation rate indicates a problem of increasing severity and therefore we should resort to shorter sampling frequency. Table 2 Action based on TDCG limits (IEEE standard C:57.104-1991) TDCG Limits, PPM

Action

< or = 720

Satisfactory operation, Unless individual gas acceptance values are exceeded

721-1920

Normal ageing/ slight decomposition, Trend to be established to see if any evolving incipient fault is present.

1921-4630

Significant decomposition, Immediate action to establish trend to see if fault is progressively becoming worse.

>4630

Substantial decomposition, Gassing rate and cause of gassing should be identified and appropriate corrective action such as removal from service may be taken.

Note : TDCG value includes all hydrocarbons, CO & H2 and does not include CO 2 which is not a combustible gas.


299

2.3.3 The relationship of evolved gas with temperature and the type of faults are shown in Tables 3 and 4 respectively: Table 3 Relationship of evolved gases with temperature Relationship with temperature Methane (CH 4)

> 120 °

CEthane (C2H6 )

> 120 °

CEthylene (C2H4 )

> 150 °

CAcetylene (C2H2 )

> 700 ° C

Table 4 Associated faults with different fault gases Associated faults with different gases Oil Overheating : C 2H 4, C 2H 6, CH4 Traces of acetylene with smaller quantity of Hydrogen may be evolved Overheated Cellulose : CO Large quantity of Carbon-Di-Oxide (CO2) and Carbon Monoxide (CO) are evolved from overheated cellulose. Hydrocarbon gases such as Methane and Ethylene will be formed if the fault involves an oil impregnated structure. Partial discharge in Oil (Corona): H 2, CH4 Ionisation of high stressed area where gas / vapour filled voids are present or ‘wet spot’ produces Hydrogen and methane and small quantity of other hydrocarbons like ethane and ethylene. Comparable amounts of carbon mono-oxide and di-oxide may result due to discharges in cellulose. Arcing in Oil : C 2H 2, H 2 Large amount of Hydrogen and acetylene are produced with minor quantities of methane and ethylene in case of arcing between the leads, lead to coil and high stressed area. Small amounts of carbon mono-oxide and di-oxide may also be formed, if fault involves cellulose.

It is well known that there is no definite interpretation method in the world, which can indicate the exact location and type of the fault. The different interpretation methods only provide guidelines to take an engineering judgement about the equipment. Apart from the DGA results various other factors are taken into consideration such as past history of the transformer, grid condition, loading patterns etc. 2.4

Ratio Methods

Several well- known methods/criteria (like Rogers ratio, IEC 60599, Dornenberg, Key gas etc.) are being used by utilities to interpret the DGA results, based mostly on the relative concentrations (i.e. ratios) of the constituent gases. These ratios generally give an indication of the existence and nature of a problem. Some of the interpretation methods used for DGA are discussed here in brief:


300 2.4.1 IEC 60599 Method

This method is applicable only when the fault gas results are ten times the sensitivity limit of the Gas Chromatograph (GC). As per IEC 60567 the sensitivity limit for the GC should be maximum 1 ppm for all the hydrocarbons and 5 ppm for hydrogen. In this method three ratios viz. C2H2/C2H4, CH4/H2 & C2H4/C2H6 are used for interpretation. Various combinations of the ratios are used for diagnosis of type of fault such as PD, Discharge of low energy, Discharge of high energy, Thermal fault < 300 deg C, Thermal fault 300 – 700 Deg C and Thermal fault > 700 Deg C. The table (as per IEC 60599) showing different type of faults depending upon the three key ratios is given in Table 5 : Table 5 DGA Interpretation Table (Source IEC 60599 – 1999) C2H2 C2H4

CH4

C2H4

Case

Characteristic Fault

PD

Partial discharges

NS

<0.1

<0.2

D1

Discharges of low energy

>1

0.1 – 0.5

>1

D2

Discharges of high energy

0.6 – 2.5

0.1 -1

1

T1

Thermal fault T < 300ºC

NS

T2

Thermal fault 300ºC < 1 < 700ºC

<0.1

T3

Thermal fault

<0.2

H2

C2H6

>2 1

2

>1 but NS

<1

>1

1-4

>1

>4

Note 1 – In some countries, the ratio C2H2 / C2H6 is used, rather than the ratio CH4 /H2. Also in some countries, slightly different ratio limits are used. Note 2 – The above ratios are significant and should be calculated only if at least one of the gases is at a concentration and a rate of gas increase above typical values (see clause 9). Note 3 – CH4 / H2 <0.2 for partial discharges in instrument transformers.CH4 /H2 <0.07 for partial discharges in bushings. Note 4 – Gas decomposition patterns similar to partial discharges have been reported as a result of the decomposition of thin oil film between over-heated core laminates at temperatures of 140 ºC and above. NS = Non- significant whatever the value An increasing value of the amount of C2H2 may indicate that the hot spot temperature is higher than 1000ºC

Though this method is quite comprehensive, still there are cases where it does not fit into any of the cases listed in the diagnosis table. These cases should be dealt through trend analysis and other interpretation methods. Again interpretation through the above method is meaningless unless it is correlated with the earlier sample results. 2.4.2 IEEE Method-C57-104/1991 2.4.2.1 KEY GAS M ETHOD Characteristic “Key Gases” have been used to identify particular type of fault. Laboratory simulations and comparison of results of DGA tests combined with observations from the tear down of failed transformers have permitted the development of a diagnostic scheme of the


301

characteristic gases generated from thermal and electrical (Corona and arcing) deterioration of electrical insulation. Table 6 shown the lists of the key gases for the conditions of arcing, corona, overheating in oil and overheating in paper in the order of decreasing severity. Table 6 Key gases associated with typical fault Fault type

Key gases

Arcing Corona

Acetylene (C2H2), Hydrogen (H 2) Hydrogen (H2)

Overheated Oil

Ethylene (C2H4), Methane (CH4)

Overheated Cellulose

Carbon Mono-oxide (CO) and Dioxide (CO2)

2.4.2.2 RATIO M ETHOD These methods are used to determine the type of fault condition by comparing ratios of characteristic gases generated under incipient fault conditions. The advantages to the ratio methods are that they are quantitative, independent of transformer capacity and can be computer programmed. The disadvantages are that they may not always yield an analysis or may yield an incorrect one. Therefore it is always used in conjunction with other diagnostic methods such as key gas method. (a) The Doernenberg Ratio method is used when prescribed normal levels of gassing are exceeded. It provides a simple scheme for distinguishing between pyrolysis (overheating) and PD (corona and arcing). In this method four ratios viz. CH4/H2, C2H2/C2H4, C2H6/ C2H2 & C2H2/ CH4 are used. Table 7 Ratios for key gases - Doernenburg Suggested fault diagnosis

Ratio 1 (R1)

Ratio 2 (R2)

Ratio 3 (R3)

Ratio 4 (R4)

CH4/H2

C2H2/C2H4

C2H2/CH4

C2H6/C2H2

1. Thermal Decomposition

>1.0

<0.75

<0.3

>0.4

2. Corona (Low Intensity PD)

<0.1

Not Significant

<0.3

>0.4

3. Arcing (High Intensity PD)

>0.1

>0.75

>0.3

<0.4

In Doernenburg’s method for declaring the unit faulty at least one of the gas concentrations (in ppm) for H2, CH4, C2H2 and C2H4 should exceed twice the values from limit L1 (Table 8) and one of the other three should exceed the values for Limit L1. Having established that the unit is faulty, for determining the validity of ratio procedure at least one of the gases in each ratio R1, R2, R3 or R4 should exceed limit L1. Otherwise the unit should be resampled and investigated by alternative procedures.


302

Table 8 Concentrations of dissolved gas Key gas

(b)

Concentration L1 (in ppm)

Hydrogen(H2)

100

Methane (CH 4) Carbon Monoxide (CO)

120 350

Acetylene (C2H2)

35

Ethylene(C2H4)

50

Ethane (C2H6)

65

The Rogers Ratio method is a more comprehensive scheme using only three ratios viz. CH4/H2, C2H2/C2H4 & C2H4/C2H6, which details temperature ranges for overheating conditions based on Halstead’s research and some distinction of the severity of incipient electrical fault conditions(Table 9). A normal condition is also listed. Table 9 Rogers ratio for Key Gases Case

R2 (C2H2/C2H4)

R1 (CH4/H2)

R5 (C2H4/C2H6)

Suggested fault diagnosis

0

<0.1

>0.1

<0.1

Unit normal

1

<0.1

<0.1

<0.1

Low-energy density arcing –PD (See Note)

2

0.1-3.0

0.1-1.0

>3.0

Arcing – High energy discharge

3

<0.1

>0.1<1.0

1.0-3.0

Low temperature thermal

4

<0.1

>1.0

1.0-3.0

Thermal <700ºC

5

<0.1

>1.0

>3.0

Thermal >700ºC

Note : There will be a tendency for the ratios R2 and R4 to increase to a ratio above 3 as the discharge develops in intensity

IEEE C57.104 - 1991 standard gives an elaborate way of analysing the type of fault using Doernenberg, Rogers’s method and TDCG limits. However it is again emphasized that DGA shall give misleading results unless certain precautions are taken. These are proper sampling procedure, Type of sampling bottle, cleanliness of bottle, duration of storage, method of gas extraction, good testing equipment and skilled manpower. 2.5

Trend Analysis

Transformers from the same manufacturer and of same type some time exhibit initially specific pattern of gas evolution which subsequently slows down (or reach a plateau) is called Fingerprints or normal characteristics, which are characteristic to the transformer and do not represent an incipient fault condition.


303

When a possible incipient fault condition is identified for first time, it is advisable to determine gassing trend with subsequent analysis giving information such as which gases are currently being generated and rate of generation of these gases. The level of gases generated in subsequent analysis provides a baseline from which future judgement can be made. In the examination of trends, key gases, TDCG, CO2/CO ratio, rate of gas generation and fingerprints (of normal trends) of particular transformer should also be considered. The ratio of gas generation is a function of load supplied by the transformers and this information is vital in determining the severity of fault condition and decision of removal of the equipment from service for further investigation. Two methods are suggested in literature for assessing the gassing rate: •

Change of concentration of gas in ppm

•

Determination of actual amount of gas generated

General guidelines for rate of gas generation for removal of transformer from service are 100 ppm/day and 0.1 cub feet (0.003 m3) gas per day A flow chart is given at Fig. 1 for step by step action to be taken based on DGA test results giving recommendations:

Fig. 1 Flow chart for DGA from IEC 60599/ 1999


304 3.0

FURFURALDEHYDE ANALYSIS REMNANT LIFE MEASUREMENT OF PAPER INSULATION

Degradation of insulating paper can be ascertained by direct or indirect methods. Direct method employs Degree of Polymerization (DP), which requires a physical paper sample from the winding. However, this method being destructive to transformer, cannot be employed as a routine condition monitoring exercise. 3.1

Degree of Polymerization (DP)

One of the most dependable means of determining paper deterioration and remaining life is the DP test of the cellulose. The cellulose molecule is made up of a long chain of glucose rings which form the mechanical strength of the molecule and the paper. DP is the average number of these rings in the molecule. For DP measurement remove a sample of the paper insulation about 1 centimeter square from a convenient location near the top of center phase with a pair of tweezers. In general, in a three-phase transformer, the hottest most thermally aged paper will be at the top of the center phase. If it is not possible to take a sample from the center phase, take a sample from the top of one of the other phases. Table 10 DP values for estimating remaining paper life

3.2

New insulation

1,000 DP to 1,400 DP

60% to 66% life remaining

500 DP

30% life remaining

300 DP

0 life remaining

200 DP

Furan Analysis

It is known that in addition to CO and CO2 the ageing process of the paper produces several oil soluble by products, most notably the furanoid compounds (FFA). When cellulose insulation decomposes due to overheating, chemicals, in addition to CO2 and CO, are released and dissolved in the oil. These chemical compounds are known as furanic compounds or furans. The most important one, for our purposes, is 2-furfuraldehyde. When DGAs are required, always request that furans testing be completed by the laboratory to check for paper deterioration. In healthy transformers, there are no detectable furans in the oil, or they are less than 100 part per billion (ppb). In cases where significant damage to paper insulation from heat has occurred, furan levels have been found to be at least 100 ppb and up to 70,000 ppb. The monitoring of furanic compounds by annual sampling of the oil and its analysis using High Performance Liquid Chromatography (HPLC) has beenis under used for condition monitoring on a routine basis for somein recent years by some utilities. Source : EPRI’s Guidelines for the Life Extension of Substations, 2002 Update, chapter 3. Table 7: DP Values for Estimating Remaining Paper Life


305

Generally FFAs are extracted from the oil either by solvent extraction or solid phase extraction and measured by HPLC by UV detector. The major FFA in oil is 2-Furfural and others are present in very low or undetected levels. 2-Furfural can be measured colorimetrically using spectrophotometer. This method is rapid and accurate and measures only 2-Furfural in oil. This technique is useful for quick screening of FFA in transformer oil. The relationship between the generation of these by-products and condition of in service paper is not well established. However, it has generally been seen that there is a linear relationship between FFA and DP. FFA may be used as a complimentary technique to DGA for condition monitoring. Table 11 Furans, DP, Percent of Life Used, of Paper Insulation Non-thermally upgraded paper 55 °C Rise Transformer 2FAL (ppb)

Thermally upgraded paper 65 °C Rise Transformer Total Furans (ppb)

Estimated Degree of Polymerization (DP)

Estimated Percentage of Remaining Life

Interpretation

58

51

800

100

Normal

130

100

700

90

Aging

292

195

600

79

Rate

654

381

500

66

Accelerated

1,464

745

400

50

Aging

1,720

852

380

46

Rate

2,021

974

360

42

2,374

1,113

340

38

2,789

1,273

320

33

Aging

3,277

1,455

300

29

Danger Zone

3,851

1,664

280

24

High Risk of

4,524

1,902

260

19

Failure

5,315

2,175

240

13

End of Expected

6,245

2,487

220

7

Life of Paper

7,337

2,843

200

0

Insulationand of the Transformer

Excessive

Testing is done for five different furans which are caused by different problems. The five furans and their most common causes are listed below: 5H2F (5-Hydroxymethyl-2-Furaldehyde) caused by oxidation (aging and heating) of the paper 2FOL (2-Furfurol) caused by high moisture in the paper 2FAL (2-Furaldehyde) caused by overheating 2ACF (2-Acetylfuran) caused by lightning (rarely found in DGA) 5M2F (5-Methyl-2-Furaldehyde) caused by local severe overheating (hotspot) (Source : Transformer Diagnostic USBR, June 2003, Page-38)

4.0

CONTINUOUS DGA MONITORING BY ON-LINE GAS SENSORS

4.1 Laboratory DGA is carried out at a predefined interval and the fault developing within that interval cannot be ascertained till the transformer has actually failed. It is important to


306

appreciate that some faults may take less than one year to progress from onset to failure; others may remain in a stable state for a much longer period but have the potential for a rapid increase. In case of sudden rise in gas levels indicated by on-line sensor, sample should be sent for immediate laboratory analysis for confirmation and to decide further course of action. One of the most widely used on line Gas Monitoring system has a membrane, which allows preferentially lighter molecules to pass through and be detected in a gas reaction cell. Some of the on-line gas monitors also provide continuous moisture measurement thereby ascertaining the wetness of the winding. 4.2 More recently various companies have developed Fourier transform infra Red (FTIR) detectors which will detect most of the gases of interest and quantify their amounts. These systems are not seen currently as an alternative to the very low cost of a laboratory based oil analysis. Their current role is to investigate particular transformer with gassing problems. However, an added benefit being gained is the greater understanding of the load temperature, time relationships for detected gases. This should eventually lead to better understanding and interpretation of the laboratory analysis. 5.0

CAPACITANCE & TAN δ MEASUREMENT OF BUSHINGS AND WINDINGS

Monitoring of insulation requires knowledge of insulation characteristics and judgement based on experience. Factors like ambient temperature, humidity, cleanliness of surface of bushings and electrostatic interference are to be considered before taking any reading so that error in measurement could be avoided and readings are accurate. To overcome interference due to charged switchyard, the polarity of the input voltage is reversed w. r. t. electrostatic interference and second measurement is made; the two averaged and interference is effectively reduced. The testing kit with its own sinusoidal source and interference suppression / cancellation unit is used which generates its own signal opposite in phase and magnitude to cancel out the interference. While carrying out this test in EHV substation, certain precautions like use of Interference Suppression Unit along with double-shielded leads, disconnection of jumpers and cleaning of surface of the bushings are taken. All phases of a particular winding is shorted together and also with neutral to minimize the effect of inductive currents during measurements. (a)

Bushing Dissipation factor and capacitance values are measured in UST (Ungrounded Specimen Test) mode.

(b)

Measurement for windings is carried out as per the following combinations: -

Winding to winding in UST mode.

-

Winding to Ground in GSTg mode with other winding guarded. While carrying out measurement for HV winding LV winding should be guarded whereas for LV winding HV winding shall be guarded.


307

In the absence of good data of temperature dependence for transformers manufactured in India, temperature correction factor may not be applied to measured values and try to take measurement as near to 20 °C as possible. Rate of change of tan δ and capacitance is very important. Normally Tan δ of bushings and windings at 20°C should be less than 0.007. Capacitance value can be within +10%, -5% of previous capacitance values. Rate of rise of Tan δ for bushings and windings should not be more than 0.001 per year. The rate of change of tan δ more than 0.001 per year needs further investigation 6.0

FREQUENCY RESPONSE ANALYSIS

6.1 Dielectric faults in transformers may be caused by mechanical displacements occurring during transportation, short circuit forces or inadequate clamping possibly caused by shrinkage of windings due to ageing. Such changes cannot be detected through DGA, winding resistance, C &Tan δ measurement etc. FRA has proved to be easy to perform in the field and provide a reliable indication of mechanical condition of transformers even if reference results are not available. 6.2 Standard Network/Spectrum Analyser, consisting of one main unit and one measurement unit can be used for FRA measurement. Connections of the instrument to the transformer using three coaxial test leads are shown in Fig. 2. It can be seen from the diagram that the swept frequency sinusoidal signal output (S) of approximately 2 V rms from the measurement unit of Analyser and one measuring input (R) are connected to the one end of a winding. While other end of the winding is connected to the other measuring input (T). The voltage are applied and measured with respect to the earthed transformer tank.

Fig. 2 : FRA test set up


308

6.3 It should be ensured that winding not being tested, are terminated in open condition in order to avoid the introduction of differences between the responses of three phases. The same procedure is followed on subsequent tests on the same or similar transformer, to ensure that measurements are entirely repeatable. 6.4 The voltage transfer function T1/R1 is measured for each winding for four standard frequency scans from 5 Hz to 2 MHz and amplitude and phase shift results are recorded on floppies for subsequent analysis. 6.5

Analysis of Measured Frequency Responses

Frequency responses recorded as above are analyzed in any of the following manner: •

Shift in the response of the winding

•

Differences between the responses of all the phases of the same transformer.

•

Differences between the responses of transformers of the same design.

In all the above cases major frequency shifts especially in low frequency range is cause for concern. As per EuroDoble Client Committee, the traces in general will change shape and be distorted in the low frequency range (below 5 kHz) if there is a core problem. The traces will be distorted and change shape in higher frequencies (above 10 kHz), if there is winding problem. Changes of less than 3 decibels (dB) compared to baseline traces are normal and within tolerances. In general, changes of +/- 3 dB (or more) in following frequency range may indicate following faults: Table 12 : Probable fault detectable by FRA Frequency Range

Probable fault

5 Hz to 2 kHz

Shorted turns, open circuit, residual magnetism or core movement

50 Hz to 20 kHz

Bulk movement of windings relative to each other

500 Hz to 2 MHz

Deformation within a winding

25 Hz to 10 MHz

Problems with winding leads and/or test lead placement

6.6 It is normally sufficient to consider only amplitude responses, although phase responses are being recorded too. In practical situation, FRA has confirmed of no winding movement and has enabled transformer to be returned to service quickly after incidents such as tap changer faults thus avoiding a costly internal inspection. This test is sensitive, immune to electromagnetic interference and very repeatable being insensitive to the disposition of test leads and not influenced by weather. A typical FRA curve with Amplitude and phase angle is shown in Fig. 3.


309

Fig. 3 : A typical FRA curve

6.7 It should be borne in mind that although FRA technique is very powerful and effective tool and capable of detecting a range of transformer faults, it is nevertheless primarily a mechanical condition assessment test and must be used in conjunction with other diagnostic tests if a complete picture of the condition of the transformer is to be obtained. 7.0

PARTIAL DISCHARGE MEASUREMENT

7.1 Partial discharges generate low amplitude, short duration current pulses and two different techniques are in common use to detect and measure these signals. One technique consists of measurements with a radio-noise meter and levels measured are referred as Radio Influence Voltage, “RIV” signals measured in micro-volt. The other method consists of measurements with a partial discharge detector and the signals are measured in pico-coulombs. Acoustic techniques have advantage over electrical method that this can be used on energized transformer and is not susceptible to interference from outside sources. Acoustic signals are usually measured using a transducer coupled to the wall of the transformer tank. Ultrasonic piezoelectric transducers (with a response in the range of 20 to 200 kHz), an amplifier, display device and source of high voltage are the basic component of typical test equipment for acoustic PD detection. 7.2 RIV measurement may indicate a possible problem but not the exact location or cause. Many faults are no threat to the transformer integrity but without knowledge of location or likely cause much time and money may be wasted investigating problems without success. DGA and PD detection when used in combination comprise a very effective diagnostic package. DGA being used initially to indicate the problem and PD detection usually on-line to confirm


310

location and identify it. Typical ranges of partial discharges for classification of defective and non-defective insulation of windings of transformers are given in the Table 3. Table 13 Classification of PD for defective and defect-free insulation of windings of transformers Defect free

10 – 50 pC

Normal deterioration

< 500 pC

Questionable

500 –1000 pC

Defective condition

1000 – 2500 pC

Faulty (Irreversible)

> 2500 pC

Critical

>100,000 –1,000,000 pC

(Source : CIGRE DOC. No. 227. Life Management Technique for Power Transformer)

8.0

POLARISATION SPECTRUM OR RECOVERY VOLTAGE MEASUREMENT (RVM)

8.1 RVM test is an important diagnostic tool; simple to carryout at site and in conjunction with other traditional insulation tests can be used effectively for real condition assessment of power transformers. 8.2 In this method, a DC voltage is applied between terminals of the windings under test for a certain pre-determined time (t c ), keeping all other windings grounded with tank. Afterwards the terminals are short circuited for a pre-selected time (t d) and opening short-circuit to activate the polarization phenomena, to build up a relaxation (return or recovery) voltage as shown in Fig. 4. Changing charging and discharging times in 10-2-104 sec (keeping t c / t d constant), a series of values for recovery voltage can be obtained. Plotting them as a function of t c , Polarization Spectrum curves can be drawn as shown in Fig. 5. 8.3 If a composite paper oil insulation dielectric is subjected to a sequence of charging / discharging process and measuring corresponding recovery voltages as explained above, then a plot of recovery voltages against charging times shall exhibit a dominant peak (Fig. 4). As the time constant associated with polarization effects is inversely proportional to dielectric losses in paper-oil, any processes or conditions that affect the dielectric losses have an inverse corresponding effect on the polarization time constant. Since chemical ageing tends to increase the dielectric losses of both paper and oil, the polarization time constant tends to shift towards lower values with ageing of paper-oil insulation. Thus the dominant time constant can be correlated with moisture content in the paper insulation as shown in Fig. 6.


311

Fig. 4 : Method of RVM testing, where SR – Initial Slope

t C – Charge time

t d – Discharge time

t p – Time of peak

8.4 The evaluation of polarization spectrum shown in Fig. 5 clearly indicates the changes in the insulation condition. The displacement of the curve peak towards small timeconstants signifies a degradation of the die-electric (paper insulation) as the age of transformer increases.

Fig 5 : Examples of polarization spectrum curves for various transformers of different age

Fig 6 : Dominant time constant as a function of moisture content and temperature


312

The moisture in paper can be evaluated if moisture content in oil and temperature of top oil is known from the following graph at Fig. 7. This is not an accurate method but it gives fairly good idea about the dryness of the insulation

Fig 7 Equilibrium water content of paper as a function of water content of impregnating transformer oil at various operating temperatures

Now-a-days testing instruments are commercially available that can precisely measure % water saturation of oil through which mositure in paper insulation can be computed. Table 14 : Moisture content in paper Source : IEEE Std. 62-1995 Insulation condition Dry (at commissioning) Moderate to Wet (Lower no. indicate fairly dry whereas large no. indicate moderately wet insulation) Wet Extremely wet

% Moisture by dry % Saturation of weight in paper (Wp) Waterin oil 0.5-1.0 %

< 5 %

< 2 %

6-20 %

2-4 %

21-30 %

> 4.5 %

> 30 %

(Source : CIGRE DOC. No. 227. Life Management Technique for Power Transformer)

9.0

ON LINE WINDING TEMPERATURE MEASUREMENT

9.1 The life of a transformer is shortened considerably if it is operated consistently at elevated temperatures. A winding temperature indicator (WTI) is used to monitor the temperature of winding by indirect methods and give alarm / trip commands to protection circuits when


313

temperature rises beyond the set limits to protect the transformer. This method uses a thermal image of the transformer and is based on the transformer load current. Calibration consists of assigning an assumed hot temperature to full load current. However it is not a direct measurement of the winding temperature and cannot reliably represent the transient conditions. 9.2 The direct measurement of winding temperature of transformer by hot probes with fibreoptic links is based on fluoroptic thermometry technology where a temperature sensitive phosphor is connected to the detector. Blue violet light pulses are sent down the fibre causing the phosphor to glow. Decay of fluorescence after each pulse varies accurately with temperature. The same optical fibre transmits the excitation pulses and returns the fluorescent signal. It has a very wide temperature range of measurement (-200°C to + 450°C) and has a high accuracy of + 0.1 °C. The optical signals do not get distorted in presence of even very high electromagnetic fields. The cables/links are non-corrosive in transformer oil and they do not influence the electric field inside transformer. 9.3 The change in decay time of fluorescent light is detected by a detector, which is directly calibrated in terms of temperature. The response time of the change is very high and of the order of 0.2 sec. So, fibre-optic point sensors for direct hot-spot measurement can measure the actual hot-spot and have a much faster response. Accurate knowledge of thermal behaviour and hot-spot temperature enables the manufacturer to refine designs and calculating procedures and the customer to fully utilize the overload capabilities of plant without reducing life expectancy and degrading the dielectric integrity. 10.0 THERMO VISION SCANNING OF TRANSFORMER A thermo vision camera determines the temperature distribution on the surface of the tank as well as in the vicinity of the Jumper connection to the bushing. The information obtained is useful in predicting the temperature profile within the inner surface of tank and is likely to provide approximate details of heating mechanism. The following temperature rises above ambient have been found to be practical during infrared inspections: Table 15 : Action based on temperature rise above ambient Temperature rise above ambient (ºC)

Recommendation (based on IEEE Std 62-1995)

0-10

Repair in regular maintenance schedule : Little probability of physical damage

11-39

Repair in near future; Inspect for physical dmages

40-75

Repair in the immediate future. Disassemble and check for probable damage

>76

Critical problems; Repair immediately

Note : Any decision to remove any equipment from service has to be taken based on test results, special tests if any (in totality) and manufacturers’ recommendation.


314 11.0 CONCLUSION

Condition monitoring uses sensors to provide raw data and warning signals from the equipment under surveillance. The diagnostic and condition monitoring together is a powerful tool for optimizing assets life. It also helps to reduce maintenance, failure and consequential losses and to assist in predicting residual life. Based on the results, refurbishment strategy, upgrading and replacement decisions can be taken. 12.0 REFERENCES 1.

Criteria for the interpretation of Data for DGA from transformer-Paul J. Griffin, Electrical insulating oils, ASTME, Philadelphia

2.

Dissolved Fault Gas Analysis Data –A practical approach to interpretation of Results- Dr. J.E. Morgan and W. Morse, Morgan Schaffer Corporation Bulletin, MS 25, 1993

3.

POWERGRID experience in dissolved gas analysis - An overview by Bhaskar V.K, De Bhowmick B.N and Kapur A.K. presented in CBIP Seminar on EHV Transformer Failure held on 22-23 December 1998 at Jabalpur, India

4.

Evolution of acceptance norm for dissolved gas analysis and application of DGA techniques in the monitoring and assessment of transformers and shunt reactors - user’s view by Agrawal S.K, Goswami M. M, Mata Prasad and Dwivedi P. K presented at TRAFOTECH 90

5.

Condition monitoring of power transformer - A POWERGRID experience by Bhaskar V.K, Tyagi R.K, De Bhowmick et al. presented at CBIP International Conference on Power Transformer held in April 2000 at New Delhi

6.

New guidelines for Furan analysis as well as dissolved gas analysis in oil-filled transformers by A. Mollmann and A. De Pablo, CIGRE 1996 paper: 15/21/33-19

7.

Essential requirements to maintain transformers in service by G. Breen, CIGRE 1992 paper 12-103

8.

A DC Expert system (RVM) for checking the refurbishment efficiency of high voltage oil-paper insulating system using polarization spectrum analysis in range of long time constants by Csepes, Bognar, CIGRE Paper 12-206, Paris 1994

9.

Experience in PD diagnostic tests on site based on the PD probe technique by E. Lemke presented at 5 th Workshop and Conference on EHV Technology, Banglore/ India, August 2-4,1995

10. Mechanical condition assessment of power transformers using frequency response analysis by John Lapworth and T Nooan 11. Recent developments in diagnostic techniques for substation equipments by Okabe, Ichihara et al., CIGRE 1996 paper 15/21/33-08 12. Residual life assessment and life extension of power transformers by Agrawal S.K. presented in 4th International Conference on Transformers, TRAFOTECH 94, IEEMA, Banglore 1994 13. State of the art condition monitoring and diagnostic techniques for power transformers and shunt reactors-POWERGRID experience by Bhaskar V.K, De Bhowmick B.N, Dr. Agrrawal S.K and Dube S.K. presented in 6th International Conference on Transformers, TRAFOTECH 2002, IEEMA, Bombay 2002 14. FRA and its use in detection of winding movement in power transformers by De Bhowmick B. N. presented at CBIP International Conference on Power Transformer held in April 2000 at New Delhi


315

15. EPRI Journal Sept. 1989 and IEEE Power Engineering Review; Sept’90 16. Robust fibre optic winding temperature sensing system published in Techcon 2003 Asia-Pacific PT&D June/July 2003. 17. CIGRE Guide for life management techniques for power transformers-20th Jan’03 prepared by CIGRE WG A2.18 18. IEEE Std.62-1995: IEEE Guide for diagnostic field testing of electric power apparatus-Part 1: Oil filled Power Transformers, Regulators and Reactors 19. IEC-60599-1999: Mineral oil-impregnated electrical equipment in service-Guide to the interpretation of dissolved and free gases analysis 20. IEEE std. C57.104-1991: IEEE Guide for the interpretation of gases generated in oil-immersed transformers

SECTION L

Capitalisation Formula for Transformer Losses

318


Rationalisation of Capitalisation Formula for Transformer Losses

319

SECTION L

Capitalisation Formula for Transformer Losses 1.0 The rate of capitalisation of transformer losses depends upon the rate of interest, rate of electrical energy per kWh, life of transformer and average annual loss factor. The annual loss factor takes into account the loading of the transformer during the year. In computing the rate of capitalisation of ‘Iron losses’, ‘Load losses’ and ‘Auxiliary losses’, following methodology is recommended : (i) Rate of interest (r); (ii) Rate of electrical energy (EC) : It is the cost of energy per kWh at the ‘Bus’ to which the transformer is to be connected. This has been taken as Rs. per kWh at 11 kV. (iii) Life of the transformer (n) : It is taken as 25 years. (iv) The transformer may be considered in service (LF) for (365X24) 8760 hours in a year. With modern techniques in design of Condition-monitoring etc, for calculation of losses no downtime is considered. (v) The cooling auxiliaries system has been considered in service for 40 percent of the time, the transformer is in service. (vi) Annual losses factor: LS = 0.2 LF + 0.8 (LF)2 where: LS is the annual loss factor LF is the annual load factor. This may be decided by the purchaser depending on the application of transformer. However assuming annual load factor (LF) as 60 percent, annual loss factor (LS) works out to 0.408. (vii) Capitalisation Formula Suggested, Capitalised Cost of Transformer = Initial Cost (IC) + Capitalised Cost of annual iron losses (Wi) + Capitalised cost of annual Load losses (Wc) + Capitalised cost of annual auxiliary losses (Wp).


320

(1+r )n −1 Capitalised cost of Iron losses per kW (Wi) = 8760 x e x r (1+r )n (1+r )n −1*LS Capitalised cost of Load losses per kW (Wc) = 8760 e x

x

r (1+r )n (1+r )n −1

Capitalised cost of Auxiliary losses per kW (Wp) = 0.4 8760 e x

x

x

r (1+r ) n

Note: (i)

Actual value can be worked out by the purchaser by considering appropriate values of r, EC, LF and LS.

(ii) For auto transformer, the load losses capitalisation shall consider the losses due to both HV and IV loaded to their rating with tertiary unloaded unless otherwise required by the purchaser. For other three winding transformers the loading combinations for capitalisation of losses shall be indicated by purchaser.

SECTION M

Specifications for Protective Schemes for Power and Distribution Transformers

322



323

SECTION M

Specifications for Protective Schemes for Power and Distribution Transformers 1.0

GENERAL

This section of the Transformer Manual covers recommended protection schemes for distribution and power transformers. This part specification does not purport to include all necessary provisions of a contract. For general requirements, ratings and tests reference shall be made to other sections of the Transformer Manual. 2.0 PROTECTION OF DISTRIBUTION TRANSFORMERS 2.1 Pole mounted distribution transformers of capacities ranging from 16 kVA to 200 kVA with voltage ratio of 11000/433-250 volts shall have the protection as given in Table 1. Table 1 Protection Voltage ratio

Capacity (kVA)

11000/433-250 Volts

16, 25, 63 100 and 200

Primary side

Secondary side

Dropout/Horn gap fuse

Moulded case circuit breaker (MCCB)

2.2 Ground mounted distribution transformers of capacities ranging from 200 kVA to 1600 kVA with voltage ratio of 11000/433, 33000/433 and 33000/11000 volts shall have the protection as given in Table 2. However, wherever circuit breakers are provided following protections are recommended: 2.2.1 IDMT type over current and earth fault relay and if required, delayed neutral earth fault protection to take care of high resistance faults in the outgoing feeders/transformer LT cable can be provided. 2.2.2 Oil temperature indicator with one electrical contact for alarm or trip shall be provided for distribution transformers of capacities 1000 kVA onwards. Winding temperature indicator with two electrical contacts for alarm and trip can be specified by purchaser as an optional item for distribution transformers of capacities 1000 kVA onwards. 2.2.3 Buchholz relay with alarm and trip contacts shall be provided for transformer of capacity 1000 kVA onwards.


324

Table 2 Protection Voltage ratio

Capacity (kVA)

11000/433-250 33000/433

315,630, 1000 and 1600 630, 1000, 1600

33000/11000

1600

Primary side

Secondary side

HRC/Expulsion fuse HRC/Expulsion fuse HRC/Expulsion fuse

MCCB/ACB MCCB/ACB CB

2.3 The 33 kV and 11 kV windings of the distribution transformers located outdoors and connected to overhead lines shall be protected by lightning arrestors. 3.0 PROTECTION OF 12 KV CLASS POWER TRANSFORMERS 3.1 12 kV Class Power transformers with capacities ranging above 1600 kVA as covered in Section 'C' of Transformer Manual shall have the following protection : l

Circuit breakers both on primary and secondary side.

l

IDMT over current and earth fault relay shall be provided on the 11 kV side.

l

l

l

l

IDMT over current and earth fault relay shall be provided on the secondary side and if required, delayed neutral earth fault protection to take care of high resistance faults in the outgoing feeders/Transformer LT cable can be provided Buchholz relay with alarm and trip contact. Winding temperature indicator with alarm and trip contacts, pressure relief device with trip contact shall be provided. However, oil temperature indicator with alarm and trip contacts, oil level indicator with alarm contact can be specified by the purchaser as optional items. Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and connected to overhead lines.

4.0 PROTECTION OF 36 KV CLASS POWER TRANSFORMERS 4.1 36 kV class power transformers of capacities ranging from 3.15 MVA and above as included in Section 'D' of the Transformer Manual shall have the following protection : l

Circuit breakers both on the primary and secondary sides.

l

High speed percentage biased differential relay with second harmonic restraint.

l

IDMT type overcurrent relay with high set elements on the primary side.


l

IDMT type over current and earth fault relay on the secondary side.

l

Oil temperature indicator with one electrical contact for alarm or trip contact.

l

Buchholz relay with alarm and trip contact.

l

l

325

Winding temperature indicator with three electrical contacts for (a) alarm (b) trip (c) control of fan for transformers above 10 MVA can be specified by the purchaser. Lightning arrestors on both primary and secondary sides when the transformer is outdoors and connected to overhead lines.

l

Oil surge protection for OLTC (if provided) diverter tank with trip contact.

l

Pressure relief device with trip contact.

l

Oil level indicator with alarm contact shall be provided

5.0 PROTECTION OF 72.5 KV CLASS POWER TRANSFORMERS 5.1 72.5 kV class Power Transformers with capacities as covered in Section 'D' of Transformer Manual shall have the following protection : l

Circuit breakers on both primary and secondary sides.

l


l

Back up IDMT over current and earth fault relay on primary side.

l

Back up IDMT over current and earth fault relay on the secondary side.

l

Oil temperature indicator with alarm and trip contact.

l


l

l

l

Winding temperature indicator with three electrical contacts for (a) alarm (b) trip (c) control of fan for transformers above 10 MVA can be specified by the purchaser. Magnetic oil gauge with low oil level alarm contact. Lightning arrestors on both primary and secondary sides when the transformer is outdoors and connected to overhead lines.

l

Oil surge protection for OLTC diverter tank with trip contact.

l


l

Restricted earth fault protection


326 l

6.0

In case of non-clearance of fault on operation of transformer protection, utilities may consider tripping of all other circuit breakers feeding the fault by devising appropriate scheme logic. PROTECTION OF 145 AND 245 KV CLASS POWER TRANSFORMERS

6.1 145 kV class power transformers with capacities as covered in Section 'E' of Transformer Manual and 245 kV class power transformers with capacities as covered in Section 'F' of Transformer Manual shall have the following protection : l


l

High speed percentage biased differential relay with second harmonic restraint

l

Restricted earth fault relay on star connected primary and secondary sides.

l

Back up IDMT over current and earth fault relay on the primary and secondary sides

l

Overflux relay.

l


l


l

Magnetic oil gauge with low level alarm contact.

l

Oil surge protection for OL TC diverter tank with trip contact.

l

l

l

l

l

7.0

Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and is connected to the overhead lines. Pressure relief device with trip contact. Winding temperature indicator with four electrical contacts for (a) alarm (b) trip (c) control of fan and pumps for transformers above 10 MVA can be specified by the purchaser Transformer fire protection trip and isolation of power supply to cooling fans and pumps Local breaker backup relays on all sides of Transformer PROTECTION OF 145 KV AND 245 KV CLASS INTERCONNECTING AUTO TRANSFORMERS

7.1 145 kV and 245 kV class interconnecting auto transformers with capacities as covered in Section 'F' of Transformer Manual shall have the following protection : l



l

High speed percentage biased differential relay with second harmonic restraint

l

Restricted earth fault relay.

l

Back up directional over current and earth fault relay on the primary and secondary sides

l

Overfluxing relay.

l


l


l

Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF).

l


l


l

l

l

327

Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and is connected to overhead lines. Pressure relief device with trip contact. Transformer fire protection trip and isolation of power supply to cooling fans and pumps.

l

Definite time over current relay for alarm.

l

Local breaker backup relays on all sides of auto transformer.

7.2 In addition to the protection mentioned in this clause, for large inter connecting auto transformer (inter connecting two different systems/utilities) with OLTC and tertiary delta winding along with neutral grounding transformer for tertiary winding, the following additional protection may be provided. The typical protection diagrams showing Relaying connection and trip logics are enclosed as Annexures I and II. (i)

Restricted earthfault relay for neutral grounding transformer.

(ii)

Definite time over current relays on secondary and tertiary side of auto transformer for alarm.

(iii)

Overcurrent relay on primary neutral.

(iv)

Overcurrent and earthfault protection on tertiary side.

(v)

Overcurrent relays on neutral of NGT.

(vi)

Buchholz relay for NGT with alarm and trip contact

(vii)

Overload trimming relays on secondary and tertiary sides for load trimming.


328

Teed protection is used wherever the transformer is charged on primary side and secondary side breaker/isolator are open. In that case backup directional overcurrent/earthfault relays on secondary side are non-operative. Hence, the instantaneous overcurrent protection provided on bushing CTs of the transformer to be wired for trip through secondary side breaker/isolator ‘b’ contact, will be provided as backup protection to transformer differential protection. 8.0

PROTECTION OF 420 KV CLASS AUTO TRANSFORMERS

8.1 420 kV class Auto Transformers with capacities as covered in Section 'G' of Transformer Manual shall have the following protection. l

High speed percentage biased differential relay with harmonic restraint.

l

Restricted earth-fault relay.

l

Neutral displacement relay or restricted earth fault relay for protection against ground faults in the tertiary winding/associated connections depending upon the tertiary earthing arrangements.

l

Backup directional overcurrent and earth fault or impedance relays.

l

Overf1ux relay.

l


l


l


l


l

Lightning arrestors on both sides of the transformer.

l


l

Transformer fire protection trip and isolation of power supply to cooling fans and pumps

l

Definite time over current relay for alarm.

l

Oil surge protection for OLTC diverter tank with trip contact

l

Local breaker backup relays on all sides of auto transformer


9.0

329

PROTECTION OF GENERATOR TRANSFORMERS

9.1 Generator transformers with capacities as covered in Sections 'E' and 'G' of the Transformer Manual shall have the following protection : l

l

Circuit breakers on HV side. Overall differential current relay covering the generator zone also, in addition to transformer differential protection.

l

Restricted earth fault relay on the HV side.

l

Overfluxing relay.

l

Neutral overcurrent relay against sustained external system earth faults.

l


l


l

l

l

Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). Magnetic oil gauge with low level alarm contact. Lightning arrestors on the HV side when the transformer is located outdoors and is connected to the overhead lines.

l


l

Oil flow indicator with one contact for alarm, wherever applicable.

l

Water flow indicator with one contact for alarm wherever applicable.

l

l

l

Transformer fire protection trip and isolation of power supply to cooling fans and pumps Oil surge protection for OLTC diverter tank with trip contact Local breaker backup relays

10.0 PROTECTION OF 765 KV AUTO TRANSFORMERS 10.1

765 kV Auto Transformers shall have the following protection:

l


l

Restricted earth-fault relay.


330 l

l

Neutral displacement relay or restricted earth fault relay for protection against ground faults in the tertiary winding/associated connections depending upon the tertiary earthing arrangements. Backup directional over current and earth fault relay with non-directional high-set feature or impedance relays.

l

Back-up neutral E/F protection with IDMT characteristics.

l

Over flux relay for both HV & MV/LV side.

l


l


l


l


l

Lightning arrestors on both sides of the transformer.

l

Pressure relief device with trip contact

l

Transformer fire protection trip and isolation of power supply to cooling fans and pumps.

l

Definite time over current relay for alarm in HV side.

l


l

Local breaker back up relays on all sides of Auto Transformer.

11.0 PROTECTION OF 765 KV GENERATOR TRANSFORMERS l

Generator transformers shall have the following protection:

l

Circuit breakers on HV and LV side.

l

Overall differential current relay covering the generator zone also, in addition to transformer differential protection.

l

Generator transformer differential protection

l

Restricted earth fault relay on the HV side.

l

l

Overhead line connection differential protection including generator transformer HV winding Overfluxing relay.


l

Neutral overcurrent relay against sustained external system earth faults.

l


l


l

l

l

Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). Magnetic oil gauge with low level alarm contact Lightning arrestors on the H.V side when the transformer is located outdoors and is connected to the overhead lines.

l


l

Oil flow indicator with one contact for alarm wherever applicable.

l

Water flow indicator with one contact for alarm wherever applicable.

l

331

Transformer fire protection trip and isolation of power supply to cooling fans and pumps.

l

Buchholz relay with alarm & trip contact for OLTC

l


l

Local breaker back up relays.

Annexure I

332


Annexure II


333

SECTION N

Specifications for Voltage Control of Power Transformers

336



337

SECTION N

Specifications for Voltage Control of Power Transformers This section covers voltage control of OFF-circuit as well as ON-load type. 1.0 VOLTAGE CONTROL (OFF-CIRCUIT TYPE) 1.1 When specified, each transformer shall be provided with off-circuit tap changing switch or off-circuit links for varying its effective ratio of transformation whilst the transformer is de-energised and without producing phase displacement. 1.2 The off-circuit switch handle will be provided with a locking arrangement along with tap position indicator, thus, enabling the switch to be locked in position. A warning plate indicating that switch shall be operated only when the transformer is de-energised shall be fitted. 2.0 VOLTAGE CONTROL (ON-LOAD TYPE) 2.1 When specified, each transformer shall be provided with voltage control equipment of the tap changing type for varying its effective transformation ratio whilst the transformers are on-load and without producing phase displacement. 2.2 Equipment for local and remote electrical and local manual operation shall be provided and shall comply with the following conditions. Local remote switch may be housed in remote control panel or in tap changer driving unit. 2.2.1 It shall not be possible to operate the electric drive when the manual operating gear is in use. 2.2.2 It shall not be possible for any two electric controls to be in operation at the same time. 2.2.3 The equipment shall be suitable if specified for supervisory control and indication on multi-way switch, make-before-break, having one fixed contact for each tap position, shall be provided and when specified, wired to the tapchanger drive gear. This switch shall be provided in addition to any which may be required for remote tap position indication purposes. Supervisory indication shall also be provided when specified in the form of contacts to close on "Tapchange incomplete". All other components of the supervisory gear if required will be specified separately. 2.2.4 Operation from the local of remote control switch or push button shall cause one tap movement only until the control switch or push button is returned to the off position between successive operations. 2.2.5 All electrical control switches and the local operating gear shall be clearly labelled in a suitable manner to indicate the direction of tapchanging.

338


2.2.6 The local control switches shall be mounted in the marshalling box, or driving gear housing. 2.3 The equipment shall be so arranged as to ensure that when a tapchange has been commenced it shall be completed independently of the operation of the control relays or switches. If a failure of the auxiliary supply during a tapchange or any other contingency such as tapchanger getting stuck would result in that movement not being completed, adequate means shall be provided to safeguard the transformer and its auxiliary equipment. 2.4 Suitable apparatus shall be provided for each transformer to give indications as follows : 2.4.1 To give indication, mechanically at the transformer and electrically at the remote control point if specified, of the number of the tapping in use on the transformer. 2.4.2 To give an indication at the remote control point that a tapchange is in progress, by means of an illuminated lamp. 2.5 For remote control, the switches, push button tap position indicator, etc., shall all be supplied as loose apparatus unless a remote control panel is specified. 2.6 All relays and operating devices shall operate correctly, at any voltage between the limits specified in the relevant Indian Standard. 2.7 The tapchanging switches and mechanism shall be mounted in oil tanks or compartments mounted in an accessible position on the transformer tank. 2.8 Any enclosed compartment not oil filled shall be adequately ventilated. Metal clad thermostatically controlled heaters shall be provided in the driving mechanism chamber and in the marshalling box. All contactors, relay coils or other parts shall be suitably protected against corrosion or deterioration due to condensation, fungi, etc. 2.9 The tap changer contacts which are not used for making or breaking current like separate selector switch contacts can be located inside main transformer tank where tapchanger construction permits such an arrangement. On load tapchangers having separate compartment for selector contacts, the oil in such compartment shall be maintained under conservator head by means of pipe connection from the highest point of the chamber to the conservator. Such connection shall be controlled by suitable valve and shall be arranged so that any gas leaving the chamber will pass into the gas and oil actuated relay. A separate buchholz relay may be provided for this compartment. 2.10 It shall not be possible for the oil in these compartments of the tapchange equipment, which contain contacts used for making or breaking current, to mix with the oil in the compartments containing contacts not used for making or breaking current. 2.11 Any 'DROP DOWN' tanks associated with the tapchanging apparatus shall be fitted with guide rods to control the movement during lifting or lowering operations. The guide rods shall be so designed as to take support of the associated tank when in the fully lowered position with oil. Lifting gear fitted to 'DROP DOWN' tanks shall include suitable device to prevent run-away during lifting and lowering operations. They shall be provided with adequate breathing arrangement. If specified the tapchanger shall be mounted in such a way


339

that the cover of the transformer can be lifted without removing connections between windings and tapchanger. 2.12 Each compartment in which the oil is not maintained under conservator head shall be provided with a suitable direct reading oil gauge. 2.13 The alternating supply for electrical operation of the control and indicating gear shall be standard 415 volts, three-phase, 3 wire, 50 Hz, along with 240 volts single phase, 2 wire 50 Hz, subject to a variation of + 10 per cent so that the equipment offered can withstand variation in A.C. 2.14 Limit switches shall be provided to prevent over running of the mechanism and except where modified in clause 2.15 shall be directly connected in the circuit of the operating motor. In addition a mechanical stop or other approved device shall be provided to prevent over-running of the mechanism under any condition. 2.15 Limit switches may be connected in the control circuit of the operating motor provided that a mechanical declutching mechanism incorporated. 2.16 Thermal devices or other means like motor circuit breakers with shunt tripcoil shall be provided to protect the motor and control circuits. All relays, switches, fuses, etc., shall be mounted in the marshalling box or driving gear housing and shall be clearly marked for purposes of identification. They shall withstand the vibration associated with tapchanger gear operation. 2.17 The control circuits shall operate at 110 V single phase to be supplied from a transformer having a ratio of 415 or 240/55-0-55 V with the centre point earthed through a removable link mounted in the marshalling box or tapchanger drive. 2.18 The whole of the apparatus shall be of robust design and capable of giving satisfactory service without undue maintenance under the conditions to be met in service, including frequent operation. 2.19 A five-digit counter shall be fitted to the tapchanging mechanism to indicate the number of operations completed by the equipment. 2.20 A permanently legible lubrication chart shall be fitted within the driving mechanism chamber, where applicable. 2.21 Loose equipments shall be supplied for mounting on the purchaser's control panel and the control panel will be additionally supplied as and when required by the purchaser. 3.0 PARALLEL OPERATION OF TRANSFORMERS WITH ON-LOAD TAPCHANGER 3.1 Besides the local and remote electrical control specified in clause 2 on-load tapchangers, when specified, should be suitable for remote electrical parallel control as in clause 3.2. 3.2 Remote Electrical Parallel Control 3.2.1 In addition to the methods of control as in clause 2, the following additional provision shall be made.

340


3.2.2 Suitable selector switch be provided, so that anyone transformer of the group can at a time be selected as 'Master', 'Follower' or 'Independent'. 3.2.3 Necessary interlock blocking independent control when the units are in parallel, shall be provided. 3.2.4 The scheme will be such that only one transformer of a group can be selected as 'Master'. 3.2.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tapchanging when transformers in a group operating in 'Parallel control' are one tap out-of-step. 4.0 ON-LOAD T APCHANGER CONTROL SCHEME 4.1 The control scheme for tapchanger can be as under : (i) Non-automatic independent-as per Scheme 1. The scheme used for independent control from local or remote panel. (ii) Non-auto/automatic independent-as per Scheme 2. The scheme used for independent control with automatic voltage control relay and line drop compensation as optional. If required non-auto condition can be availed. (iii) Non-automatic simultaneous parallel operation-as per Scheme 3. The scheme used for non-automatic simultaneous parallel operation. (iv) Non-auto/automatic simultaneous parallel operation-as per Scheme 4, Sheet 1 and 2. The scheme used for automatic simultaneous parallel operation with facility for use with non-auto condition also. Sheet I of the Scheme represents main control scheme and with sheet 2 line drop compensation control scheme can be achieved as optional. 4.2 General Local control items shall be mounted inside the on-load tapchanger driving mechanism or marshalling box. Remote control items are to be mounted on remote control cubicle installed in the control room. All the control items are to be mounted in easily accessible position and clearly labelled. All the control item shall be of best quality and or class most suitable for working under the conditions specified and shall withstand the variation of temperatures and atmospheric condition arising under working conditions so also withstand vibrations. All the control items shall be wired and connected as per 'Schematic Diagram of tapchanger control equipment given in Scheme 1. 4.3 Motor On-load tap changer driving gear Motor shall be of squirrel cage totally enclosed type and shall comply with Indian Standard IS: 325. It shall be suitable for direct starting and continuous running from 415 volts 3-phase or 240 volts single phase 50 Hz supply. Motor shall be capable of continuous operation at any frequency between 48 and 51.5 Hz together with any voltage within 10 per cent of nominal value. Motor shall have ball or roller bearing and vertical spindle motor shall have bearing capable of withstanding thrust due to the weight of the moving parts. The stator windings shall be adequately braced and suitably impregnated to render them non-hygroscopic.


341

342



343

SCHEME 4 (Sheet 1)

344



345

346


4.4 Overload Protection Relay The overload protection relay shall be of robust, adjustable triple-pole construction. It should provide accurate and reliable protection against overload, single phasing, over heating and short circuit. The relay should be provided with temperature compensating device to off-set the effect of ambient temperature variation. For single phase motor, over-load protection device with feature similar to those of the three-phase motor as far as these are applicable shall be provided. 4.5 Contactors/ Relays Contactors/Relays shall be of robust and compact construction and shall comply with Indian Standard IS : 2959. The electromagnetically operated air break type contactor with sufficient number of contacts shall be suitable for mounting on a vertical supporting structure. The contactors shall be suitable for operation at 110 Volts A.C.-15 per cent to + 10 per cent 50 Hz. Main and auxiliary contacts of contactor shall be suitably rated. For sufficient long life these contacts shall be break type and shall make contacts practically bounce-free. 4.6 Control Supply Transformer The control supply transformer shall be single phase having ratio 240/55-0-55 or415/55-055. Its insulation shall be suitably impregnated to render it non-hygroscopic. 4.7 Control Selector Switches All the control selector switches shall be of robust and compact construction and shall comply with Indian Standard IS : 4064 and 4047. The control switches shall be suitable for on-load switching of resistive and inductive loads. The switches shall incorporate multi air break type wiping contacts housed in an assembly of packets moulded from anti tracking material. The knob of the handle of the switch shall be suitably designed so that while operating a firm grip is obtained. 4.8 Remote Tap Position Indicator Remote tap position indicator mounted on remote control panel shall show accurately same tap position as indicated by local tap position indicator on on-load tapchanger. The remote indication can be by means of an analogue indicator, or digital indicator or by means of lamp indications. Transmitter switch in the driving gear shall be make before break type. This switch in the driving gear shall be mounted in accessible position so that it can be cleaned and maintained regularly. The remote indicator mounted on control panel shall not be affected by normal auxiliary voltages supply variation.


347

4.9 Indicating Lamp Necessary indicating lamps provided shall be of low watt consumption and of filament type or Neon or LED type. Lamps shall be of such construction so that these can be replaced very easily. 4.10 Space Heater Space heater of adequate capacity and robust construction shall be provided inside each control cabinet to prevent moisture condensation. Space heaters shall be rated for 240 volts 1 phase, 50 Hz supply. Heater shall be complete with miniature circuit breaker or "ON-OFF" switch. HRC fuse on phase and link on the neutral. Mounting of the heater and its location shall not cause localised intensive heating of control equipment and wiring. 4.11 Wiring All the wiring shall be carried out for motor circuit with 1100 volts grade PVC insulated stranded copper conductors of size 2.5 sq mm and for control circuit with 650 volts grade PVC insulated copper conductor of size 1.5 sq mm suitable for tropical atmosphere. All wiring shall be in accordance with relevant IS. Engraved core identification ferrules, marked to correspond with the wiring diagram shall be fitted at both ends of each wire. Ferrules shall fit tightly on the wires and shall not fall off when the wire is removed. All wiring shall be terminated on terminal blocks through suitable lugs. Insulated sleeves shall be provided at all the wire terminations. All wiring shall be neatly bunched and cleated without affecting access to equipment mounted within the cabinet. One piece moulded 1100 V grade terminal blocks complete with insulation barriers, terminal studs, washers, nuts and lock nuts shall be used. Terminal blocks shall be numbered for identification and grouped according to function. 10 per cent spare terminal blocks for control wire termination shall be provided on each panel. Terminal board rows should be spaced adequately apart to permit convenient access to wires and terminations. Terminal boards shall be so placed with respect to the cable gland plate (at a minimum distance of 70 mm) as to permit satisfactory arrangement of multicore cable tails without undue stress or bends. Opening of door should not disturb or stress the wire termination. 4.12 Voltage Regulating Relay (i) Introduction: Voltage Regulating Relay is used for regulating the secondary voltage of power transformers with on-load tapchangers. The required dead band settings are set by setting the nominal value, and lower and upper levels independently. The time delay setting on the front panel eliminates the relay operations for momentary fluctuations of the regulated voltage thus reducing the number of operations of the tap changer. " When the regulated voltage falls below the specified undervoltage limit, the control\ relays are automatically blocked, i.e., there is no voltage correction, and a pair of contacts is made available for alarm.

348


(ii) General Description: Voltage regulating relay should be designed for maximum operational simplicity for regulating the secondary voltage of power transformer with onload tapchangers. The deadband (band width) can be set by setting the nominal value adjustment (NVA) to the required value "110 V + 10 per cent". The desired time delay can be set on the front panel and the control action will take place only if the voltage continues to remain outside the deadband after the time delay has elapsed. For voltage corrections requiring more than one tap change, time delay is initiated again before further tapchange. The relay is reset automatically after the voltage is brought within the selected deadband. For repeated short duration voltage fluctuations on the same side of the deadband, the time delay is effectively reduced to provide a voltage time integral response of the regulator. Operation of the Raise Control relay is automatically inhibited when the voltage falls below the specified under-voltage limit. One pair of normally open relay contacts are provided to effect the tapchanger, Raise and Lower operation and to trigger an alarm in case of undervoltage conditions. (iii) Specifications Auxiliary Supply

: -15% 50 Hz + 10%

PT Supply (regulated Voltage)

: 110 V + 10 per cent 50 Hz

Sensitivity (Dead Band) : Nominal value adjustable (NVA) and Nominal Value Range between + 0.75 to + 2.5 per cent Time Delay Setting

: Fixed, i.e., (Voltage independent) time delay continuously adjustable from 10 to 110 secs.

Time Delay Resetting

: Instantaneous resetting with voltage deviation occurring in opposite direction.

Under Voltage Blocking

: Internal blocking at 80 per cent of regulated value. Restoration at 85 per cent of regulated value.

Control Relays

: One pair of normally open potential free contacts of suitable rating.

Control Operation

: Single pulsed operation of sufficient duration to initiate tapchanger.

Operating Temperature

: – 5o to + 50°C

Option

: Line drop compensator with resistive and reactive compensation of either polarity upto 20 per cent adjustable in steps or continuously and suitable for operation with 1 Amp. current transformer. If required suitable interposing current transformer to be used to get 1 Amp. secondary current. 1 pair of NC (UV) contacts provided.


349

4.13 Line Drop Compensator (LDC) : Description I The Line Drop Compensator is an optional unit designed to match with the Automatic Voltage Regulator Relay. The unit is housed in the same enclosure or separately mounted. The voltage at the generating end and at the receiving end are not the same due to the drop across the line. The LDC is used to compensate for this line drop, and the amount of compensation required is calculated as a per cent of the nominal voltage knowing the length of the line, its resistance/unit length, its reactance/unit length and the rated current, and set on the front panel. The line current is stepped down to 1 Amp. and fed to the LDC. The resistive and reactive drops are simulated by having 90o phase shifted voltage and their polarity selected by polarity switches. The net compensation is then fed to the stepped down PT voltage. II Specifications Resistive Compensation

: 0-20 per cent of the regulating value continuously adjustable.

Reactive Compensation

: 0-20 per cent of the regulating value continuously adjustable.

Input Rated Current

: 1 Amp. 50 Hz.

Power Consumption

: As required (CT burden will depend on power consumption).

Accuracy

: 10 per cent.

Max. Overcurrent

: 50 per cent of rated current (1.5 Amp.)

Polarity Selection

: Both positive and negative compensation.

III Operating and Connection Requirements Connection to the LDC unit are made through the rear panel terminals. The line current is stepped down to 1 Amp. 50 Hz and fed to the LDC. The net compensation is fed to the AVR circuit. internally or external connections. The required amount of percent R and per cent X compensation can. be set on the front panel of the LDC. The polarity selector switches provide both positive and negative compensation. The per cent R and per cent X settings can be calculated from the following formulae: √ 3ILRL Per cent R =

x

100 per cent

VL √ 3ILXL Per cent X = 100 per cent VL x

350 where


IL = the primary rated current of the line. VL = the voltage between lines of the power transformer. XL = the line reactance in ohms/phase. RL = the line resistance in ohms/phase.

Note : When LDC unit is not to be used, keep per cent R and per cent X settings to Zero, i.e., on min. position.

5.0 NOTES ON TAP CHANGER SCHEME 5.1 Note on Schematic Diagram of Tap Changer for Parallel Control The parallel scheme is prepared keeping in mind that any one of the on-load tapchanger is selected to the master position and the other to the follower position. This is being done by selector switch SSS provided on control cubicles for each transformer. With selector switches SSS in their respective position as above bus wires N 504 and N 505 become source of supply to the OLTC control circuits of all the transformers. Selecting Local/Remote switches CSS in remote position one can press Master unit push button RPB or LPB, for desired raise or lower operation, a direct impulse for Raise or Lower goes to Master unit OLTC. Also through Master unit selector switch SSS the bus wires N 506 or N 507 energise and transmit Raise or Lower impulse to follower OLTC unit through their own selector switch SSS. 5.2 Out-of-Step Circuit Out-of-step circuit is designed on the principle of odd/even position of on-load tap changers. Through the link provided on first unit between bus wire N 504 and N 513 bus wire N 518 becomes source of supply to out-of-step relays of all the units. Under normal working condition the out-of-step relays of all the units are energised. The out-of-step relays of all the follower units get supply from bus wire N 511 or N 512 through their own odd/even switch OSS. The bus wire N 511 or N 512 gets supply from bus wire N 515 through Master units selector switch SSS and odd/even switch OSS. The out-ofstep relay of Master unit gets supply from bus wire N 518 through selector switches SSS and contacts of out-of-step relays of follower units. When any of the on-load tapchanger from group lags behind by one step or moves ahead by one step, for what so ever the reason may be, creates condition of out-of-step of transformers running in parallel. Under the out-of-step condition the supply to the out-of-step relay is cutoff and hence it gets de-energised and one of its contact energises a time delay relay TDR which initiate visual and audible alarm. 5.3 Automatic Control Automatic control feature is used to keep voltage constant at desired level, 110 volts 2 wire PT supply should be arranged as input to AVC relay. OSS switch is to be set on auto mode by which AVC relay circuit comes into operation. The setting are to be done as required prior to commissioning the relay. 5.4 Line Drop Compensation The feature to be used along with AVC relay to compensate for the voltage drop across the line and to maintain desired voltage at receiving end. Input to LDC device through secondary current 1 Amp. from line CT and output from LDC is connected with AVC. Suitable adjustments for line resistive and reactive compensation to be made on LDC to suit.

SECTION O

Specifications for Fire Protection of Power Transformers

352



353

SECTION O

Specifications for Fire Protection of Power Transformers 1.0

GENERAL

1.1

Introduction

The hazard of fire originating in or spreading to power transformers has always been recognised in the Power Industry. With the increasing size of generating units and associated transmission and distribution networks the number of transformers or large capacities has increased phenomenally thus necessitating more stringent protection measures to prevent fire risk to transformers and damage to equipment. This section of the transformer specification discusses the various aspects of transformer fire protection. 1.2

Strategy

The strategy for safeguarding against fire is to emphasize fire prevention rather than fire fighting. Nevertheless, it is equally important to provide adequate fire fighting arrangements. This applies also to associated equipment like bushings, circuit breakers, instrument transformer, cables, etc., since a fire originating in these can easily spread to the power transformer. In effect, all the transformer installation should be provided with the prevention as well as fire detection and fighting systems. 1.3

Other Factors

Factors like proximity of the transformer to buildings and other equipment such as switchgear play an important role in design of the fire prevention scheme. It is desirable that these equipments be segregated from the transformer installation or be provided with fire prevention measures to avoid spread of fire to the transformer installations. 2.0

DESIGN CONSIDERATIONS

A majority of fires originating in transformers are due to inadequate design and installation, apart from faulty operation and maintenance practices, Proper installation, house keeping and maintenance can reduce fire hazards to a great extent. Hence, fire hazards must be given utmost attention while designing, selecting and installing power transformers and correct operation and maintenance procedures must be adhered to strictly. 2.1

Bushings

Bushings are often the source of transformer failures and consequent fires. This is due to the fact that dielectrie stresses in bushings are very high and sometimes oil tightness may not be ensured. Only bushings of proven design, which have been fully type tested and have passed all acceptance tests shall be used, Bushings shall be provided with test taps and regular


354

(annual) checks of bushing tan delta shall be carried out. Also, the oil level, in oil filled bushings, shall be checked daily. 2.2

Tapchanger

Tap changer, particularly on-load type are a potential fire risk. The selection of tap changer design, and proper maintenance of its mechanism are important. In no case should an offload tap changer be operated when the transformer is energised from any of its windings. 2.3

Cable Sealing Ends

The level of compound in the sealing ends should be checked periodically. 3.0

INSTALLATION REQUIREMENTS

The general recommendations for safeguarding power transformers are given below and specific recommendations are given in clause 4. The requirement relevant to transformers, in increasing order of importance of installation are: (a)

Soak pits

(b)

Drain pits

(c)

Barrier walls

(d)

Fire detection system

(e)

Fire hydrant

(f)

Deluge, spray or mulsifyre system

In case of remote controlled or unattended substations, automatic fire detection and fighting system must be provided. 3.1

Outdoor Transformers

(a)

Soak Pit and Drain Pit

The transformers foundation shall be surrounded by a suitable soak pit enclosed by a 150 mm high non-combustible curb. This soak pit shall be filled with coarse crushed stones about 25 mm in diameter to a minimum depth of 300 mm. The volume of the soak pit minus the volume of the stones should be sufficient to contain the entire oil content of the transformer if the oil content is less than or equal to 5 kl. In case the oil content is more than 5 kl, the volume of soak pit minus the volume of stones should be sufficient to contain at least one third of the total oil content. The excess should be led through two or more hume/concrete pipes (min. 150 mm dia.) from bottom of pit to a central remote burnt oil tank.


355

Control cables emanating from transformers shall be led through the soak pit through hume RCC pipes. The marshalling kiosk of the transformer shall be installed outside the pit and away from the potential fire hazard zone. (b) Barriers between Transformers Barrier wall of brick or reinforced cement concrete shall be provided for separation of transformers wherever adequate space is not available (Refer clause 4). The barriers shall extend at least 300 mm above the highest transformer bushing and pressure relief vent and lengthwise 600 mm beyond the transformer including any radiators and tap changer enclosures. (c) Fire Detection, Hydrant and Deluge Systems Selection of these systems should be based on the importance of an installation. The fire detection system only detects a fire and sends an alarm, whereas the other systems are active fire fighting systems. Selection guidelines are given in clause 4. 3.2 Indoor Transformers (a)

(b)

(c)

3.3

For indoor installation with oil filled transformers rated at more than 75 kVA, each of the transformer should be located in a vault. Transformer vaults should preferably have a minimum fire resistance rating of 3 hours but where transformers are protected by water spray, Carbon dioxide systems, construction with a 1 hour rating is adequate. Facilities for remote monitoring of vault temperature shall be provided. It is recommended that a trapped floor drain be provided which discharges burning oil to a safe location. A fire door and a noncombustible curb at least 100 mm high should be provided at each doorway. Switchboards etc., should be physically separated from the vaults and the latter should never be used as offices, filing cabinets, work rooms etc. Adequate ventilation should be provided in the transformer vault. Self cooled transformer should be separated by 600 to 750 mm to permit free air circulation. It is recommended that air filters be provided on all vents wherever the installation is located in dusty, high polution areas. The vent should automatically close in the event of a fire in the vault. General

The requirements laid down in Section 5 of Tariff Advisory Committee's "Regulations for the Electrical Equipment of Buildings" and Section 7.9 of IS : 10028 (part II) shall be followed for all transformer installations. In addition, the following measures are recommended for cables : (a)

The power cables entering the transformer shall be coated with fire resistant material in the immediate vicinity of the transformer cable box entry so as to prevent spreading of fire from or to the transformer cable.

(b)

Cable trenches shall be filled with sand to prevent spread of fire.

It is recommended that trenches of more than 1000 cm2 cross-sectional area be provided with incombustible barriers at intervals not exceeding 45 metres. The barriers shall be at least 50 mm in thickness and of the same height as the cable trench. The cables shall be carried


356

through holes in the barriers which shall be made good thereafter to prevent passage of fire beyond the barriers. 4.0 INSTALLATION REQUIREMENTS: SPECIFIC A summary of various recommendations for fire protection and fighting systems for indoor and outdoor transformers is given below: 4.1 Fire Protection for Outdoor Power Transformers

(a)

Size (each) Under 10 MVA

Number One or more

Transformer Protection Hydrant protection

(b)

10 to 100 MVA

One only (See Note 2)

Hydrant protection with water spray equipment

More than one

1. Provide a 7.5 m clearance between units and 60 mm hose with two portable spray nozzles: or 2. Masonry barriers between units with 60 mm hose and two portable spray nozzles: or 3. Fixed automatic water spray with separate piping for each unit.

(c)

Above 100 MVA

One only (See Note 2) More than one (See Note 3)

(d)

Above 200 MVA

One only (See Note 2) More than one (See Note 3)

Hydrant protection with water spray equipment 1. Fixed automatic water spray (mulsifyre) separate riser and piping for each unit; and 2. Provide a 7.5 m clearance or masonry barriers between units. Hydrant protection with water spray equipment, however, fixed water spray protection may be desirable in addition. 1. Fixed automatic water spray with separate piping for each unit; and 2. Provide a 15 m clearance or masonry barriers between units.

Notes 1. The enclosure should consist of a masonry barrier with wing walls of the same height extending 0.6 to 1 m beyond the transformer, including any radiators and tapchanger enclosures. The enclosure should also be provided with a roof of equal fire resistance to the walls. 2. Where there are important or high value bus structures exposed to a transformer oil fire and/or electric service or production could be interrupted for an extended period resulting in a large loss, a fixed automatic water spray system should be provided to minimize the physical damage from fire and reduce the downtime for repairs. 3. Multiple transformer of 100 MVA and above may be protected as single units if separated by a minimum of 35 m. For those separated by a distance between the minimum clearance shown and 35 m transformer protection may be either a barrier or fixed water spray. 4. Cables, isolated bus duct, or cable tray penetrating an exposed wall should be sealed with a fire barrier or stop. Ventilation louvers should be relocated to an unexposed area. 5. Wherever water spray nozzles are provided, the nozzle should be separated from the transformer by over 4 m for voltages below 250 kV. For voltages beyond this and for solid hose streams, this distance varies with factors such as water pressure, wind velocity and direction, size of nozzle etc.


4.2

Fire Protection for Indoor Power Transformers Typical size of Transformer

Type of Building/Transformer Protection (See Note 2) encl. (See Note 1)

(a)

Upto 100 kVA

Combustible

(b)

Upto 30 MVA

Non-combustible Non-combustible

Combustible

(c)

Above 30 MVA & upto 100 MVA

Any

(d)

Above 100 MVA

Any

Notes 1. 2. 3.

5.0

357

Automatic sprinklers. Alternatively, use dry type transformers. Usual first aid protection. 1. Fire resistive vault 2. 40 mm fire hose with two water spray nozzles; and 3. Usual portable extinguishers. 1. Fire resistive vault, and 2. Automatic sprinklers, water spray, CO2 system 1. Fire resistive vault, and 2. Automatic sprinklers, water spray, CO2 system. 1. Fire resistive vault 2. Automatic sprinklers water spray, CO2 system 3. Nitrogen injection system

Combustible and non-combustible buildings are generally classified based on type of construction materials used and type of occupancy. This protection applies for all new installations and at existing locations when a large loss possibility exists without improvement. If transformers are of unusual importance or large (above 10 MVA) located within fire resistive transformer houses or vaults, install fire resistive walls between units to reduce the exposure to adjacent transformers unless automatic fixed extinguishing systems of 7.5 m clearances are provided.

RECOMMENDED MAINTENANCE AND TESTING PRACTICES

It is essential to monitor certain parameters to check the healthiness of the transformer and to minimise fire risks. The parameters to be checked and their frequencies shall be as brought out elsewhere (Section K) in the transformer manual. Some of the parameters more relevant to transformer fires are discussed below. 5.1

Oil Leakage

Oil leakage from transformer tank, bushings or radiators may become sources of major fires. It is recommended that all transformer installations be inspected daily for leakage of oil. Any leakage detected should be immediately attended to. In case of excessive leakage the transformer should be de-energised and repair work carried out. 5.2

Hot-oil Circulation

During hot-oil circulation in the transformer, it must be ascertained that all combustible materials are kept at a safe distance from the transformer. The transformer shall be covered with non-combustible materials. Under such condition, it is essential that the transformer is kept under close watch.


358 5.3

Terminal Equipment

Sparks from improper terminal connectors and neighbouring fuses etc., falling on the transformer can cause great fires. To prevent such occurrences it is recommended that the terminal connectors be regularly inspected for over-heating/sparking. Infra-red temperature scanners can be used for this purpose. It is also recommended that fuses be installed at a distance from the transformer such that sparks generated during their operation will not reach the transformer. 5.4

Transformer Oil

The condition monitoring of transformer oil can give valuable insights into the healthiness of transformers. It is recommended that dielectric strength, acidity and oil tan delta (90°C) be monitored continuously and detailed investigation be carried out whenever any of these characteristics indicate signs of deterioration of oil quality. 5.5

Housekeeping

The importance of good housekeeping and cleanliness in reducing fire hazard cannot be over-emphasised. Many fires have been caused by oil drips and collection of rags in dirty cluttered surroundings. It is very important to remove possibilities of such fires, by ensuring that the installations are spacious and the vacant spaces are periodically cleaned to remove obstructions to ventilation and movement of personnel. 6.0

FIRE FIGHTING EQUIPMENT

In addition to the fire protection systems described above, it is essential to provide primary fire fighting equipment for every transformer installation. The equipment required for indoor and outdoor installations should be as per following guidelines : Typical size of transformer

Upto 20 MVA

Upto 50 MVA

Above 50 MVA

For the first two units 45 ltrs. 2 ltrs. Sand & foam type foam type water extingui- extingui- buckets shers shers 2 4 2 each (Indoor) 4 each (Outdoor) 3 6 2 each (Indoor) 4 each (Outdoor) 4 8 6 each

For every additional two Units or part thereof 45 ltrs. 2 ltrs. Sand & foam type foam type water extingui- extingui- Buckets shers shers 1 2 2 each (Indoor) 4 each (Outdoor) 2 2 2 each (Indoor) 4 each (Outdoor) 2 2 6 each

Notes 1.

The sand and water buckets shall be of 9 litre capacity each and shall have round bottoms.

2.

Sand pits of adequate sizes shall be provided in central location for catering to any sand requirements. These pits should always be kept filled with dry sand.


7.0

359

FIRE FIGHTING SYSTEMS

Some of the major types of fire fighting systems are discussed below : 7.1

Automatic Mulsifyre System

This system is widely used for fire fighting of outdoor transformers. Fire detectors located at various strategic points are used to sense high temperature near the transformer. If the temperature exceeds the set value the automatic mulsifyre system sprays water at a high pressure on the surface of the transformer to control fire on any burning oil spilled over. Various subsystems are used to make a complete mulsifyre system. (a)

Main Hydrant: This is used to carry the water to various parts of the switchyard or transformer substation and forms the backbone of the system. Sturdy corrosion free pipes and valves should be used for this purpose. The materials should be able to withstand fire for a reasonable duration.

(b)

Fire Detectors: Fire detectors can either be thermocouples or specially designed bulbs which burst when a high temperature is applied and release any valves or checking device to start the water spray.

(c)

Ring Mains and Nozzles: Ring mains which surround the transformer are provided to feed the water to the nozzles at various levels. Since the water pressure is high, the ring mains should be designed to withstand this pressure. Nozzles should be located such that the water spray, in the event of a fire, envelopes the entire surface of the transformer. The whole system should be periodically checked to detect any leakages.

(d)

Pumps: Pumps are provided to fill the hydrants initially and to maintain its pressure. Pumps driven by electrical motors are a standard provision; however, the standby pumps should preferably be diesel engine driven. It is recommended that the main and standby pumps in a pump house be segregated.

7.2

Sprinkler and Hydrant System

This system is similar to the mulsifyre system but water in this case is sprayed on the transformer body at lower pressure, Hence the name sprinkler. This system is normally not capable of extinguishing large fire. For this reason, it is desirable to connect an alarm to the sprinkler system. Whenever the sprinkler operates an alarm is given to signal requirement of additional fire fighting arrangement. The auto-starting pump valves and alarm must be periodically checked. 7.3

Foam System

This system is used to cut off the oxygen supply to the burning parts and thereby extinguish the fire. For this it is essential that the entire surface of the burning part or oil be covered with foam. The system includes a foam compound chamber, which contains the compound. In the event of a fire, this compound comes in contact with water and air to form foam. This foam is then


360

sprayed onto the burning parts. The compound chamber should be corrosion proof and the valves should be periodically tested to ensure their healthiness. 7.4

Other Systems and Selection

Various other systems are available for fire fighting of transformers notable among them being: (a)

Carbondioxide and Halon system for indoor or enclosed installations, and

(b)

"Drain and Stir" method for outdoor installations in which on detecting fire, oil is partially drained from the transformer and nitrogen gas is bubbled into the transformer tank to quench the fire.

8.0

STANDARDS AND GUIDELINES PROTECTION : REFERENCES

(a)

IS: 10028 (Part II)

(b)

Tariff Advisory Committee

(c) (d) (e)

BS 5306 Part 4 NEPA-15 NEPA 70

9.0

ACKNOWLEDGEMENT

(a)

(b) (c) (d)

(e) (f)

RELEVANT

TO

TRANSFORMER

FIRE

: Code of practice for selection, installation and maintenance of transformer. : Regulations for the electrical equipment of buildings. : Carbondioxide fire fighting systems. : Water spray fixed systems. : Fire protection of transformers and transformer vaults.

In the preparation of the manual, recommendations of the following have been used : Insurance Technical Bureau, of : Fire and explosion hazards liquid-filled London-Research Report electrical equipment. ITB-R84/108-Part I. Schadenspiegel No.2, 1980 : Deluge systems as fire protection for open air West Germany. transformers. Fire Technology, vol. 14, No.2, May 1978, NEPA, USA. Factory Mutual Engineering Corporation Manual, USA. -April 1978. ISI Publication. National Fire Protection. Association, Massachusetts, USA.

: Fire resistant fluids for industrial transformers. : Loss prevention data on transformer.

: National Building Code of India. : National Electrical Code.

(g)

National Fire Protection Association, Massachusetts, USA.

: National Fire Code 1981.

(h)

Tariff Advisory Committee.

: Building Regulations.

SECTION P


SECTION P Specifications for Transformer Bushings upto 800 kV Voltage Class 1.0

GENERAL

This specification covers outdoor capacitance graded Oil Impregnated Paper (OIP) bushings with values of highest voltage for equipment (Um) from 52 kV upto 800 kV voltage class and with values of rated current (Ir) upto 5000A and solid porcelain and oil communicating type bushings for voltage class < 36 kV for use in oil filled transformers and reactors. The Synthetic Resin Bonded (SRBP) type and Resin Impregnated Paper (RIP) type bushings are not covered in this section. These bushings, if required by the purchaser shall be supplied as per purchaser’s specification. In case of Bushings other than those specified, dimensions will be subject to agreement between manufacturer and purchaser. The specification establishes essential details and dimension to ensure interchangeability and adequate mounting of the bushings. 1.1 The porcelain components shall be sound, free from defects, thoroughly vitrified and smoothly glazed. 1.2 Unless otherwise specified, the glaze shall be brown in colour. The glaze shall cover all exposed porcelain parts of the bushings except those areas which are required to be left unglazed. 1.3 The design of the bushing shall be such that stresses due to expansion and contraction in any part of the bushing shall not lead to deterioration. 1.4 Cement if used in the construction of the bushing shall not cause fracture by expansion or loosening by contraction. Cement thickness shall be as small and even as practicable. 1.5

All exposed ferrous metal parts shall be hot dip galvanized wherever possible.

1.6

No arcing horns shall be provided on the bushings.

1.7

Any stress shield shall be considered as an integral part of the bushing assembly.

1.8 52 kV to 800 kV voltage class bushings shall be Oil Impregnated Paper (OIP) type condenser bushings. The bushings below 52 kV voltage class shall be of porcelain and oil communicating type unless otherwise specified. 1.9

Each bushing shall have marked upon it the manufacturer’s identification mark.


364

1.10 The duration and rated short time current of the bushing for various voltage ratings shall be as specified. 1.11 Limits of temperature rise shall be in accordance with IEC 60137-2003. 1.12 Permissible variation in the value of capacitance, and maximum value of dielectric dissipation factor (tan delta) of bushing and the test tap on transformer bushing shall be as per IEC 60137- 2003. 1.13 Bushings made of solid porcelain and oil communicating type and those having insulation which does not flow under service conditions shall be suitable for mounting at any angle of inclination. Unless otherwise specified, those having a liquid insulation shall be suitable for mounting at any angle of inclination to the vertical, not exceeding 30°. 1.14 The cantilever strength of the bushing shall be in accordance with IEC 60137 -2003 unless otherwise specified. 1.15 The profile of the porcelain and spacing of the petticoats shall suit the duty specified. The petticoats shall preferably be of aerodynamic type conforming to IEC 60815 to enable hot line washing during service. 1.16 OIP Bushings shall preferably be of hermetically sealed. 2.0

STANDARDS

2.1 The electrical characteristics of the bushings shall be in accordance with IEC 601372003. 3.0

SOLID PORCELAIN, OIL COMMUNICATING AND OTHER TYPE BUSHINGS UPTO 36 KV VOLTAGE CLASS

3.1 The dimensional parameters of the bushings upto and including 36 kV voltage class have already been standardised in IS : 3347 and shall be referred to. 3.2

Rated Voltage, Current and Basic Insulation Level

3.2.1 The rated voltage, current and basic insulation levels of the bushings shall be in accordance with IS: 2099. 4.0

OIP CONDENSER TYPE BUSHINGS FROM 52 KV TO 800 KV VOLTAGE CLASS

4.1

Interchangeability

4.1.1 All the bushings from 52 kV to 800 kV voltage rating shall have dimensions as mentioned in clause 4.3 to enable interchangeability with different makes of bushings manufactured in conformity with this specification.


365

4.1.2 The standardized dimensions shall be kept in view by the transformer manufacturers as well, while designing the transformers, so that the transformer can accept any bushing of the parameters and dimensions specified herein. 4.1.3 Other ratings of bushings, not covered in the specification, shall be supplied, if required. 4.2

Basic Insulation Level, Voltage, Current Ratings and Creepage Distances

The basic insulation level, rated voltage and current and creepage distances for various voltage class bushings shall be as under : Voltage rating

BIL (kVP)

SIL (kVP)

Power frequency withstand voltage (kV)

800

2100

1550

880

420

1425

1050

630

245

1050

460

145

650

275

72.5

325

140

52

250

95

Current Rating (Amps) 1250 1600 800 1250 2000 2500 800 1250 1600 2000 800 1250 400 800 1250 800 1250 2000 3150 5000

Creepage distance (mm) 16000 10500

6125

3625 1810

1300

4.2.1 The minimum value of creepage distance specified is 25 mm/kV of the rated voltage of bushing upto 420 kV and 20 mm/kV for 800 kV class. 4.2.2 For areas with very heavy or extremely heavy pollution the minimum creepage distance shall be as specified by the user. 4.3

Standardised Parameters of Bushings

4.3.1 To render the bushings interchangeable, certain parameters of the bushings have been identified and their maximum and minimum dimensions have been standardised. The identified parameters shall be as indicated in Table 1.


366

Table 1 Sl. No.

Parameter

Identification of Parameter

1

2

1.

L1

Length between top of air end terminal and bottom seat of flange.

2.

L2

Length between bottom seat of flange and bottom of the oil end shield/ stress relieving electrode/ oil end terminal whichever is the longest.

3.

L3

Length between top of air end terminal and bottom of the live metallic tank.

4.

L4

Flash over length.

5.

L5

Length of the air end terminal.

6.

L6

Length for bushing current transformer (BCT) accommodation.

7.

D1

Outside diameter of live metallic tank.

8.

D2

Maximum diameter of oil immersed end.

9.

D3

Outside diameter of fixing flange.

10.

D4

Fixing flange holes pitch circle diameter.

11.

D5

Diameter of fixing hole.

12.

N

Number of fixing holes.

13.

D6

Maximum diameter of oil end shield/stress relieving electrode.

14.

D7

Minimum inside diameter of central tube.

15.

D8

Diameter of air end terminal.

3

4.3.1.1 The standardised dimensions for various voltage and current rating bushings shall be as indicated in Figs. 1 and 1A. 4.4

Lead Arrangement

The lead arrangement, inside diameter of the central tube, rod and jointing rod for different current ratings and voltage class bushings shall be as under. 4.4.1 Definitions 4.4.1.1 DRAW-LEAD CONDUCTOR Conductor consisting of one or more flexible leads in parallel drawn into the central tube of the major insulation body of the bushing. (preferred up to 1000 A). 4.4.1.2 DRAW ROD

OR

TUBE CONDUCTOR

Conductor consisting of a round rod either removable and drawn into the central tube of the major insulation body of the bushing or fixed rod or tube and not removable. (preferred from 1250 A up to 5000 A).

367

Fig. 1

Fig. 1(A) Standard dimensions for OIP Condenser Bushings

Fig. 4



368 Voltage rating (kV) 800 420

245

145 72.5

52

where,

Current rating (Amps) 1250 1600 800 1250 2000 2500 800 1250 1600 2000 800 1250 400 800 1250 800 1250 2000 3150 5000

Type of lead

DR DR DL DL DR DR DL DR DR SS DL DR SS SS SS SS SS SS SS SS

Dia. of the Jointing rod/rod (mm) 45 45 45 45 50 50 35 45 45 50 35 35 -

Dia. of the central tube (mm) 60 60 60 60 60 60 48 48 60 60 38 38 -

DL- Draw Lead DR- Draw Rod SS- solid Stem

4.5

Bushing Current Transformer (BCT)

4.5.1 To accommodate the bushing current transformers, space provided on various voltage class bushings shall be as under: 800 kV : 420 kV : 245 kV : 145 kV :

72.5kV : 52 kV

:

400 400 300 600 100 300 600 100 300 100 300

mm mm mm mm mm mm mm mm mm mm mm

4.5.2 Bushings with space as necessary for the accommodation of bushing current transformers specified shall be also offered.


369

4.6 Lead Joint 4.6.1 Draw lead and draw rod type leads shall have joint as shown in Fig. 2. The dimensional details, material composition, finish, bolts spacing etc., have also been shown in the figure. The joint shall be provided at the bushing fixing flange level. The bottom portion of the joint shall be in flush with the bottom of the flange. The complete joint with the top portion of the lead upto the joint with nuts and bolts shall be supplied alongwith the bushing. The free portion of the joint shall be brazed / crimped by the purchaser with the lead to be supplied alongwith transformer to make the complete lead. 4.6.2 In case of draw rod type lead arrangement, the complete rod upto the joint with free end of the rod forming one portion of the joint with necessary nuts and bolts shall be supplied along with bushing. The remaining portion of the rod with one end to match the joint shall be arranged by the purchaser through the transformer supplier. 4.7

Oil End Terminal and Shield Fixing Arrangement for 245 kV Solid Stem Type Bushing

The oil end terminal and shield fixing arrangement for 245 kV Solid stem type bushing shall be as shown in Fig. 3. The entire oil end terminal arrangement along with corona shields shall be supplied along with bushing. 4.8

Oil End Terminal for 72.5 kV/ 52 kV/ Solid Stem Type Bushing

For 72.5 kV/ 52 kV bushings the palm type terminal at the oil end of the solid stem type bushings shall be supplied in accordance with Fig. 4 as applicable. 4.9

Air End Terminal

The lengths and diameters of the air end terminals for the various bushings shall be as under: Bushings upto 800 amps – 30 mm x 125 mm (D x L) Bushings of 1250 Amps – 60 mm x 125 mm (D x L) (Upto 3150 Amps) Bushings of 5000 Amps - 90 mm x 125 mm (D x L) 4.10 Test Tap OIP bushings shall be provided with test taps of proven design unless other wise specified. 4.11

Oil Level Indicator

4.11.1 The bushings shall be provided with oil level indicators as under: 800 kV bushing – Magnetic oil level gauge 420 kV bushing – Magnetic oil level gauge 245 kV and below – Oil sight window The oil level indicator shall be so designed and dimensioned that oil level shall be clearly visible from ground level.

370


Fig. 2


Fig. 3

371

372


Fig. 4

SECTION Q

Specifications for Dry Type Transformers

374



375

SECTION Q

Specifications for Dry Type Transformers 1.0

SCOPE

1.1 This section of the specification covers the different types of dry type transformers. This section does not purport to include all the necessary provisions of a contract. For general requirements, tests, erection, maintenance and commissioning, reference shall be made to sections A, J & K of the Manual. 1.2 It is not the intent to specify completely all details of design and construction of the equipment. However, the equipment shall conform in all respect to high standard of design, engineering and workmanship and be capable of performing in continuous commercial operations. 2.0

STANDARDS

2.1 Except where specified otherwise herein, all material, equipment and construction shall conform to Indian Electricity Act and rules and latest versions of Indian standards specified below : 2.2

List of Standards

(a)

IS – 11171

:

Dry type transformers

(b)

IS – 2026 (Part-I)

:

Power transformers - General

(c)

IS – 2026(Part-II)

:

Power transformers temperature rise

(d)

IS – 2026 (Part-III)

:

Insulation levels and dielectric tests

(e)

IS – 2026 (Part-IV)

:

Terminal markings, tappings and connections.

(f)

IS - 2026 (Part-V)

:

Transformer / Reactor Bushings - Minimum external clearances in air

(g)

IS – 12063

:

Degree of protection provided by enclosures

(h)

IEC – 60076-11

:

Power transformers dry type transformers


376 3.0

TERMINOLOGY

3.1

Dry Type Transformer

A transformer in which mineral oil or any liquid is not employed either as a cooling or insulating medium. Cooling will be by natural circulation of air or by forced air cooling. (AN or AN/AF as per IS 11171). Two types of dry type transformers are generally available, i.e., (a)

Encapsulated Winding Dry Type Transformer: A dry type transformer having one or more windings encapsulated with solid insulation.

(b)

Non-Encapsulated Winding Dry Type Transformer: A dry type transformer having none of the windings encapsulated with solid insulation.

4.0

STANDARD RATINGS FOR 3 PHASE TRANSFORMERS kVA 16 25 63 100 160 200 250 315 500 630 800 1000 1250 1600 2000 2500 16 25 63 100 160 200 250 315 500 630 800 1000 1250 1600 2000 2500 630

Voltage ratio 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 6600/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 11000/433 33000/433


800 1000 1600 1250 2000 2500 1000 1600 1250 3150 4000 5000 6300 8000 10000

5.0

377

33000/433 33000/433 33000/433 33000/433 33000/433 33000/433 33000/11000 33000/11000 33000/11000 33000/11000 33000/11000 33000/11000 33000/11000 33000/11000 33000/11000

RATED FREQUENCY

The standard frequency shall be 50 Hz with a tolerance of +/-3 percent. 5.1

Operation other than the Rated Voltage and Frequency

5.1.1 Transformer built in accordance with this specification may be operated at its rated kVA at any voltage with in +/-10 percent of the rated voltage at that particular tap. 5.1.2 The transformer shall be capable of delivering rated current at a voltage equal to 105 percent of the rated voltage. Note : The slight temperature rise increase which would correspond to the 5 percent over voltage due to high no load loss is disregarded.

5.1.3 A transformer for two or more limits of voltage or frequency or both shall give its rated kVA under all the rated conditions of voltage or frequency or both; provided an increase in voltage is not accompanied by decrease in frequency. 6.0

ELECTRICAL CHARACTERISTICS AND PERFORMANCE

(a)

Thermal classification of insulation and permissible temperature rises should confirm to class ‘F’ or class ‘H’ as per relevant clause of IS 11171.

(b)

Impedance voltage and short circuit performance against thermal and dynamic requirements are applicable as per relevant clauses of IS 2026 & IS 11171.

6.1

Protective Housing for Dry Type Transformer

Dry type transformer shall be provided with suitable protective sheet steel housing, if required by the site conditions with minimum IP 43 degree of protection for the enclosure for out door and IP33 for indoor transformers. The housing shall have ventilation louvers/ opening provided with wire mesh screens and shall be supplied with suitable lifting lugs. Safety limit switches shall be provided and wired in such a way that the incoming supply may be disconnected whenever any one of the sides of the enclosures are opened with the transformer in energized condition.


378 6.2

Off-Circuit Links

The off-circuit tapping links shall be provided on the HV side with appropriate register plate to show the link location. The links shall be provided with a separate door for access. 6.3

Insulators

HV & LV insulators shall be of epoxy mould type / porcelain 6.4

Termination

LV & HV termination is bus bar or cable entry. Separate neutral bushing shall be provided, if LV with cable box. All the bus bars are to be supported on insulators. 6.5

Transformer Fittings

Each transformer shall be fitted with following accessories (i)

Inspection covers (if taps are available)

(ii)

Off circuit links in the primary for voltage variations

(iii)

Rating, diagram and tap connection plates

(iv)

Terminal marking plate

(v)

Two nos. earthing terminals with lugs

(vi)

Lifting lugs and haulage lugs

(vii)

Winding temperature detectors with solid state type temperature signaliser with digital read out and requisite sets of remote signaling contacts for alarm and trip operation.

(viii) Under carriage with bi-directional rollers with locking and bolting devices. Suitable arrangement for core and winding assembly to draw out the same. (ix)

Marshalling box complete with all instruments, accessories and fittings as required for the transformer.

(x)

Danger plate indicating “entry prohibited under energized condition” of the transformer.


7.0

379

INSULATION LEVELS Highest voltage for equipment Um kV (rms) < 1.1 3.6 7.2 12.0 17.5 24.0 36.0

Rated short duration power frequency withstand voltage kV (rms)

Rated lightning impulse withstand voltage kV (peak) List 1 List 2

3 10 20 28 38 50 70

-20 40 60 75 95 145

-40 60 75 95 125 170

Choice between List 1 and List 2 as per relevant clause of IS 11171 & IEC 60076 Part 11 (2004)

8.0

EXTERNAL CLEARANCES

The following clearances are to be maintained in air between line to earth for the respective voltage : Highest voltage for equipment Um kV (rms) < 1.1 3.6 7.2 12.0 17.5 24.0 36.0

9.0

Minimum external clearance between line to earth mm 25 60 90 110 170 210 280

TESTS AND TEST CERTIFICATES

The transformer shall be subjected to all the routine, type and special tests as per IS–11171 and 2026 / IEC 60076 as agreed upon between the purchaser and the manufacturer. 9.1

Type Tests

The following shall constitute the type tests: (a)

Measurement of winding resistance

(b)

Measurement of voltage ratio and check of voltage vector relationship

(c)

Measurement of impedance voltage (principal tapping), short circuit impedance and load loses

(d)

Measurement of no-load losses and current

(e)

Separate source voltage withstand test


380 (f)

Induced over-voltage withstand test

(g)

Lightning impulse test

(h)

Temperature rise test

9.2

Routine Tests

The following shall constitute the routine tests: Measurement of winding resistance (a)

Measurement of voltage ratio and check of voltage vector relationship

(b)

Measurement of impedance voltage (principal tapping), short circuit impedance and load loses

(c)

Measurement of no-load losses and current

(d)

Separate source voltage withstand test

(e)

Induced over-voltage withstand test

9.3

Special Tests

The following tests may be carried out by mutual agreement between the purchaser and the supplier : (a)

Short circuit withstand test

(b)

Measurement of acoustic sound level

(c)

Partial discharge measurement

(d)

Mechanical tests: IP test on enclosure

10.0

SURFACE TREATMENT AND PAINTING

Surface treatment and painting shall be done as per clause 1.6 of Section A of this Manual.


SECTION R

Cable Boxes for SF 6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above

381

382


Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 383

SECTION R

Cable Boxes for SF 6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 1.0

SCOPE

The purpose of this specification is to establish electrical and mechanical interchangeability between cable terminations and the gas-insulated enclosure and to determine the limits of supply. This technical specification covers the connection assembly of cables to SF6 gas insulated terminations (cable boxes) in single phase or three-phase arrangements, where the cable terminations are gas filled and there is a separating insulating barrier between the cable insulation and the gas insulation of the transformer/ switchgear. This also covers the connection assembly of extruded insulation cables to gas insulated enclosure, where the cable terminations are of dry type. In this arrangement, the cable termination design comprises an elastomeric electrical stress control component in contact with a separating insulating barrier between the cable insulation and the gas insulation of the transformer/ switchgear. The cable termination does not include any insulating fluid. 2.0

STANDARDS

IS 2026 (Part 1)

:

Power Transformers - General.

IS 2026 (Part 3 )

:

Power Transformers - Insulation level and dielectric tests.

IS 2026 (Part 4)

:

Power Transformer - Terminal marking, tapping and connections

IS 2099

:


IEC 60137

:


IEC 60141-1

:

Tests on oil filled and gas pressure cables and their accessories – Part-1: Oil filled, paper or polypropylene paper laminate insulated, metalsheathed cables and accessories for alternating voltages upto and including 500 kV.

IEC 60141-2

:

Tests on oil filled and gas pressure cables and their accessories – Part-2: Internal gas pressure cables and accessories for alternating voltages upto 275 kV.

IEC 60517

:

Gas insulated metal enclosed switchgear for rated voltages of 72.5 kV and above.

IEC 60694

:

Common specifications for high voltage switchgear and control gear standards.


384 IEC 60840

:

Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) upto 150 kV (Um = 170 kV) – Test methods and requirements.

IEC 60859

:

Cable connections for gas insulated switchgear for rated voltages of 72.5 kV and above.

IEC 61639

:

Direct connection between power transformers and gas insulated switchgear for rated voltages of 72.5 kV and above.

3.0

LIMITS OF SUPPLY

The limits of supply for fluid filled cable terminations shall be according to Fig. 1 and for dry type cable terminations shall be according to Fig. 3. To limit the voltage under transient conditions, between parts 6 or 11 and part 13 of Fig. 1 for fluid-filled cable terminations and non-linear resistors (part 15) of Fig. 3 for dry type cable terminations may be connected across the insulated junction. The number and characteristics of the non-linear resistors shall be determined and supplied by the cable termination manufacturer, taking into consideration the requirements of the user and the switchgear manufacturer. 4.0

RATING

When dimensioning the cable connection assembly, the following rated values shall apply: (a)

Rated voltage (72.5 kV – 100 kV – 123 kV – 145 kV 300 kV – 362 kV – 420 kV – 550 kV)

– 170 kV – 245 kV –

(b)

Number of phases in one enclosure (One or three phases for Ur ≤170 kV and single phase for Ur >170 kV)

(c)

Rated insulation level (ref. IEC 60694)

(d)

Rated normal current and temperature rise (2000 A at a temperature 900 C is standardised for interchangeability)

(e)

Rated short time and peak withstand currents (ref. IEC 60517; cl. 4.5, 4.6 and 4.7)

(f)

Rated duration of short circuit

Fig. 1 : Fluid-filled cable connection assembly - Typical arrangement


Fig. 3 : Dry type cable connection assembly - Typical arrangements


388


5.0

DESIGN CONSTRUCTION REQUIREMENTS

5.1

Pressure Withstand Requirements

The maximum recommended design pressure (absolute) for the outside of the cable termination is 0.85 MPa. In addition, the bushing and cable termination shall be capable of withstanding the vacuum conditions when the transformer connection enclosure is evacuated as part of the gas filling procedure. The transformer connection enclosure shall satisfy the requirements as per IEC 60517 for the design pressure. The maximum operating gas pressure (absolute) of a direct connection assembly shall not exceed: • The design pressure of the transformer connection enclosure plus 0.1 MPa when the design pressure is lower than 0.75 MPa (gauge); •

0.85 MPa (absolute) when the design pressure equals or exceeds 0.75 MPa (gauge).

5.2

Mechanical Forces on Cable Terminations

The manufacturer of the cable termination shall take into account, in the three-phase connection, the total dynamic forces generated during short circuit conditions. These forces consist of those generated within the cable termination and those coming from the main circuit of switchgear. The maximum additional force to be applied from the switchgear to the connection interface transversely and being transferred from the main circuit end terminal shall not exceed 5 kN. For single-phase connections, even taking into account lack of symmetry, it is considered that the additional force is small. However, a total mechanical force of 2 kN applied to the connection interface transversely should be assumed. It is the responsibility of the manufacturer of the switchgear to ensure that the specified forces are not exceeded. 5.3

Mechanical Forces Applied on the Bushing Flange

In addition to the maximum operating gas pressure specified 5.1, the flange of the bushing attached to the transformer connection enclosure is subjected, in service, to the following loads: • part of the weight of the switchgear not supported by the switchgear's own supporting structures; • part of the wind load, if applicable, not supported by the switchgear's own supporting structure's; • expansion/ contraction stresses due to the temperature variations of the switchgear enclosures. For the evaluation of these stresses, it shall be considered that, on the transformer side the variation height of the bushing flange due to temperature variation does not exceed ±0.0008 times the transformer tank height in the case of a steel tank.


These loads result in the simultaneous application, at the centre of the bushing flange, of: • a bending moment M 0 ; •

a shearing force Ft ;

• a tensile or compressive force Fa. The bushing and the transformer shall be capable of withstanding, in service, the values of M 0, Ft, and Fa specified in Table 1, and it shall be the responsibility of the switchgear manufacturer to ensure that these values are not exceeded. Table 1 : Moment and forces applied on the bushing flange and transformer Rated voltage kV 72.5 – 100 123 – 170 245 – 300 362 – 550

Bending moment M 0 kNm 5 10 20 40

Shearing force Ft kN 7 10 14 20

Tensile or compressive force Fa kN 4 5 7 10

Except where specified otherwise by the customer, the relative positions and levels of the transformer and switchgear foundations respectively shall be considered as not varying. 5.4 Vibrations The vibrations generated inside the energised transformer are transmitted by the oil and the tank wall of the transformer to the bushing rigidly fixed on this wall and to the switchgear. The switchgear manufacturer and the transformer manufacturer shall agree to take into account these vibrations. 6.0

STANDARD DIMENSIONS AND SPECIAL REQUIREMENTS

6.1 Fluid Filled Cable Terminations Standard dimensions for fluid filled cable connection enclosures, main circuit end terminals and cable terminations applied to single phase enclosures are shown in Fig. 2. With the given four standard sizes, the voltage range (Ur) from 72.5 to 550 kV is covered. 6.2 Dry Type Cable Terminations Standard dimensions for dry type cable connection enclosures, main circuit end terminals, and cable terminations applied to single phase enclosures are shown in Fig. 4. With the given four standard sizes, the voltage range (Ur) from 72.5 to 550 kV is covered. Figure 3 shows the two types of dry type cable termination. Type A incorporates an elastomeric electrical stress control component inside the insulating barrier. For type B, the elastomeric electrical stress control component is located is located externally. 7.0

TESTS

7.1 General The testing of the cable termination and the gas insulated metal enclosed switchgear is to be performed for cable terminations in accordance with IEC 60141-1 for oil filled cables, IEC 60141-2 for gas filled cables, IEC 60840 for cables with extruded insulation and IEC 60517 for switchgear. In addition, this specification gives recommended arrangements for dielectric tests and for the tests after cable installation.


7.2

Dielectric Type Tests

7.2.1 Dielectric Type Tests of Cable Terminations The dielectric type test of the cable termination fitted with a representative cable shall be performed in an enclosure filled with insulating gas at a pressure according to the values specified in Table 2. If a shield is an integral part of the cable termination design, it shall be mounted in its service position during the test. An additional test shield may be used to screen the exposed connection interface, if required by the cable termination manufacturer, provided it does not overlap the connection interface by more than the distance l2 in Fig. 2 for fluid filled cable termination. Table 2 : Gas-pressure limits for dielectric type test of cable-terminations Range of rated voltages Ur kV 72.5 - 100

Minimum SF 6 functional pressure °C (absolute) at 20°C MPa 0.10

123 - 170

0.30

245 - 300

0.35

362 - 550

0.40

Note : The gas pressures are intended as a guideline for the manufacture of the cabletermination. Higher values are permissible. If a gas other than SF6 is used the minimum functional pressure should be chosen to give the same dielectric strength.

7.2.1.1 DIELECTRIC TYPE TEST OF SINGLE-PHASE CABLE-TERMINATIONS The cable-termination is surrounded by a metal cylinder connected to earth, the internal diameters of which are 300 mm, 300 mm, 480 mm and 540 mm respectively for the four standard sizes of cable connection enclosure (d 5 in Fig. 2 for fluid-filled cableterminations and Fig. 4 for dry type cable-terminations). 7.2.1.2 DIELECTRIC TYPE TEST OF THREE-PHASE CABLE-TERMINATIONS The non-symmetrical arrangement for Ur <170 kV with a cylinder of 650 mm internal diameter (Fig. 5) is intended to simulate the conditions for a three-phase cable connection. Cable-terminations, which are intended for a single-phase cable connection, may be tested in the same cylinder of 650 mm internal diameter to cover all expected sizes of cable connection enclosures. 7.2.2 Dielectric Type Test of Cable Connection Enclosures The cable connection enclosure and main -circuit end terminal may be subjected to t h e dielectric type test according to IEC 60517 without the cable-termination.

392


Dimensions in millimetres Fig. 5 Non-symmetrical arrangement for dielectric tests of cable terminations

The gas pressure for the dielectric type test may be specified by the manufacturer of the switchgear in accordance with Table 1. 7.3 Tests After Cable Installation If parts of the switchgear directly connected to the cable connection assembly c a n n o t withstand, at rated filling density for insulation gas, the test voltage specified for the cable test (IEC 60141 and IEC 60840), or, if in the judgement of the switchgear manufacturer, it is not acceptable to apply the test voltage to the switchgear, the switchgear manufacturer should make special provisions for the testing of the cable, for example disconnecting facilities and/or increasing gas pressure in the cable connection enclosure. Note : It should be noted that increasing the gas pressure is not a reliable method of improving the electrical strength at the surface of an insulator when tested with d. c. voltage.


If required by the user, the switchgear manufacturer shall provide the location for a suitable test bushing and provide the user with all necessary information for mounting such a bushing to the cable connection enclosure. For cases where electrical clearances are inadequate, the term bushing shall include a suitable insulated connection and test terminal. The requirement for the test bushing shall be specified by the user in the enquiry. 8.0

INFORMATION ORDERS

TO

BE

GIVEN

WITH

ENQUIRIES,

TENDERS

AND

Refer to IEC 60840, IEC 60141 and clause 9 of IEC 60517. In addition, the user and the manufacturers shall consider the installation requirements of the equipment. Manufacturers shall state the specific requirements for civil, electrical and installation clearances applicable to the switchgear, cable-termination and cable. 9.0

RULES FOR TRANSPORT, MAINTENANCE

STORAGE,

ERECTION,

OPERATION

AND

Refer to IEC 60694, clause 10. The cable-termination manufacturer should ensure that during manufacture, handling, storage and installation of the cable-termination, provisions should be made to ensure that the requirements given in 5.2 of IEC 60694 can be satisfied after final assembly of the connection. The cable-termination manufacturer should supply the necessary information to enable these requirements to be satisfied, if the cabletermination is to be installed by others.

SECTION S


396



397

SECTION S

Guidelines for Repair of Power Transformers at Site 1.0

INTRODUCTION

Transformers are amongst the most efficient equipment made by mankind and as with all manmade equipment the power transformer also fails. The reasons for the failures can be attributed to design, manufacturing, operational and maintenance issues. Design, manufacturing and process validation is ensured during the routine and type testing of the transformers at the manufacturer’s works. Over the years the manufacturers have had better understanding and reduction in the knowledge gap regarding transformer technology and this led to improved and compact designs. The inclusion of newer tests in the test schedule verify the enhanced knowledge base. With rapid growth of the network, increased fault levels and faster ageing, the equipment are subjected to increased operational stress. Increased consumer awareness, privatization, competition and profitability has forced utilities and generation companies to focus on longer life cycles and increased revenue generation from every investment made. With the increased cost of acquisition the options of refurbishment and repairs at site have gained prominence and are commercially lucrative also due to the faster turn around for product to be back into action. In this chapter effort has been made to focus on various aspects that need to be kept in mind when planning for repair at site. The focus areas are diagnosis of the fault and localization of the fault zone to determine nature and extent of repairs. Based on this assessment the logistic requirements for the repairs the quantum of work involved and various technicalities involved in the process are to be carried out. 2.0

ON-SITE REPAIRS

Performing on-site repairs on a transformer has multiple advantages and the major ones are described in this section. 2.1

Advantages of Site Repairs

2.1.1 The damaged equipment can be brought into service much faster than repairs at the factory by way of saving in to & fro transportation time as well as completion of repair-tocommissioning carried out in single set-up. 2.1.2 On-site repair costs considerably less than repair at the factory particularly, for large power transformers in remote areas.


398 3.0

REASONS FOR RELUCTANCE TOWARDS SITE REPAIR OF POWER TRANSFORMERS

3.1 The reluctance of the utilities/customers for encouraging repairs at site are listed below : (a)

Validation techniques of the repair work carried out at site (customers generally have a mind-set that high voltage test is the only validation procedure and this testing will not be available in case of repair at site).

(b)

Concern about drying out of the transformer at site, after the repairs have been carried out and the validation of the end point of drying out.

(c)

Non-availability of the logistics arrangement which the owner has to necessarily make for effecting the repair at site and lack of expert manpower to assist in carrying out the work.

(d)

Other reasons could be : (i)

Exorbitant hourly rates of technical manpower deputed by the reputed manufacturers generally with no ceiling, for carrying out repairs at site.

(ii)

Unavailability of suitable storage facility for oil, equipment for drying out of power transformer at site and other logistics at site.

(iii)

Guarantee and warrantee requirement to match that of new transformer or in case the repairs are carried out under controlled conditions at the factory.

(iv)

Unavailability of suitable tooling for carrying out transformer repairs.

(v)

Lack of understanding of changes in maintenance methods as transformer insulation ageing would have occurred.

(vi)

Lack of covered high roof space for repairs, since repairs at site need dry and clean conditions.

(vii)

Lack of indepth knowledge about transformer manufacturing, practices and various stage checks.

(viii) Play safe attitude and not to take any chance on performance after repairs. (ix)

Demonstrated high voltage withstand capacity (as guarantee) of repairs carried out.

3.2

From the manufacturer end the reluctance is because of the following reasons :

(a)

Diversion of manpower and tooling from the regular manufacturing activity.

(b)

Lack of confidence on alternative drying out methods and the availability of suitable instrumentation for quantification.


(c)

399

Other reasons are (i)

Their engineers would have difficult time in delivering results and perform in conditions not under their control.

(ii)

Though no guarantees may have been agreed but reputation of the company remain at stake.

(iii)

Accuracy and quality of available history & available performance data.

(iv)

Disruption in production activity if manpower is sent to site.

(v)

Repair activity is not treated as a regular business activity.

(vi)

The quantum of work is double i.e., once for dismantling and the second for assembly which is generally not well appreciated by the end user.

(vii)

Difficulty of duplication of drying out process at site and reliability of instruments/the data collected.

3.3 Repairs of power transformers at site is controversial and is still a subject of discussion amongst engineers. While examples of successful repairs of power transformers up to 400 kV class exist, still most of the utilities and the manufacturers are shy of taking up this activity at site. One of the most common reasons for the utilities not promoting this activities is the non-availability of high-voltage test equipment at site for HV testing after repairs. The fact is, that this is a type test and a manufacturer during the manufacture of the power transformer even at his works is not using any high voltage tests in the production process or as part of in-process quality checks. 4.0

SITE REPAIR CATEGORIES AND REQUIREMENTS

4.1

Repairs can be categorized in to :

(a)

Minor repairs.

(b)

Major repairs

4.1.1 Minor Repairs Generally repairs involving low volume of oil draining and attending to minor defects in the transformer like replacing a defective bushing, bushing CT, gasket, bolt or burnt-out connection at a bushing or small repairs of diverter switches. 4.1.2 Major Repairs These repairs generally involve draining of main tank oil for accessibility to the problem area and are mostly in the core-coil assembly i.e., Winding, Core, OLTC selector switch. It involves, total oil draining, storage, oil filling under vacuum and during out activities for restoration of the transformer to service.


400 4.2

Site Repair Requirements

4.2.1 Power transformer repair at site is in no way any different from the repair process followed at the factory. The repair will be successful if all the quality check parameters adopted at factory are religiously duplicated at site and so also the tooling and the environment. 4.2.2 The information required for decision on carrying out repair at site is given below : (i)

Nature of the fault.

(ii)

Location of the fault.

(iii)

Extent of damage due to the fault.

(iv)

Scope of work based on information collected.

(v)

Material required for carrying out the repairs.

(vi)

Sequencing of activities during the repairs.

(vii)

Time required for various repair activities.

(viii) Facility and tooling needed at site for repairs. (ix)

Suitable equipment for drying out transformer at site.

(x)

Validation method of the repair to be carried out.

4.2.3 The scope of work determination is the most important activity for site repair as this will govern the material, tooling and facilities required for the successful completion of the work. The whole scope of work can be divided into following main activities : l

Pre repair work assessment.

l

In process stage checks and review of repairs work assessment.

l

Post repair activities including the final in-process checks.

l

Drying out and commissioning activities.

4.2.4 After the fault detection techno-economic assessment is done in respect of down time and cost of repairs at site, to the overall time (including transportation time) and cost of repairs at the factory and it is generally found that the down time is an important aspect than the rather pure economics. As soon as the service interruption has taken place restoration of the supply becomes the prime activity. To determine the health of the transformer and its suitability for the immediate restoration requires that the condition of the equipment be determined to the


401

maximum possible accuracy. The manufacturer can also contribute in this assessment. It has been generally observed that after the disruption has occurred the condition determination is not done systematically as required for correct assessment. Most of the time it is attributed to lack of equipment, test instruments and at times required expertise of the available technical persons. 5.0

DIAGNOSTIC TESTING OF THE TRANSFORMER

5.1 Record of the system condition at the time of outage as well as immediately before is one of the most important activity. The load conditions at the time of disruption i.e., the load current, voltage, the status of the various equipments and protection in service, any fault history etc. and also weather condition needs to be recorded. Many failures in transformers have been found to be due to the abnormal system operation and/or protective device malfunction therefore the status of all the protections is extremely important and need to be correctly recorded. 5.2 Immediately after the interruption, temperature of the top oil may be recorded, and the oil samples must be collected for measuring moisture content in oil, Dissolved Gas Analysis (DGA) and for furfural content specifically for old transformers (in service for more than 10 years). As these tests are trending tests, the earlier test readings will act as reference and help in reaching some conclusion. 5.3 The fault detection comprises of the combination of the following electrical tests after the transformer has been shutdown to determine the cause of the failure and the location therefore. l

Magnetising Current.

l

Magnetic Balance.

l

Voltage Ratio.

l

Single Phase Short Circuit Current.

l

Winding Resistance.

l

Vector Group.

l

OLTC Contact Resistance.

For diagnostic testing it is mandatory to repeat all the tests in all the possible tap positions as fault can exist in isolated condition even with normal operating condition. These electrical tests are indicative of the location of the fault and to further accurately establish the fault some invasive tests may need to be done. These invasive tests are generally done at the winding level after isolating the OL TC to further localize the fault and if OLTC is the culprit then also these invasive tests help in locating the fault.

402


Condition assessment helps in determining the scope of work and the tests to be followed for determining the healthiness of the various aspects of the transformer are to be so selected that at least two diagnostic tests should indicate the same parameters/findings. 5.4 Most of the manufacturers recommend that the testing to determine the condition of the equipment after operational disruption be done in single phase and/or at reduced or elevated voltage values to exactly determine the extent of the problem. Example 1. A fault at 240 volts may be showing no abnormality but when carried out at 2 kV level can give a totally different value. Magnetising current test and magnetic balance test results have been known to show variations in values in the case of interturn and interdisc insulation failures. Example 2. A high winding resistance value in a winding having the OLTC connected can mean improper contact for bushing termination or defective joint or improper OLTC contact in the selector switch or the diverter switch. The improper diverter switch contact will reflect in multiple test results either in the odd position or the even position for in-tank OLTC construction based on the Dr. Jansen principle. For different OLTC construction the result may reflect in only one position and may be confusing and require invasive testing. Example 3. A 160 MVA power transformer in prefect working condition for 15 years is taken in for overhauling due to low IR values of the insulation. After drying out it is found that the winding resistance results of the transformer are erratic and indicated some problem. A vigorous motion given to the tap connections restores winding resistance values to near normal values. On investigation, stripping the taping of the tap lead joint revealed a burnt out joint and the best part was that the DGA history of the transformer indicated no problem of this type. The other finding here was that the transformer was being routinely tested for winding resistance only in three positions first, nominal and the last position. 5.5 For transformers in the network with history of tripping under fault conditions the FRA is an invaluable tool for the determination of the condition of the transformer. The FRA record of the transformer is to be maintained meticulously and after the transformer faces a tripping, the FRA test is to be conducted to determine any change in the geometry of the assembly or loss of clamping pressure of the winding. This helps in detecting small changes so that the repair/refurbishment/overhauling/replacement plan can be done properly to avoid unpleasant surprises. 6.0

REPAIRING OF TRANSFORMERS

6.1 Even some 400 kV class transformers have been successfully overhauled at site involving changing of old gaskets, changing of oil, drying out of the job for IR value improvement and are in successful operation. Though not in the repair category it clearly demonstrates the adequacy of the drying out method, if properly done at site. 6.2 The repair process needs either the involvement of the manufacturer’s engineers or skilled manpower experienced with similar activity at some manufacturing location. Different repair activities require different materials, tooling and facilities. The facility requirement is also governed by the manufacturing methodology adopted for the transformer


403

manufacturing. Large power transformers are generally manufactured in three different configurations (a)

Cover mounted construction

(b)

Cover free construction.

(c)

Cover free and bell cover construction.

These constructions have their own advantages and disadvantages and the choice of the method of construction is either specified by customer or is decided by the manufacturer. The manufacturer is conversant with all the construction configurations and has facilities and tooling to enable manufacturing of all the configurations. 6.3

Procedure of Major Repairs

Major repairs in the core coil assembly are categorized in to three main types : (a)

Core components.

(b)

Winding and its connections.

(c)

OLTC selector switch and its components.

For various components/mountings fitted on the transformer, the major repairs are in : (a)

High voltage bushings.

(b)

OLTC diverter switch and its components.

These repairs require draining of oil, internal inspection for determining the extent of the problem and remedial action thereof including the components required to be procured. Generally the problem solving does not take much time compared to the time required for the pre repairing and post repairing activities leading to successful commissioning. Winding repairs are the most complicated of all the repair works carried out and generally require replacement of the defective coils or the whole composite coil of the winding. For winding replacement the maximum tooling and logistics arrangement is required including the new winding from the manufacturer or third party supplier. Winding replacement activities are preferably to be carried out under the guidance of manufacturer though such guidance is now a days available from third party sources also. During the repair activity some improvements or refurbishment can be also done to increase the reliability of the equipment and ensure trouble free operation.


404 6.4

Facility Planning for Site Repairs

6.4.1 Most power stations are adequately equipped for handling large and heavy weight equipment/transformer and generally have a Service-bay equipped with crane and working area. In case of non-availability of overhead crane facility, adequate crane capacity needed for the repairs should be determined and hired out. At this point of time the need is to assess the following : (i)

Location where the refurbishment is to be carried out, preferably covered space.

(ii)

Adequate space availability for dismantling the transformer and storage of the dismantled transformer parts.

(iii)

Crane of adequate capacity with enough working space by height clearance.

(iv)

The selected area should be dry, away from any open source of water, dust free and shall allow additional covering if necessary.

(v)

Power supply point of adequate capacity for lighting and supply for various equipment operations like hydraulic crimping tools, brazing machines, pneumatic spanners heating, filter machines etc.

(vi)

The power supply should not be very near either as it is a fire hazard in case of any problems with the supply wire or electrical equipment failure. Generally the supply should have ELCB protection facility.

6.4.2 Lifting Facilities for Tank Cover Transformer manufacturers use various types of tank covers and each one has its own typicality of handling. Some of the points to be taken care of when planning for the crane and lifting facilities are given below : l

For Top Cover Type Transformers

(i)

Weight of the top cover.

(ii)

Dimension of the cover.

(iii)

Dimension of the active part.

(iv)

Weight of the active part.

(v)

Height to be lifted for minimum clearance.

(vi)

Generally it is preferable to have vertical overhead cranes, in case other cranes are used then adequate boom height should be available and the clearance of the boom from the active part during lifting (i.e., if 40 ft high working height is required then with the mobile cranes the same distance is to be available from the top of the active part lifted such that the active part is clear of the tank top and not from the boom top).


(vii)

405

The top cover should be inspected for unbalance load distribution and all mountings on the top cover should be removed.

(viii) The mobile crane should be rated at least 75 % more than the maximum load to be lifted. (ix)

Four nos. slings having total load lifting capacity atleast twice the maximum load to be lifted.

(x)

The OLTC mounting to be checked and dismantling procedure selected.

l

For Bell Cover Type Transformer

(i)

Weight of the bell cover.

(ii)

Dimension of the bell cover.

(iii)

Height to be lifted for clearance of at least 30 cms from the top most part of the active part assembly.

(iv)

Lifting height is to be calculated from the top of the bottom part of the bell cover and not from the ground level.

(v)

Generally it is preferable to have vertical overhead cranes, in case other cranes are used then adequate boom height should be available and the clearance of the boom from the active part during lifting (i.e., if 40 ft high working height is required then with the mobile cranes the same distance is to be available from the top of the active part lifted such that the active part is clear of the tank top and not from the boom top).

(vi)

Top cover should be inspected for unbalance load distribution and all mountings on the top cover should be removed.

(vii)

The mobile crane should be rated at least 50 % more than the maximum load to be lifted.

(viii) Four nos. slings having total load lifting capacity atleast twice the maximum load to be lifted. (ix)

l

The OL TC will be mounted on the top cover but resting fork will be fitted on to the end frame. For Top Covers with the Active Part Fitted to it

These are nearly similar to the transformers with the top cover type and the main consideration is that the weight to be lifted is now the weight of the active part plus the weight of the top cover. Also in these type of transformers OLTC etc. are all mounted on to the top cover and weight eccentricity can be experienced. The transformer is to be first


406

inspected from inside and then the slings of different lengths may have to be used for balancing the same during the lifting of the job. 6.4.3 Other Facilities and Related Requirements The place selected to carry out the repair and/or refurbishment activity should preferably be covered, but in case of unavailability of such space then a temporary cover around the transformer may have to be built such that the top of the same is removable and the crane hook can be brought in after removing the cover and the required activity is carried out. Also at some places it will be preferable to build a temporary cover such that the same can be rolled out to expose the transformer for work requiring crane access and then rolled back to cover the transformer again. The area/location for repair/refurbishment activity of the transformer should have adequate space so as to store the removed fitments from the transformer at the same place. Generally when the transformer is disassembled for repair/refurbishment lot of fittings and mountings on the transformer cover will have to be dismantled. Internally the active part may be earthed to the tank and in some of the older designs of temperature sensors the WTI CT are mounted as part of the active part and the CT connections for the heater are terminated at the end of the pocket as the heater is part of the pocket and not part of the meter. Also the place should not be near to any source of open water and also no water dripping should occur near by. The other facilities for site repair of the transformers and related requirements are tabulated below : Sl. No. Facility

Require ments

1.

(i)

Lighting

(ii) (iii) 2

Fans & ventilation

(i)

(ii) (iii) 3

Access method

(i) (ii)

Should be provided with cool lighting as the internal environment is going to be warm and oil saturated. All light fittings should be provided with glass cover, Hand lamps are to be explosion proof type. The routing of the supply wires to the various light fittings and facilities should be preferably new wires and without joints, and all termination's to be properly secured. Ventilation is of paramount importance. As the transformer exposed parts are to be protected from moisture. Ingress it is desirable that the internal environment be maintained hot and dry. Humidity control will ensure faster final drying out. The enclosure environment can be kept dry by injecting dry air through a dry air generater. The routing of the supply wires to the various light and fittings should be preferably new and without joints, and all termination's to be properly terminated. Access to the enclosure is to be highly regulated, preferably as far as possible dirt carrying objects like shoes etc. are to be kept out of the enclosure, The access opening should not be directly open type, it should be at such a place that at least two covers should overlap each other.

Guidelines for Repair of Power Transformers at Site 4

Cleanliness

5

Enclosure

6

Power

7

Storage

8

Safety

407

(i) Shoes should be left out side. (ii) The area within the shed is to be covered with tarpaulin in case it is not hard surface and should be cleaned with vacuum cleaners only. (iii) The t ransformer areas where people are moving while working are to be cleaned at least 2-3 times daily with vacuum cleaning and any oil wet smears are to be cleaned with oil washing. (i) The enclosure size should be such that it should accommodate all the materials being removed from the active part assembly at the same place. (ii) The structure should be rigid and the moving part should be light and arrangements should be such that after placing it should be easily tied to the main structure tightly. (iii) Height of the enclosure should be 1½ man height above the topmost part of the transformer tank. (i) The power supply should be made available at two places near the supply work area but not within the enclosure. (ii) Half of all the fittings are to be supplied from one of the source and the other half from the other source. (iii) The light and power fittings from one source are to be fitted all around the enclosure and the second set in the same fashio n. (iv) In case any one source failure the work enclosure will not get dark and adequate light and air circulation will be maintained. (v) No live wire insulated or otherwise is to be routed over the transformer active part during the repair process and are to be only routed along the enclosure body. (i) All insulation materials are to be wrapped in polyethylene, bound and tagged and stored in the sequence of their removal from the active part (e.g. : The part removed first from the winding is to be kept nearest to the winding and in front while the last opened object is to be located at the farthest so that it is also the last object assembled). (ii) The windings not to be handled are to be wrapped preferably wi th polythene during the working hours and removed(when subjected to any hot oil bath or treatment. (iii) All common blocks, winding clamping ring, winding pressuring /packing blocks, washers are to be tied and stacked to-gather and preferably kept soaked in oil drum. (i) As the transformer active part insulation is a combination of paper soaked in oil it is a potential fire hazard and should never be left unattended. (ii) All lighting fixtures are to be of cool daylight t ype. (iii) No electrical connections are to be made by twisting wires, they are to be made by male female connectors. (iv) Joints of wires taped with insulation tapes should be of bright colours so that any heating could be detected easily by discolouration. (v) The whole enclosure is a no smoking zone. (vi) Fire extinguishers both. of Dry chemical powder and CO2 gas type are to be located such that there are no obstacles in the path. (vii) Asbestos sheets should be available handy to cover any ignited surface. (viii) No inflammable material should be stored near the work area. (ix) There should be no fire source near the work area.


408 9

Safety aspects related to working inside the transformer

Working within the transformer require special skills, some of the common and important safety measures are: (i)

All tools being taken inside the transformer tank must be tied at one end with some object outside the transformer tank or to the wrist of the person going to work inside the transformer. (ii) For climbing in and out of the transformer it is mandatory that rope ladders be used for climbing in and out of the transformer. At no point of time metallic ladders should be used as slipping can cause damage to manpower or the active part. (iii) Person working on or within the transformer should not carry anything metallic in the pockets like loose change, pens etc. All watches etc. are to be removed before entering the transformer tank. (iv) The clothes worn during the visit inside the transformer should not contain any metallic buttons etc. Also the belts are to be left outside as generally they contain metallic buckles. (v) Movement within the transformer should be very carefully done, so that body weight is not put on any cleates or various supports used within the transformer. (vi) The active part should not be disturbed without marking the present position at two or three reference points so that the various clearance are maintained as per the. manufacturer recommendations. (vii) No body weight should be put on the various connection leads of the winding to the bushing, tap coil to the OLTC etc. (viii) Any new material being fitted within the transformer particularly insulation materials made of pressboard or premawood should be first dried in a oven at 90o C for minimum of 12 hours preferably in a vacuum oven and then soaked in filtered oil for an equal duration. Thicker items require longer soaking time.

7.0

QUALITY ASPECTS DURING THE REPAIR OF THE TRANSFORMER AT SITE

7.1

The Joints Quality

The connection phase consists of joining of the winding to various cables and their further termination on to the OLTC/OCTC or to the various bushings. For the termination of the winding to cables and the subsequent connection two process have been followed : l

Jointing by brazing.

l

Jointing by crimping.

As the repair unit is generally oil soaked the repair by crimping is a better method. For jointing by brazing resistance brazing for smaller dimension cables and copper conductors can be followed but for large dimension cables and multiple parallel conductors flame brazing is followed. The flame brazing process is well established and highly skilled workers are required with adequate provision for safety to avoid any incident of fire hazard. The process also entails surrounding the nearby oil soaked insulation with asbestos sheets. After the brazing has been carried out the joint is to be cleaned of all burn marks and the brazing material spatters the joint is filed to bring about smooth finish so as to avoid any sharp points and subsequent damage to the taping.


409

The crimping method is a cleaner and a safer process because no heating is involved but it requires custom designed ferrules and sockets to be crimped on and the crimping machine and joint qualification be carried out before the start of the jointing process. 7.2

Shrinkage Arresting after Drying Out

This is a grey area in repairs but a mandatory process is to be followed for any repair jobs for the successful completion of the repair process. Various manufacturers have their recommendations regarding the methods to be followed for this process. A standard recommendation could be that after completion of drying out process the transformer is to be oil filled and the insulation soaked in for duration of 24 hours. After that the oil be drained and the job extracted and the coils clamped to required height. The job is then retanked and the drying out process repeated for 2 more cycles before final oil filling and circulation and preparation of the job for testing and final commissioning. The above mentioned recommendations are based on site experiences. 8.0

TOOLS REQUIRED FOR SITE REPAIRS

A tentative list of tools required for the activities related to transformer repairing at site is given below : 8.1 Sl. No.

Tools Tools

1.

Special tools for dismounting of the OL TC.

2.

D shackles -1 ton

3.

D shackles -5 ton

4.

D shackles – Max. load /3, multiple quantities

5.

Slings - 1 ton (wire rope/nylon)

6.

Slings – 5 tons (wire rope/nylon)

7.

Slings – Max load/3, (wire rope) 4 nos.

8.

Pump – 1/2 hp

9.

Pump – 3 hp + starter

10.

Hose 25 m long minimum (multiple quantities)

11.

Extension board – 3 ph & 1 ph

12.

Spanners DE type (size : 8-9 to 32-36

13.

Spanners ring type (size : 8-9 to 32-36)


410 14.

Ring & DE spanner 55 size

15.

Box spanner (size : 8-9 to 32-36)

16.

Torque spanner handles

17.

Sledge hammer – 5 kg

18.

Hammer - 1 kg

19.

Mallet

20.

Screwdrivers -various sizes

21.

Hacksaw frame standard size

22.

Hacksaw blades HSS type

23.

Tommy bars -20 mm dia 3 ft’ long

24.

Chisels: 1 in wide, 2' wide

25.

No. punch (8 8) various sizes

26.

Flat files (small & regular

27.

Half round files (small & regular)

28.

Round files (small & regular

29.

Welding machine

30.

Drilling machine

31.

Grinding machine

32.

Gas cutting set with cylinders

33.

Pipe 3” dia 5 ft. long

34.

Knives for paper cutting etc.

8.2

x

Special Tools

1.

Winding lifting arrangement

2.

Lead cutting tools

3.

Brazing machine

4.

Brazing rods

5.

Brazing torch nozzles and oxy-acetylene cylinders

Guidelines for Repair of Power Transformers at Site 6.

Hydraulic Crimping Tools (cutting tools & crimping dies)

7.

Stools for yoke unlacing of adequate height and wt. capacity

8.

Nylon slings for tying the winding to lifting frame

9.

Wooden / ms pallets for lamination storage

10.

Fans

11.

Torches

12.

Vacuum cleaners

13.

Lamination stack tying arrangement

14.

Lamination stack tying arrangement

15.

Asbestos sheets for heat insulation during brazing.

16.

Dry air generator and compressor setup.

17.

Scaffolding arrangement for access to the active part during

8.3

Test instruments

1.

2 kV test set

2.

Meggar 1 kV

3.

Meggar 5 kV motorised

4.

Ratio meter

5.

Winding resistance kit

6.

Multimeters

7.

Dew point meter

8.

Tandelta kit

9.

Clampmeters

10.

Variac 1 ph and 3 ph

11.

Continuity tester.

411


412 9.0

CONSUMABLES REQUIRED FOR REPAIR/REFURBISHMENT JOBS ARE

Sl. No.

Materials

1.

Crepe Paper Rolls -25 mm wide

2.

Crepe Paper Rolls -15 mm wide

3.

20 mil Paper Rolls

4.

PCB Sheets -1.5 mm thk. (impregnated)

5.

PCB Sheets -3.0 mm thk. (impregnated)

6.

Cotton Tape Rolls-l/2"

7.

Cotton Tape Rolls-I"

8.

Newar Tape Rolls-

9.

Permali studs & nuts -10 mm

10.


11.


12.


13.

25 thk' PCB/PW off cuts (impregnated)

14.

PVA glue

15.

Fibre glass tube-core bolts

16.

Fibre glass washer-core bolts

17.

Oil drums with 209 litres each as per IS : 335

18.

Common Blocks (various sizes)

19.

Terelene Tapes

20.

Cotton waste

21.

Cotton cloth (markin type)

22.

Sand paper

23.

Blanking plates-various sizes

24.

Plastic sheets/polythene

25.

Carbon Tetra Chloride (cleaning agent)

26.

Wooden coil bobbins

Guidelines for Repair of Power Transformers at Site 27.

Empty oil drums

28.

Wooden boxes with locks

29.

Plastic bags (small)

30.

Plastic bags (medium)

31.

Undelible markers

32.

Tags (fibre)

33.

String roll

34.

Plastic buckets and mugs

35.

Tarpaulins (large)

36.

Themocoal foam -8 mm thk. Profil

10.0

413

DRYING OUT OF TRANSFORMERS AT SITE AFTER REPAIRS

Various methods are prevalent for carrying out the final drying out activity at site after the repairs have been completed. Some of them are briefly discussed below : 10.1

Drying out by Hot Air Blowing and Vacuum Pulling

In this method the active part is heated up by blowing in hot air into the tank and the temperature of the active part is raised to a higher temperature of around 70o to 75o celcius and then vacuum pulling is done for extraction of moisture from the insulating materials. Multiple cycles of heating and vacuum pulling, are carried out till the termination criteria is reached. Oxygen present in the hot air has a tendency to oxidize the thinner insulation faster than the thicker insulation. This method is not recommended for CCA above 66 kV class. 10.2

Drying out by Hot Air Blowing

In this method hot air is blown into the main tank and the active part is heated up, the temperature of the hot air is themlostatically controlled at the blower end at around 70o to 75o celsius and the drying process is continued in this manner. This method is adapted for transformers whose tank cannot withstand high vacuum. The hot air remo ves moisture from the insulation of the active part and the vapour pressure difference ensures removal of moisture from the insulating material to the hot air. The presence of hot oxygen in hot air tends to oxidize the thin insulation faster than the thick insulation. The process is continued till the termination criteria is achieved. This method is recommended only for 33 kV class CCA and below. 10.3

Heating in Inert Environment and Vacuum Pulling

In this the transformer tank is first evacuated and then filled with dry N2 gas and the active part heated up by applying external heat to the tank body. The heating can be by means of Halogen or infrared lamps or by winding some turns of cable on the tank body and passing

414


low voltage and high current through the cable. The temperature of the active part is raised between 80o to 90o celsius and then vacuum pulled on the tank for faster extraction of moisture from the active part insulation. Due to the inert atmosphere this is the most widely adopted method and this ensures that the oxidation of the insulation paper is minimized. Multiple heating and vacuum cycles are repeated till the proper drying out criteria is achieved. 10.4

Heating by Hot Oil Circulation and Vacuum Application

In this method the heating of the active part is carried out by circulation of hot oil. The maximum temperature of the oil can be 60o celsius. On reaching the desired temperature the oil is drained very fast from the tank and vacuum applied on the tank. The fast draining is necessary to minimize the heat loss during the oil draining operation. The heating of the oil is carried out by high vacuum oil filter machine and during the process the moisture carried by the oil is removed during the passage through the filter machine. Multiple heating and vacuum cycles are repeated till suitable drying out criteria is achieved. 10.5 Heating by Oil Spray and Vacuum Pulling In this method hot oil is sprayed on to the active part through pipes mounted inside the tank with multiple nozzles in them. During this process vacuum is also applied into the tank to remove the moisture from the insulating material. The heating through oil spray is limited to again up to 60 degrees for the oil but this method is least prevalent as the installation of spray pipe is required inside the tank, but widely followed in the eastern European countries. The main advantage of this process is the simultaneous flushing of the moisture from the insulation by the oil and also the cleaning of the insulation due to the oil flow on the insulation. 10.6

Drying Out by Hot Oil Circulation

Generally used for small transformers upto 10 MVA and 33 kV class of transformers where the insulation mass is not much and the tank is not designed to withstand full vacuum. Generally special transformers like furnace and rectifier transformers are also in this category of transformers and the oil does the heating and also the moisture removal from the insulation. 10.7 Termination Criteria for the Drying Out Process The termination criteria for the drying out process is expressed differently by different manufacturers but they are all aimed towards achieving good dry insulation which ensures trouble free operation of the equipment. The moisture content in the insulation is kept at around 0.5% of the insulation weight for new transformers at the time of manufacturing. But for older transformers the moisture content is kept between 1 to 2 % of the weight of the insulation, as beyond this value there is a chance of the thin paper insulation getting brittle during the heating and vacuum pulling process. Some of the common methods of measurement of the termination criteria are :


(i)

415

Measurement of Insulation Resistance Values

Measurement of the insulation resistance values through the drying out process and this is generally followed for small transformers and those being dried out by hot air or hot oil circulation. The IR curve has a tendency to follow the bath tub curve and the subsequent calculation of the Polarization Index values form the Insulation resistance values. (ii)

Dew Point Measurement

The dew point of the dry gas generally N2 is taken before the same is injected into the tank and the value should be around –60o celsius. It is then held inside the tank for 24 hours to achieve vapour phase balance and then again measure the dew point value. Dew point values at around –25o celsius at a temperature 40o celsius are considered satisfactory but lower the value better is the drying out. The process needs to be repeated with fresh dry gas everytime till the termination criteria is achieved. (iii)

Tan Delta Value of the Insulation

Generally followed for transformers being dried out with oil circulation and oil being used for heating of the insulation. Voltage should not be applied to the active part under vacuum. The tandelta measurement should be done after oil filling, circulation and standing time. Tan delta values of 0.5 are considered to be good values for termination of the process. (iv)

Rate of Moisture Extraction

Another method is the measurement of the water extraction rate during the vacuum pulling process applied after the heating phase. The rate of water extraction after certain duration is measured and the acceptable value of around 30 to 50 ml/hour/ton of insulation would be considered sufficient for termination of the process. One school of thought advocates the measurement of the water extraction rate of 100 ml/hr, reading repeated for 3 consecutive hourly readings irrespective of the quantum of the insulation. (v)

Recommendation of the Manufacturer

Termination criteria as recommended by the manufacturer can be considered as the final say in the matter and multiple measurements can be carried out to confirm the achievement of the termination criteria. The factory based drying out process using HAV or vapour phase method has the termination criteria of 30 gms /hour/ton of water extraction rate. 11.0

REFURBISHMENT OF POWER TRANSFORMERS

Refurbishment can be defined as an activity undertaken to improve the existing condition of the transformer or transformer insulation.

416


The options available for the transformer refurbishment are as under: (i)

Over loading the existing transformers with additional coolers and increased condition monitoring with insulation refurbishment.

(ii)

Refurbishment of the transformer.

11.1

Requirement for Option (i) above

(a)

For the new loading, recalculation of the temperature rises of the top oil and the windings temperature for the 145, 245 & 420 kV class transformers.

(b)

Check the extent of the excess over the permitted / standardized values.

(c)

If additional cooling keeps the temperature within the permitted value then the existing winding can be used with minimum risk.

(d)

Parallely the diagnostic testing to determine the residual life of the transformer insulation, the present condition of the insulation and the mechanical condition of the active part should also be done.

11.2

Requirement for Option (ii) above

When the calculated temperature rise of the windings exceed permissible limits and extra cooling is not effective but the condition of the insulation indicate high residual life, the best effective situation could be : (a)

Use the same core with necessary insulation change.

(b)

Use of the same tank but with changing of the gaskets.

(c)

Use of latest methods to reduce the stray losses.

(d)

Replacement of the old winding by new winding & new insulating material.

(e)

Suitable up-gradation of off circuit tap switch or OLTC.

(g)

Suitable modification to use new bushings.

(f)

Suitable testing process could be identified to check and prove the various parameters after refurbishment and refurbishment could be carried out even at site to minimize the time and cost implications.

While planning for overhauling/refurbishment of transformers it is necessary to determine at the onset the scope of work to be carried out. The refurbishment activity can be categorized roughly into following : (a)

Improving the insulation condition of the transformer with no replacement of oil and the condition assessment and RLA indicates significantly good condition.


417

(b)

Improving the insulation condition of the transformer with replacement of oil as the oil condition is beyond improvement by filtration and RLA indicates good life condition.

(c)

Improving the insulation condition of the transformer and also minor changes to accessories for up-rated operation.

(d)

Improving the insulation condition of transformer with major overhauling and repairs of the transformer for up-rated operation.

11.3 The Main Activities/Benefits of Refurbishment one listed below : 11.3.1 Activities •

Inspection, testing and repair / replacement / up-gradation of protective devices fitted on the transformer (Temp. indicators, Bucholz relay etc.)

•

Changing of old gaskets and stoppage of leaks from the main tank and accessories.

•

Inspection and up-graqation / repair / replacement of bushings, bushing leads.

•

Inspection and up-gradation / repair / replacement of off -load or on -load tap changer diverter switch, selector switch and motor drive unit.

•

Fitting of new / latest condition monitoring systems.

•

De-sludging and tightening of the various joints with an eye not to introduce any other problem in the transformer during the over hauling.

•

Generally most of the HV windings have wraps and removal of the same during inspection for assessing the condition of winding could be beneficial in terms of desludging and suspected deterioration due to heating or PD.

•

Re-fitting of the wraps (removed for inspection) as per the original is of utmost importance particularly for transformers with DOF cooling.

•

If the oil parameters are within limits then vacuum re-conditioning of the oil.

•

Reconditioning or modification of the conservator or sealing system.

•

Drying out of the transformer with heating and vacuum cycles in its own tank.

11.3.2 Benefits •

Slowing the ageing rate of the paper insulation and oil.

•

Humidity in the paper insulation to be kept in the range of 1% -2%.

•

Improving or sustaining the short circuit withstand capacity of the winding (ageing reduces the withstand capacity) generally by coil re-tightening.

CBIP Publication No 295

Recommend Documents