M a inte na nc e Pla n HV Ma c hi hines nes
ABB Limited
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Level 1 - Inspection When is Level 1 Inspection to be conducted? 1. After every 5% consumption of insulation insulation life based on thermal degradation model, or on alarms/deteriorating thermal or vibration conditions or any other abnormal event as reported by the customer or on completion of 10000 equivalent operational hours and intervals of the same, whichever is earlier. Objective: 1. To assess the thermal thermal life of the insulation. 2. To identify bearing related defects, and assess the impact of conditions that generate forces on the bearings 3. To identify rotor rotor winding related and other electromagnetically electromagnetically related defects 4. To determine the requirement of Level 2, Level 3 or Level 4 Inspection/Maintenance Inspection/Maintenance Scope of work: The following data is to be sent by customer. 1. Recording of temperatures – Winding, Core, Ambient, Bearings 2. Review of operational data – Starts/stops (hot/cold), overloads, overloads, Op. Hrs., (See Attached Question Form in Appendix I) 3. To perform a vibration and stator line current current analysis on the motor or alternatively, To analyze data collected and submitted by the customer from vibration and stator line current spectra. It is to be noted that the above vibration analysis is in addition to routine vibration measurements that are performed by plant personnel. Expected downtime: 0 day (Online Analysis) Deliverables: ABB will analyze the data submitted and will submit a report report that will include information relating to the following 1. Extent of the thermal thermal life degradation of the insulation insulation 2. Recommendations for bearing maintenance 3. Maintenance plan based on insulation insulation condition, bearing bearing condition or rotor windings 4. Analysis of machine machine losses for detection of possible possible thermal abnormalities (for (for ABB Machines only)
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Level 1 - Maintenance When is Level 1 maintenance to be conducted? 1. During machine operation operation on basis of Level I inspection analysis or on information of abnormal increase in winding temperatures from site personnel. Objective: 1. To take necessary action based on Level-1 inspection inspection or based on customer intimation from site. Scope of work: 1. Cleaning of filters, where present 2. Changing of filters, where possible on-line. (New filters will be supplied by customer)
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Level 2 - Inspection When is Level 2 inspection to be conducted? 1. On the basis of Level – 1 analysis, for every 10 % consumption of the insulation life based on the thermal degradation model for the insulation. 2. or on alarms/deteriorating thermal or vibration conditions or any other abnormal event as reported by the customer 3. or after every 20,000 equivalent hours of operation or 2 years, whichever is earlier. Objective: 1. To assess the machine condition by performing measurements on the stator windings. 2. To identify bearing related defects, and assess the impact of conditions that generate forces on the bearings 3. To identify rotor winding related and other electromagnetically related defects 4. To develop a maintenance plan for life extension of the machine windings. Scope of work: Before stooping the machine for a Level 3 inspection, all data as required for a level 1 inspection are to be collected and analyzed. After stooping and disconnection of the machine from the power supply Testing the Machine a) Stator Windings i)
Polarization-Depolarization Current Analysis
ii) Capacitance & Tan Delta Analysis iii) Non-linear Analysis iv) Partial Discharge analysis v) Winding resistance For detailed scope of work, see Appendix II, Part A Expected downtime: 1 day Pre-requisites: 1. The machine terminals should be disconnected from the main bus bars. Deliverables: 1. Condition Assessment report for the stator windings for a more accurate calculation of the insulation degradation. 2. Recommendations for bearing maintenance 3. Maintenance plan based on insulation condition, bearing condition or rotor windings. 4. Maintenance plan with tentative Level 3, 4 Inspection schedules.
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Level 2 - Maintenance When is Level 2 maintenance to be conducted? 1. During the Level 2 inspection, when the machine is shutdown Scope of work: 1. Cleaning/changing of filters (where present) 2. Bearing Maintenance a) Sleeve Bearing i)
In position dismantling of the bearings.
ii) Checking of initial bearing clearances. iii) Inspections of bearing housings for leaks, tightness of fastenings and guide support. iv) Checking of wear and damage of shaft seals and hemp packing and cleaning of drain holes in seals. v) Inspection of bearing seat surfaces on shaft and in bearing housings vi) Replacement of bearing oil as per the original grade recommended by the OEM, if required. b) Anti-friction Bearing i)
Remove external grease caps.
ii) Check tightness of lock nuts. iii) Inspect condition of grease retainers, spacers, lock washers and other external components for signs of wear and damage iv) Replenish grease as per recommended procedures.
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Level 3 - Inspection When is Level 3 inspection to be conducted? 1. If Level – 2 analysis indicates a 25 % reduction in estimated insulation life. 2. or on alarms/deteriorating thermal or vibration conditions or any other abnormal event as reported by the customer 3. After first 40,000 equivalent hours of operation or 5 years, whichever is earlier. Objective: 1. To assess the machine condition. 2. To identify bearing related defects, and assess the impact of conditions that generate forces on the bearings 3. To identify rotor winding related and other electromagnetically related defects 4. To determine the time for performing Level – 4 inspection/maintenance. 5. To plan suitably for Level – 4 inspection/maintenance with aim to reduce the shutdown time. Scope of work: Before stooping the machine for a Level 3 inspection, all data as required for a level 1 inspection are to be collected and analyzed. After Stoppage, Disconnection and end cover removal: 1. Visual inspection of a) Terminals/Bushings and HV connections b) Bearing condition c) Endwinding portion of stator winding d) Supports/ Tie-ups/blocks along Stator e) Rotor bars/winding/Rotor coil supports f)
Balancing weights
g) Slip rings (if present) h) Brush rocker assembly (if present) i)
Fans
j)
Air gaps
2. Testing the Machine: a) Stator i)
Polarization-Depolarization Current Analysis
ii) Capacitance & Tan Delta Analysis iii) Non-linear Analysis
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iv) Partial Discharge analysis v) Winding resistance measurement b) Rotor (Wound Rotor or Synchronous Machine Rotor) i)
Polarization-Depolarization Current Analysis
ii) Winding Resistance/Impedance Measurement c) Exciter & Exciter Windings (Synchronous Machines) i)
IR Measurement
ii) Winding Resistance/Impedance Measurement iii) Diode Check d) RTD & Space Heater Check (see relevant parts of Appendix II Parts A & B for details) Expected downtime: 3 days Pre-requisites: 1. The machine terminals should be disconnected from the main bus bars. 2. The inspection windows should be opened where present 3. The end-covers/end cover need to be opened, where possible. Deliverables: 1. Condition Assessment report for the stator windings for a more accurate calculation of the insulation degradation of the stator as well as, condition assessment of the rotor and exciter. 2. Recommendations for bearing maintenance 3. Maintenance plan based on insulation condition, bearing condition or rotor windings 4. Maintenance plan with tentative Level 4 Inspection schedule.
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Level 3 – Maintenance When is Level 3 maintenance to be conducted? 1. Level 3 maintenance is to be conducted when the machine is stopped for the level 3 inspection. 2. During the Level 3 maintenance the machine is shutdown and end covers are removed. Scope of work: 1. Cleaning of filters (where present) 2. Bearing Maintenance a) Sleeve Bearing i)
In position dismantling of the bearings.
ii)
Checking of initial bearing clearances.
iii)
Inspections of bearing housings for leaks, tightness of fastenings and guide support.
iv)
Checking for wear and damage of shaft seals and hemp packing and cleaning of drain holes in seals.
v)
Inspection of bearing seat surfaces on shaft and in bearing housings
vi)
Recording of bearing insulation resistance.
vii)
Re-fitting of end covers and bearings after completion of electrical maintenance.
viii)
Replacement of bearing oil as per the original grade recommended by the OEM.
b) Anti-friction Bearing i)
Clean and inspect all bearing components for signs of wear.
ii) Clean and inspect bearings to the maximum extent possible. iii) Check lock nuts for tightness. iv) Check fit of outer race in housing/on end cover. v) Re-grease prior to assembly vi) Replace bearing if warranted. 3. Brush Rocker Assembly Maintenance (where present) i)
Cleaning of brush rocker assembly using solvents
ii) Dry-out of the brush rocker assembly after cleaning iii) Replacement of brushes (if found damaged) iv) Brush seating v) Integrity check of connections 4. Stator Maintenance i)
Cleaning of the overhang portion of the winding.
ii) Dry-out of the stator windings
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iii) Polarization-depolarization Current Analysis (repeated post cleaning-drying to evaluate efficacy of cleaning and drying process). 5. Stator Terminals i)
Inspections of all line and neutral connections
ii) Checking of tightening torque on all connections iii) Checking/Installation of Thermal Tags 6. Rotor Maintenance (synchronous machines) i)
Cleaning of the overhang portion of the winding, slip rings and other accessible areas.
ii) Dry-out of the rotor windings iii) IR – PI measurement post cleaning and drying
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Level 4 - Inspection When is Level 4 to be conducted? 1. If Level –1, 2 or Level – 3 analysis indicates a 50 % reduction in estimated life. 2. or on alarms/deteriorating thermal or vibration conditions or any other abnormal event as reported by the customer 3. After first 80,000 hours of operation or 10 years, whichever is earlier. Objective: 1. To assess the stresses on the machine and identify the effects of the same on the insulation system. 2. To identify bearing related defects, and assess the impact of conditions that generate forces on the bearings 3. To identify rotor winding related and other electromagnetically related defects 4. To determine the residual life of the stator windings. 5. To carry out maintenance activities as recommended by Level - 2 & Level - 3 plans. 6. To increase its life of the stator insulation system. Scope of work: Before stopping the machine for a Level 4 inspection, all data as required for a level 1 inspection are to be collected and analyzed. After Stoppage, Disconnection and rotor removal: 1. Visual inspection of a) Stator windings and insulation b) Slot portion of the winding and wedge condition c) Supports/ Tie-ups/blocks along Stator d) Stator core laminations e) Rotor bars/winding/Rotor coil supports f)
Balancing weights
g) Slip rings (if present) h) Shaft i)
Air gaps
j)
Terminals/Bushings and HV connections
k) Bearing condition l)
Cooling system, ducts/tubes
m) Seals, Gaskets n) Brush rocker assembly (where present) o) Fans p) Lubrication system
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q) Instrumentation and protection system associated with the unit 2. Residual Life Assessment of the Machine: (See the relevant sections in Appendix II, III and IV for details) a) Stator i)
Polarization-Depolarization Current Analysis
ii) Capacitance & Tan Delta Analysis iii) Non-linear Analysis iv) Partial Discharge analysis v) Winding resistance vi) Wedge Mapping vii) Flux Loop Test viii) Coupling Resistance Measurement (where possible) ix) Corona PD Probe Test x) Stress Analysis using FEM b) Rotor (Wound Rotor and Synchronous Machines) i)
Polarization-Depolarization Current Analysis
iii) Recurrent Surge Oscillograph (RSO) Test (turbo rotors only) ii) Winding Resistance/Impedance Measurement c) Exciter & Exciter Windings (Synchronous Machines) i)
IR Measurement
ii) Winding Resistance/Impedance Measurement iii) Diode Check d) RTD & Space Heater Check Pre-requisites: 1. The rotor needs to be threaded out, and suitably mounted for inspection Deliverables: 1. Condition Assessment report for the stator windings for a more accurate calculation of the insulation degradation. 2. Recommendations for bearing maintenance 3. Maintenance plan based on insulation condition, bearing condition or rotor windings 4. Maintenance plan with scope definition for Level 4 maintenance.
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Level 4 - Maintenance When is Level 4 maintenance to be conducted? 1. During the outage of the machine for Level-4 inspection Scope of work: 1. Cleaning of filters and/or replacement (condition based) 2.
Bearing Maintenance a) Sleeve Bearing i)
In position dismantling of the bearings.
ii) Checking of initial bearing clearances. iii)
Inspections of bearing housings for leaks, tightness of fastenings and guide support.
iv)
Checking for wear and damage of shaft seals and hemp packing and cleaning of drain holes in seals.
v)
Inspection of bearing seat surfaces on shaft and in bearing housings
vi)
Recording of bearing insulation resistance.
vii)
Re-fitting of end covers and bearings after completion of electrical maintenance.
viii)
Replacement of bearing oil as per the original grad recommended by the OEM.
b) Anti-friction Bearing i)
Clean and inspect all bearing components for signs of wear.
ii) Clean and inspect bearings to the maximum extent possible. iii) Check lock nuts for tightness. iv) Check fit of outer race in housing/on end cover. v) Re-grease prior to assembly vi) Replace bearing if warranted. 3. Brush Rocker Assembly Maintenance (where present) i)
Cleaning of brush rocker assembly using solvents
ii) Dry-out of the brush rocker assembly after cleaning iii) Replacement of brushes (if found damaged) iv) Brush seating v) Integrity check of connections
4. Stator Maintenance i)
Cleaning of the overhang portion of the winding.
ii) Dry-out of the stator windings
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iii) Epoxy spray impregnation of windings iv) Heating of stator for curing of applied epoxy resin v) Providing a final coat of a moisture resistant anti tracking varnish on windings vi) Polarization-depolarization Current Analysis (repeated post cleaning-drying to evaluate efficacy of overhauling process. 5. Stator Terminals i)
Inspections of all line and neutral connections
ii) Checking of tightening torque on all connections iii) Checking/installing thermal tags
6. Rotor Maintenance (Wound Rotor and Synchronous machines) i)
Cleaning of the rotor winding and slip rings.
ii) Dry-out of the rotor windings iii) IR – PI measurement post cleaning and drying iv) Epoxy spray impregnation of windings v) Heating of rotor for curing of applied epoxy resin vi) Providing a final coat of a moisture resistant anti tracking varnish on windings
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APPENDIX I Namep late Details
Tag ID Sr. Number Type Manufacturer Frame kW/HP Voltage Current Frequency Speed Power Factor Efficiency Insulation Class Enclosure Cooling Test Certificate Details
No Load Current (A) Wdg Resistance (m? ) Winding Cap (pF) Starting Torque Pull Out Torque Pull Up Torque pf at 25% load pf at 50% load pf at 75% load pf at 100% load Start-Stop Details
Starting Current Starting Time (NL) Starting Time (Load) Number of Starts/stops Total Operational Hours
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Load Cycle
(ON TIME)
kW-1 Current-1 kW-2 Current-2 KW3 Current-3 kW4 Current-4 Load Cycle
(OFF TIME)
kW-1 Current-1 kW-2 Current-2 KW3 Current-3 kW4 Current-4 Operating Details
SET - 1
SET - 2
Voltage Current kW Speed Frequency B e ar i n g N u m b e r s
NDE DE Type Vibrations H DE V DE A DE H NDE V NDE A NDE
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RTD Temperatures Winding 1 2 3 4 5 6 7 8 Core 1 2 Bearing NDE DE Body Maximum Ambient Av Day max Av Day min
Other Details
Breaker Type Dist from Machine Relay Type Surge Arrestor at
Machine end Breaker end
Unusual Conditions
Last Rewound
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APPENDIX II PART A
S t at o r W i n d i n g :
a) DC Charging-Discharging Current Analysis or Polarization Depolarization Current Analysis (PDCA): PRINCIPLE: [1]
[2] [3]
[4]
A highly regulated dc step voltage with a stability of < 1 v/sec, is applied to the winding/windings using a highly regulated electronic power supply in the range of 100 Volts to 2500 Volts, depending on the machine's working voltage. The voltage is maintained for a period of not less than 1000 seconds. The current flowing through the insulation is monitored during the charging period. After all relevant data is obtained, the windings are discharged through a micro ammeter and discharge currents are monitored after the initial winding capacitance discharge (< 5 secs), over a total time period that will not be less than the charging time period. The charging and discharging currents are plotted on a log-log scale and analyzed in the time and the frequency domains.
The following parameters are computed from the measurements performed: (1) Ion Migration Time Constant (2) Slow Relaxation Time Constant (3) Interfacial Polarization Time Constant. (4) Mobility Index of the Binding Resin. (5) Ion Concentration Index. (6) Dispersion Ratio. (7) Surface Resistivity of the Endwinding. (8) Insulation Resistance & Polarization Index. (9) Charge Storage values (10) Aging Factor On the basis of the above an assessment of the winding insulation is made with regard to
General insulation quality Sensitivity of the insulation system to moisture absorption Presence of contamination in the windings Condition of the binding resin
b) Tan delta and Capacitance Test: PRINCIPLE: Tan ? and Capacitance will be measured both below and above discharge inception voltage, at voltage levels that will be determined on the basis of discharge inception with the aim of accessing the maximum required data for analysis.
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Capacitance and tan delta measurements will be performed using a transformer ratio arm bridge. Measurements will be performed at increments that will not exceed 0.2 V L. Maximum test voltage will be (1/ ? 3)*VL, r.m.s. Resulting Data will be analyzed to obtain the f ollowing parameters: 1. 2. 3. 4.
Discharging void volume ratio (if discharges are present) Effective phase of occurrence of discharges Characterizing constants if variations are due to stress grading Effective area involved in slot discharges (if slot discharges are present).
c) Partial Discharge Test: PRINCIPLE: Partial discharge pulse patterns will be monitored and recorded using a transformer ratio arm bridge with appropriate coupling capacitors. The p.d. pulse patterns will be analysed with regard to pulse count, pulse magnitude, polarity dependence and phase to identify the nature of discharges which can then be classified as: (i) Internal Discharges (ii) Surface Discharges (iii) Slot Discharges
d) Non-linear Insulation Behavior Analysis: PRINCIPLE: To study the non-linear behaviour and characteristics of insulation material. The tan delta and capacitance measurements vary with voltage even in absence of partial discharges and one of the most obvious reasons for such a behaviour is the presence of nonlinear field stress grading system at slot ends. Other reasons are space charge/interfacial polarization due to contamination, electrostatic forces on delaminated insulation surface, partial discharges etc. It is evident that both the voltage supply across insulation and the current passing through the insulation have harmonics, which cause increase or decrease in the measured tan delta and capacitance values. Thus, it becomes necessary to understand this time varying effect of insulation impedance on the capacitance and tan delta measured. Non-Linear Analysis provides a detailed understanding of these non-linearities and supplements the tan delta analysis. The analysis provides additional insights into the a ging of insulation. The machine insulation is tested by applying a known voltage across the insulation and monitoring the voltage and the current flowing through the insulation, by capturing several waveform cycles of the voltage and the current on a digital storage oscilloscope. The insulation is tested at predetermined voltage levels upto a maximum of (1/? 3)*VL, r.m.s. The instantaneous admittance of the insulation is calculated and the admittance patterns analysed for specific harmonic patterns. The extent of harmonics, predominance of odd or even harmonics, high or low frequency harmonics is analysed to provide information on: The integrity of the stress grading system used at the slot ends The contribution of the slot stress grading system, contamination and ageing to the non-linear behaviour
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-
To confirm that observed anomalous tan delta variations can be physically related to the above non-linear phenomena
e) Winding Resistance Measurement on Stator winding : Winding resistance is measured to identify the existence of any shorts, breaks (open circuit) or high resistance joints in the stator winding.
f) RTD Checks: Resistance Temperature Detectors will be checked for ohmic resistance, and the insulation resistance.
Part B
g) Corona Probe Measurements: A probe in the form of an RF coil is mounted on the end of an insulating rod and is used for collecting data at the D-end and the ND-end of the stator winding, with regard to partial discharge activity in the various slots when the end covers are removed. The data collected is viewed on a Digital Storage Oscilloscope and relevant data is stored.
T e st s o n R o t o r :
a) IR & PI test: IR & PI test will be carried out on rotor winding using 500 V megger.
b) AC Impedance Test: For detecting the presence of shorted turns in the rotor winding by comparing obtained impedance values with earlier measured values.
c) Winding Resistance Measurement: Winding resistance is measured to find any inter turn shorts and breaks (open circuit) or for high resistance joints.
d) Charging – Discharging Current Analysis : This test is performed on the rotor to distinguish, in the case of suspected leakage to damage and/or contamination in the rotor windings, whether such problems are localized (damage related) or global (contamination related). Test is performed same way as DC Absorption test on stator winding, at test voltages not exceeding 500 V.
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PART C Other stator checks
a) Flux loop core check: A loop is toroidally wound in the stator core in order to develop a flux level in the stator cotre that is as close as possible to the rated flux, with the power supply that is made available by the customer at site. The stator core temperatures are monitored after a minimum of 30 minutes after the loop is excited, and the surface temperature of the core noted. The increase of surface temperature over the avaerage temperature of the core is noted. This temperature difference should not exceed 10 degree C with the rated flux passing through the core.
b) Wedge Mapping Test: Wedge looseness check will be performed by tapping method and a wedge tightness map is prepared. If ripple springs are provided in the wedges, wedge deflection fixture or other automated techniques are used f or looseness check.
c) Coupling Resistance Measurement: Contact resistance will be measured between the Bar insulation and the Ground (Slot wall) for both the top and bottom bars using a probe, and a chart showing the coupling resistance reading at each slot will be prepared.
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APPENDIX III ADDITIONAL INFORMATION ON MEASUREMENTS PERFORMED STATOR a) Polarization-Depolarization Current Analysis: While performing IR and PI measurements in machines with modern day insulation systems, it is often noticed that good/acceptab le IR and PI values are obtained in spite of the machine windings being excessively contaminated. This is mainly due to the fact that IR and PI measurements are based on “leakage current” detection and are largely reflective of charge transport rather than charge storage mechanisms. Charge storage analysis is useful since it is possible to identify whether charge is stored in normal “traps” within the insulation or within contaminants that are likely to be present in the insulation. Such analysis is therefore aimed at increasing the success rate of identifying the presence of contamination in the insulation. The PDC Analysis is used to quantify and characterize charge storage mechanisms, and is therefore a reliable indicator of presence of contamination. The machine insulation (individual phase as well as three phases combined with respect to ground) is charged for more than 15 min and later discharged through a resistor for a period of time equal to the charging period. The charging and discharging currents are measured at certain fixed intervals of time. The depolarization current is mathematically split into three parts using regression analysis. The three parts are representative of space charge and interfacial polarization phenomenon in slot and end winding region. These phenomena are assumed to have negligible spatial interdependence. These three curves are then analyzed mathematical to calculate three time constants T1, T2, T3 and charge values Q1, Q2 and Q3 attributable to the respective polarization phenomena. Other calculated particulars include: Dispersion ratio: It is ratio of capacitance calculated due to charge storage in the insulation to the geometrical capacitance of the winding and is reliable indicator of contamination. This is generally to be used in conjunction with Q3 or the charge stored due to interfacial polarization on the endwindings. Q1/Q2 Ratio: Knowledge of the distribution of charge storage is important. This ratio gives information on the proportion of charge storage in slot region and is an indicator of problems such as lack of contact of coil with slot. Aging Factor: It provides an indication of aging due to de-polymerization mechanisms of resin that could occur in close vicinity of the electrodes. Besides, the analysis also provides information on the extent of contact of coil with slot, and leakage current sources.
b) Tan delta and Capacitance Test: The PDCA test described above is more sensitive to the surface condition of the insulation. For an in depth understanding of insulation characteristics there is a need for ac measurements. Tan delta measurements have traditionally been conducted at various voltage levels up to the rated voltage of machine, by using a Schering or a Transformer Ratio Arm bridge. The rationale of this method has been to assess the extent of partial discharge activity that occurs in the air spaces in the insulation, which is reflected in the tan delta tip-up measured with increase in the test voltage. However, correlation of the insulation condition with tip-up measurements has not been easy due to the following:
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? ? Guidelines
have been established for testing of coils during quality control checks. These tests are generally performed using guard electrodes in order to discount the losses that occur in the stress grading system employed at the slot ends. While performing measurements on machines in-service, it is not possible to use guard electrodes. The standard norms are therefore no longer applicable. Also, the losses that occur in the stress grading system can be large enough to completely overshadow the losses that are generated by the discharging of air spaces in the insulation. It is therefore not unusual that “tip-up” in tan delta values measured below and above discharge inception, might hardly be perceptible.
? ? Partial
discharges that occur near the voltage peak increase the tan delta tip-up to a considerable extent with very little resultant variation in the capacitance measured. Partial discharges, on the other hand that occur near the voltage zero, increase the measured Capacitance considerably, while having little effect on the tan delta tip-up. This phase dependence of partial discharges on the measured tan delta would imply that despite considerable partial discharge activity tan delta tip-up could be very low, thus defeating the very purpose for which such measurements were intended. Unfortunately, norms generally mention only tan delta values, totally ignoring the change in the capacitance measured.
? ? The
presence of harmonics in the power supply voltage, either due to problems in the voltage source or due to the non-linear nature of the capacitance measured, give rise to variations in the phase of occurrence of partial discharges and therefore have a major impact on the tan delta measured.
? ? The
occurrence of partial discharges in large air spaces (say between the coil and the slot) results in a tan delta curve, which peaks at twice the discharge inception voltage. While measuring machines where there is a combination of loss effects, the tan delta curve will exhibit a decrease with voltage increase, once more making interpretation difficult. Also, in the event of interfacial polarization effects, in the stress grading systems used in the slot and at the slot ends, there will be a reduction in tan delta with voltage increase.
For these reasons, both tan delta and capacitance values are measured with increase in the measured voltage. The maximum voltage employed is the phase to ground voltage. The curves obtained are analyzed both below and above discharge inception voltage to reveal presence of partial discharges and its general location. Unlike conventional tests, values obtained are also analyzed below discharge inception voltage to reveal physical abnormalities like lack of contact of coil with core, interfacial polarization, presence of contamination and looseness of coils/wedges. The variations in tan delta and capacitance due to polarization effects in the stress grading systems, contaminants, and other air spaces are estimated above discharge inception and subtracted from the variations due to the occurrence of partial discharges. An effective maximum change of capacitance is calculated, taking into account the phase of occurrence of the partial discharges. The discharging air space volume is estimated at a given test voltage and is proportional to this maximum change of capacitance calculated. The modification of normal properties of the stress grading system, are also estimated. The test also does not trend the absolute values are they are sensitive to several physical phenomenon, but calculates certain parameters like effective phase shift, discharging void volume content, which make the test independent of previous readings.
c) Partial Discharge Test: Partial discharges have been known to accelerate the aging process. They cause erosion of insulating material and propagate throug h treeing mechanism. Partial discharges in stator winding insulation could be considerably large with little damage in the insulation system, due to the presence of mica, which has a very high discharge resistance. In present day insulation systems, the possibility of internal discharges or discharges that occur within the main insulation are rare. PD phenomenon can be found,
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??
Within the main ground-wall insulation as a result of de-lamination of voids cau sed by missing or incompletely cured bonding material
??
Within the slot when contact is lost between the conducting surface on the coil and the core. This is slot discharge and can cause serious burning of coil surface and slot fillers.
??
In end-winding region due to surface co ntamination, called surface discharge.
??
In the region where coil exits the slots due to sharp change of potential along the surface of coil between the portion grounded to the stator core and ungrounded portion.
Partial discharge causes erosion of the insulating material at the tip of spark, carbonization, ozone formation and even nitrogen-based acids through some chemical reactions. Hence its detection and effective classification is crucial to identify the above-mentioned locations of pd phenomenon. Partial discharge pulse patterns will be monitored and recorded using a transformer ratio arm bridge with appropriate coupling capa citors. The p.d. pulse patterns will be analyzed with regard to pulse count, pulse magnitude, polarity dependence and phase to identify the nature of discharges which can then be classified as: (i) Internal Discharges (ii) Surface Discharges (iii) Slot Discharges
d) Non-linear Insulation Behavior Analysis: The test is supplementary test to both PDCA and Tan Delta & Capacitance Analysis. Traditional measurements performed on stator winding insulation indicate variation in capacitance and tan delta values with voltage, even in absence o f partial discharges. One of the most obvious reasons for this variation is the presence of non-linear field stress grading system employed at the slot ends. Other reasons include space charge and interfacial polarization phenomenon, due to variety of reasons including contamination of the windings, aging of the insulation, and effects of electrostatic forces on delaminated stator insulation. Besides, partial discharge activity results in change of instantaneous capacitance with voltage and hence is also a contributor of such nonlinear behavior. In this test, an AC high voltage is imposed on the insulation system, and the current drawn by the insulation is subjected to a special non-linear analysis. Due to charge storage mechanisms, this current is replete with harmonics. The relative content of harmonics in the admittance of the insulation are estimated, predominant harmonics and the pattern of harmonic magnitudes is indicative of anomalies in the insulating system such as ionic activity in slot region, presence of contamination and the occurrence of partial discharges. The test also provides a clearer indication of aging of insulation (if any).
e) Winding Resistance Measurement on Stator winding: Winding resistance is measured to identify the existence of any shorts, breaks (open circuit) or high resistance joints in the stator winding.
f) Wedge Mapping Test: Wedge looseness is a dangerous condition as coils are not restricted from moving in the slot, leading to coil surface erosion due to its rubbing with core and eventually partial discharges in slots or slot discharges. While the effects of looseness namely slot discharges, and other erosion of the coil surface, can be detected by the diagnostic tests, wedge checks are performed to identify looseness as the very initial stages, so that further damage cou ld be precluded.
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One of the methods used is by tapping each wedge in all the slots, after dividing each wedge into three imaginary parts, with a light hammer and listening to the emanating sound, while manually feeling the wedge for minor movement. A map is prepared to represent an overall picture of wedge tightness. For large alternators, this method has been a utomated using an automated tapping h ammer at 10 Hz, and picking up the vibrations on the tapped wedge with accelerometers. While the automated method is used even in cases where ripple springs are employed in stator slots, a wedge deflection test is also adopted. This check is done by applying pressure on the wedges using a known force and measuring the deflection of the wedges, thereby determining the spring stiffness.
g) Flux Loop Test: The Hot spot & Electromagnetic Core Imperfection detector is a test for detection of core faults such as inter-laminar short circuits particularly in large generators, where it can be rather cumbersome to perform a standard loop test. Defects in the inter-laminar insulation cause fault currents to flow locally in the core. These currents can produce dangerous local over heating or hot spots in the damaged areas and the damage to the core may become progressively worse. In extreme cases sufficient heat is generated to melt small parts of the core and even modest rises in core temperature adjacent to the winding can result in the premature f ailure of the winding insulation. In this method, the stator core is excited to a flux level as close as possible to the rated flux and the temperature rise of the core is noted to detect hot spots.
h) Coupling Resistance Measurement: Contact resistance will be measured between the Bar insulation and the Ground (Slot wall) for both the top and bottom bars using a probe, and a chart showing the coupling resistance reading at each slot will be prepared.
i) Corona PD Probe Measurements: In this test the contact resistance/capacitance between the coil outer surface and ground in the slot is measured for both the top and bottom bars and a chart showing the coupling resistance along each slot is prepared. This test provides information on extent of contact of coil side with core and therefore the extent of coil looseness due to side clearances or any other deterioration or damage to the discharge protection coating. This test is used to quantify the lack of contact of coil with core problem that is detected in the PDCA and C-Tan Delta Analysis and provides valuable data inputs for FEM modeling and stress calculations.
ROTOR (if the rotor has insulated windings) a) Polarization-Depolarization Current Analysis: Similar to the test as described above for Stator Windings, with the difference being that the test is carried out using 100 - 500 V megger.
b) Winding Resistance / Impedance Test: For detecting the presence of shorted turns in the rotor winding by comparing obtained impedance values with earlier measured values o r for detecting high resistance joints.
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APPENDIX IV Life Assessment Approach by Operating Stress Evaluation using Fine Element [FEM] Technique During operation, an electrical rotating machine is subjected to thermal, electrical, mechanical and ambient stress either singly or in combination, ultimately resulting in aging of insulation. The tensile curve determined from operational data and critical stress value are assumed for the particular class of insulation. The tensile curve is corrected based on the diagnostic tests, while critical stress levels are assumed. This helps in remnant life estimation with enhan ced accuracy. The next accuracy stage is to determine the actual stresses within the machine. These stresses can be evaluated using finite element techniques. Testing engineer collects the data regarding geometry of the coil, materials used, operating temperatures, arrangement of blocks and ties etc. These are later modeled using FEM software that calculates the Von-Mises stresses. From the knowledge of the operating stress, machine health and aging extent, a weak link in the insulation can be identified. Thus, the life can be calculated with highest accuracy. Based on design data and actual measurements of the key dimensions of the machine, these stresses can be evaluated using finite element techniques. Figure 2 shows thermal stress on a coil and figure 3 shows high electric stress in regions around a crack in the electrical insulation. From the knowledge of the operating stress, machine health and aging extent, a weak link in the insulation can be identified and plotted.
Figure 2. Finite element analysis (thermal) of generator stator coil In this approach also, failure is defined in terms of a specific loss of an insulation property, an alternative, - seemingly obvious approach, - would be, to define the life of stator winding insulation directly in terms of its electrical breakdown in operation. Stator winding insulation is exposed to a combination of stresses, - electrical, mechanical, thermal and environmental - which act upon the insulation and result in a global or local weakening of the insulation structure.
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Air
Core
Insulation
Insulation
Figure 3. Finite element analysis of a crack in in sulation
The dominant nature of mechanical stresses have been observed: ?? ?? ??
during multi-factor aging experiments from operational data, - starts/stops have found to correlate well with failures at the slot ends. from studies/surveys on the causes of failures of stator winding insulation.
Defects that form and develop under the influence of mechanical stresses have been known to have an ability to be modeled.
STRESS ANALYSIS: THERMO MECHANICAL STRESSES: The change in the winding temperature from a cold to hot condition and from a hot to cold condition constitutes a thermal cycle. Thermosetting insulation systems generally are stable dimensionally with increase/decrease in temperature. However, the copper of the bars tends to expand on application of heat. The restraining force is generally offered by the end-winding bracing supports that are used to limit winding movement due to electro-magnetically generated forces. These constraints result in a mechanical strain at certain bracing support locations. Also, the conductor bends and twists due to a change in the direction of expansion of the coil, resulting in additional development of stresses. The developed stresses are computed using a three-dimensional finite element (FEM) package. STEP 1: A 3D model of the coil is constructed based on the geometry measurements made. The ties are modeled at the end-windings as per their location from the slot en ds.
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Modeling of ties
3D model of coil based on Geometrical data
0
STEP 2: The temperatures are then incorporated in the model. A 70 C is assumed for the portion 0 of coil in the slot region (straight portion) and a temperature of 60 C is assumed for the end-winding portion of coil.
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STEP 3: Based on diagnostic testing earlier we make use of 4 parameters (i) Charge storage Q1 and Q2 in slot region (ii) Void Volume Content (iii) Coupling Resistance (iv) Partial Discharges To arrive at a Contact Index number. STEP 4: The boundary conditions are then fixed for the coil model. The contact index number determines the percentage area of the coil that is not making proper contact with the stator core and is modeled as without friction, while the remaining portion is modeled as with friction. The coil motion is restricted in radial and transverse direction (i.e. along X and Y axis) but coil movement can take place axially (along Z direction). The coil motion is also restricted at the region of ties. The following figure shows the coil with boundary conditions. STEP 5: The FEM Stress analysis is then done and the material data regarding Poissons ratio and Modulus of elasticity, the differential coefficient of expansion are fed during the analysis. The FEM plot as shown in Fig.2 above is made and the magnitude and location of maximum and minimum stresses are calculated and displayed in the plot. STEP 6: Based on the operational data such as the operational hours and starts-stops data we calculate the critical stress. The values are then calculated for aged insulation at 90 deg C. STEP 7: The Arrhenius curve as discussed in first method is then converted into the “Ultimate Tensile Strength” curve and is plotted against the operational hours. This is done by calculating the percentage life used up as per the Arrhenius curve and then assuming a 50% reduction in tensile strength for the life that is used up. The ultimate tensile strength curve also takes into account the thermal cycle fatigue caused by fluctuations in day and night temperatures. STEP 8: LIFE ESTIMATION From the operational data, we have the present operating hours, which is marked on the ultimate tensile strength curve. We then plot the developed stresses as constant on this curve. The stress curve (constant) line intersects the ultimate tensile strength curve at some point. This point gives us the operational hour beyond which there is a high risk of failure. The difference between the operational hours at intersecting point and the present operating hours determines the residual life in terms of operational hours. (as shown in figure below)
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UNIT 4 70 ULTIMATE TENSILE STRENGTH
68 66 64 DEVELOPED STRESS 62 60
PRESENTS EQUIVALENT OPERATIONAL HOURS
58 56 54
EQUIVALENT OPERATIONAL HOURS AT INTERSECTING POINT
REMAINING LIFE
52 50 1 4 0 0 0 0
1 4 5 0 0 0
1 5 0 0 0 0
1 5 5 0 0 0
1 6 0 0 0 0
1 6 5 0 0 0
1 7 0 0 0 0
1 7 5 0 0 0
1 8 0 0 0 0
1 8 5 0 0 0
1 9 0 0 0 0
1 9 5 0 0 0
2 0 0 0 0 0
2 0 5 0 0 0
2 1 0 0 0 0
2 1 5 0 0 0
2 2 0 0 0 0
2 2 5 0 0 0
2 3 0 0 0 0
2 3 5 0 0 0
2 4 0 0 0 0
2 4 5 0 0 0
2 5 0 0 0 0
2 5 5 0 0 0
2 6 0 0 0 0
2 6 5 0 0 0
2 7 0 0 0 0
2 7 5 0 0 0
2 8 0 0 0 0
2 8 5 0 0 0
2 9 0 0 0 0
2 9 5 0 0 0
3 0 0 0 0 0
Operating hours
We are not satisfied by just calculating the residual life of the machine. We use our expertise and experience to understand the root cause of problem and recommend suitable actions to eliminate/reduce the problems and increase the life. The various ways in which remaining life of the machine can be improved are illustrated below:
Ultimate tensile strength n i a r t s / s s e r t S
Developed stress Improvement Improvement in life in life by restoring by improving strength droop strength rate droop
Years
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The problems such as contamination, machine running hot, lack of contact of coil with core and partial discharges etc. cause reduction in ultimate tensile strength. The early detection of problems and appropriate maintenance actions, like cleaning/overhauling of the machine can help in restoring the tensile strength. By improving the cooling eff iciencies, better heat dissipation, removal of blockages in ventilating ducts, the tensile strength can be improved as shown by blue line above.
Ultimate tensile strength n i a r t s / s s e r t S
Developed stress Improvement improvement in life in life by restoring by reducing stress original stress through redesign developed
Years
The problems such as looseness of coils and partial discharges cause acceleration of aging problem and hence the developed stresses may not be constant but increasing as shown by solid red line. Placing the coils tight in slot by inserting adequate side packers, rewedging etc can arrest the looseness and prevent increase in developed stresses, causing improvement in remaining life. A further improvement can be achieved by reducing the developed stress (blue line) by rewinding/revarnishing options.
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APPENDIX V GENERAL TEST REQUIREMENTS TO BE PROVIDED BY THE CUSTOMER AT SITE AND OTHER NOTES 1. Machines will have to be offered for test (by the customer) with the terminals made available, and with cables/bus bars disconnected at the machine end. Disconnection and reconnection wherever required will have to be carried out by the customer. 2. Any dismantling / assembling of Rotors or decoupling required for testing is not in ABB scope. 3. To comply with safety requirements, the customer will have to ensure that one other competent person of the customer in addition to our test engineer is present during testing, and that there is adequate lighting in the test area. 4. Stators will be tested up to a maximum of line to ground voltage, which is generally considered to be a safe test voltage level. If the insulation of the machine fails during test, it could only be attributed to a major defect in the insulation of the machine, and as such ABB will not be held responsible for such a failure. 5. Power supply board with a minimum of three domestic 30 amp sockets and switches (single phase, 3-pin, 230 V / 3 phase 440 V) at work site. 6. Suitable/bench or any other adequate arrangements to set up the test equipment as close as is possible to the machine to be tested. 7. A suitable trolley or other adequate lifting and shifting arrangements to move the test equipment to the test sites. 8. Space for the safe storage of test equipment while not in use. 9. Prior permission/gate pass for the testing team and equipment to be arranged by the customer. 10. Manpower for shifting of equipment to be arranged by the customer. 11. The customer will assist in providing ABB with all available data requested on the machines to be tested, including p revious tests, drawings etc., if required. 12. Customer shall provide following facilities free of cost at the site where required: 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
Machine parts placed at repair site, free from any incumberance Workshop facilities Crane facilities with operator Oxy-acetylene plant DC welding generator Adequate power and lighting with switching arrangement, as required Stands for placement of components, especially rotating arrangement for rotor with roller Clean tarpaulins as required
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