TABLE OF CONTENTS ACKNOWLEDGEMENT INTRODUCTION
4
COMPONENTS OF THE OF THE SUBSTATION
6
SITE SELECTION AND LAYOUT OF 110 OF 110K V SUBSTATION
7
AN OVERVIEW OF PUNNAPRA OF PUNNAPRA SUBSTATION
9
FUNCTIONING OF THE OF THE SUBSTATION
10
OPERATIONS TO BE CARRIED OUT
13
LIGHTNING ARRESTOR
16
WAVE TRAP
17
CAPACITOR BANK CAPACITOR BANK
18
ISOLATOR
19
EARTHING SYSTEMS
20
RELAYS
32
BATTERY AND BATTERY AND BATTERY CHARGER BATTERY CHARGER
35
BUSBAR
37
CIRCUIT BREAKERS
38
POWER LINE POWER LINE CARRIER COMMUNICATION CARRIER COMMUNICATION
44
CONSTRUCTIONAL DETAILS OF TRANSFORMER5 OF TRANSFORMER5
46
CLASSIFICATION OF TRANSFORMERS OF TRANSFORMERS
52
SPECIFICATION OF TRANSFORMERS OF TRANSFORMERS
56
INSTRUMENT TRANSFORMERS
59
CAPACITOR VOLTAGE CAPACITOR VOLTAGE TRANSFORMER
70
REFERENCE
72
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CIRCUIT BREAKERS
38
POWER LINE POWER LINE CARRIER COMMUNICATION CARRIER COMMUNICATION
44
CONSTRUCTIONAL DETAILS OF TRANSFORMER5 OF TRANSFORMER5
46
CLASSIFICATION OF TRANSFORMERS OF TRANSFORMERS
52
SPECIFICATION OF TRANSFORMERS OF TRANSFORMERS
56
INSTRUMENT TRANSFORMERS
59
CAPACITOR VOLTAGE CAPACITOR VOLTAGE TRANSFORMER
70
REFERENCE
72
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ACKNOWLEDGEMENT I have taken efforts in this project; however, it would not have been possible without the kind support and help of many individuals and organizations. I would like to extend my sincere thanks to all of them. I wish to express my sincere gratitude to Fr.Cyriac Kochupurayil , Principal and Lizz Joseph, H.O.D of Electrical and Electronics department of Carmel Polytechnic College, Punnapra for providing me an opportunity to conduct this industrial visit. I am extremely thankful & indebted to the numerous 110kV substation Engineers, who provided vital information about the functioning of their respective departments thus helping me to gain an overall idea about the working of organization. I am highly thankful for the support and guidance of each of them. I am highly indebted to my project guide, Mr. ………………………………… ( Executive Engineer), Mr. ……………………………… (Assistant Executive engineer) , Mr ……………………….. (Assistant Engineer), Mr. M r. …………………………… (Station Engineer) for giving me their valuable time and helping me to grasp various concepts of switchyard equipments and their control instruments and their testing. I would like to express my gratitude towards my parents, classmates & my friends for their kind co-operation and encouragement which helped me in the completion of this report. Last but not the least, thanks goes to the Almighty, who has been always the savior and who is leading everyone to the enlightening of knowledge and wisdom. Name S5 Electrical Carmel Polytechnic College
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INTRODUCTION A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Substations generally have switching, protection and control equipments, and transformers. Substations are of different types. A transmission substation connects two or more transmission lines and a distribution substation transfers power from the transmission system to the distribution system of an area. In Kerala, the major substations include one 400 KV sub-station, and seventeen 220 KV substations. The department of Electrical and Electronics Engineering of Carmel Polytechnic College, Alappuzha gives a chance to their students to spend two weeks in industrial companies. This training gives the student the opportunity to see what they have studied and how to deal with practical life. My training program was in the period from 6th May 2013 to 20th May 2013 at 110kV substation, Punnapra. The present day electrical power system is AC i.e.; electrical power is generated, transmitted and distributed in the form of alternating current. The electric power is produced at the power station, which are located at favorable places, generally quite away from the consumers. It is delivered to the consumer through a large network of transmission and distribution. At many places in the line of power system, it may be desirable and necessary to change some characteristics (e.g. Voltage, AC to DC frequency, power factor etc) of electric supply. This is accomplished by suitable apparatus called substation for example, generation voltage (11kV/6.6kV) at the power station is stepped up to high voltage of transmission of electric power. Similarly near the consumer’s localities, the voltage may have to step down to utilization level. This job is again accomplished by suitable apparatus called substation.
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A substation may include transformers to include voltage levels between high transmission voltage levels and lower distribution voltages, or at the interconnection of two different transmissions. The word substation comes from the days before the distribution system became a grid. As central generation station became larger, smaller generation plants were converted distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of the power station. Substations generally have switching, protection and control equipments, and transformers. Devices such as capacitors and voltage regulators may also be located at a substation.
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COMPONENTS OF THE SUBSTATION TRANSMISSION PATH INSIDE THE SUBSTATION
A: Primary power line's side B: Secondary power line's side
1. 2. 3. 4. 5. 6.
Primary power lines Ground wire Overhead lines Potential or Voltage transformer Disconnect switch Circuit breaker
7. Current transformer 8. Lightning arrestor 9. Main transformer 10. Control building 11. Security fence 12. Secondary power lines
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SITE SELECTION AND LAYOUT OF 110kV SUBSTATION
110kV substation forms an important link between Transmission network and Distribution network. It has a vital influence of reliability of surface. Apart from ensuring efficient transmission and distribution of power, the substation configuration should be such that it enables easy maintenance of equipment and minimum interruptions in power supply. Substation is constructed as near as possible as the load centre. The voltage level of power transmission is decided on the quantum of power to be transmitted to the load centre. Main points to be considered while selecting the site for grid substation are as follows:
The site chosen should be as near to the load centre as possible.
It should be easily approachable by road or rail for transportation of equipments.
Land should be fairly leveled to minimize development cost.
Source of water should be as near to the site as possible. This is because water is required for various construction activities (especially civil works), earthing and for drinking purposes etc.
The substation site should be as near to the town/city but should be clear of public spaces, aerodromes and military/police installations.
The land should have sufficient ground area to accommodate substation equipments, buildings, staff quarters, space for storage of material, such as store yards and store sheds etc. with roads and space for future
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expansion.
Set back distances from various roads such as National highways, state highways should be observed as per regulations in force.
While selecting the land for the substation, preference is to be given to government land over private land.
The land should not have water logging problem.
Far away from obstructions, to permit easy and safe approach termination of high voltage overhead transmission lines
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AN OVERVIEW OF PUNNAPRA SUBSTATION
Connector MVA:
{2*63 + 16 + 16(EDTA II) +12.5 traction}
= 170.5 MVA 110kV SIDE
PLPU NO I
PLPU NO:II
PUED NO I
63 MVA II
PUED NO:II
16 MVA
63 MVA I
TRACTION
.
66kV SIDE
63 MVA I
ALP NO: I ALP NO: II 10 MVA I
63 MVA II
MVKA NO: I MVKA NO: II 10 MVA II
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FUNCTIONING OF THE SUBSTATION RESPONSIBILITIES AND DUTIES
1. Operating crew of substation comprises of one Assistant Engineer as operator and one Overseer as Shift Assistant. Operator on duty shall carry out all the operations required for normal functioning of the substation as per the directions followed. 2. Sub Engineer (Electrical/Maintenance) attends to all maintenance work connected with lines and equipments of substation including routine and breakdown maintenance. He will assist the AE in the preparation of monthly returns and allied Db works. 3. Station Engineer holds overall charge of the substation.
OPERATIONS IN GENERAL
The following operating instructions may be strictly followed for the smooth operation of the substation: 1.
The operator, taking over the shift charge shall record the time of taking over the duty with name and signature. He / She shall also record the name of shift assistant in the diary and log book.
2.
Handover the charge with clear explanation in brief regarding the substation and feeders such as PW/IC/NBC in force, trouble noted in any of the equipments etc. Handing over time and dated signature
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with the name of the relieving operator should invariably be recorded. 3.
An operator should primarily check protective and alarm circuits of the individual feeders and also the control supply system including the battery system. Then the overall inspection of the control room and yard equipments should be conducted. Check and confirm the reliability of emergency lights and accessibility of fire fighting equipments.
4.
Read carefully previous operations and make a thorough picture regarding the substation feeder positions. Record all entries with time and sequence of operations performed. The tripping and any major events requiring special attentions should be recorded in red ink and scheduled interruptions like switch off and permit to work should be recorded in green ink.
5.
Message book and phone call register are to be maintained by the operator on duty. Phone message received and transmitted shall be recorded with date and time and confirm the authenticity of the person at the other end. Confirm that the messages are communicated to the right person to whom it is intended and act according to the seriousness of the matter contained therein.
6.
Visit the yard frequently and watch the various equipments and their functions carefully.
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7.
Promptly record hourly and half hourly readings with utmost care.
8.
The operator on duty shall see that the substation equipments and panels in the control room are kept clean.
9.
Station clock timings should be checked and corrected if necessary at 3pm on every day, with 220kV substation Kalamassery.
10.
Check the specific gravity and the cell voltage of the pilot cells of the station battery and record them in the log sheet by the 1 st shift assistant operator every day.
11.
Take suitable steps to avoid overloading of equipments and feeders.
12.
Maintain the system voltage within the statutory limits with appropriate tab selections as far as possible.
13.
Carry out various routine operations symmetrically as scheduled below separately.
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OPERATIONS TO BE CARRIED OUT
FAULTS ON TRANSFORMERS
If the circuit breaker of a transformer has tripped, the alarms may be accepted, the relay indications carefully checked and noted. If the tripping is an Overload, switch off all the outgoing feeders from the transformers. Reset the relays and test charge the transformer on no load. Then charge the outgoing` feeders one by one and ensure that the load is not more than the capacity of the transformer. If the tripping is for any other reason other than the over current, the transformer may be charge only after consulting the higher officials. INCOMING FEEDERS
If the incoming feeders are tripped on over current relay, reduce the load on the transformer by switching off outgoing feeders from the transformer. Reset the relay and charge the incoming feeder. Then charge the incoming feeder one by one. If the incomer is again tripped, the outgoing feeder last charged may be kept open and other feeders charged suspecting fault on the particular feeder. The load on the transformer may closely be watched and if found exceeding the admissible limit, the distribution authorities may be directed to limit the current. OUTGOING FEEDERS EXCEPT AUXILIARY
In case an outgoing feeder is tripped, accept the alarm, note the relay indication, reset the relay and accept the alarm and test charge the feeder. If the feeder trips instantly or any apparent
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fault or heavy fluctuations in the supply system, flashing the cubicle are noted, the feeder may be declared as faulty after confirming that the fault exists on the feeder beyond the outdoor isolation point by isolating the AB switch and charging the cable portion from the control room. Inform the distribution section to rectify the fault. If a feeder trips on OC relay, only three test charging may be attempted. Avoid further test charging until confirmation from distribution authority is received that the load on the feeder has been reduced. AUXILIARY FEEDER
The method in the case of other outgoing feeder may be adopted in this case also. But as the station supply is taken from the beach feeder, when the feeder is faulty, open the AB switch in the 11kV outdoor structure and charge the breaker for taking the auxiliary supply, inform the matter to distribution section.
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STUDY OF SUBSTATION EQUIPMENTS
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LIGHTNING ARRESTOR Whenever an incoming comes to a substation, initially the line is connected through a lightning arrester. This is for the protection of the station. Generally a lightning arrester seems like a set of insulators connected together with a ring in the top. This ring is called grading ring. The purpose of grading rings is that in case of heavy voltage surges the charge is distributed uniformly through the ring and then the discharge occurs. An ammeter is connected with the maximum current passed through it. The ammeter is reset. The ammeter in the arrester carrying the topmost conductor will have maximum current passing through it. Metal Oxide Varistors
Metal Oxide Varistors have been used for power system protection since the middle of 1970’s. The typical lightning arrestors also known as surge resistors have a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrestor, the current from the surge is diverted around the protected insulation in most cases to the earth.
SPECIFICATION METOVAR META OXIDE SURGE ARRESTOR
Rated voltage Rated frequency Long duration discharge Discharge current Max continuous operating voltage Pressure relief current Type
96kV 50Hz Class 3 10kA 81kV 40kA Metovar
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WAVE TRAP Wave trap is also known as line trap. It is an instrument used for tripping of the wave. The function of this trap is that it traps the unwanted waves. Its shape is like that of a drum. It is connected to the main incoming feeder so that it can trap the waves which may be dangerous to the instruments in the substation. The wave trap traps the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom / tele protection panel in substation control room through the coupling capacitor and LMU. This is relevant in Power Line Carrier Communication (PLCC) systems for the communication among various substations without dependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals. Line signals sent on the line from the remote substation and diverting them to the telecom / teleprotection panel in the substation control room. The wave trap offers high impedance to the high frequency communication signals thus obstructs the flow of the signals to the substation bus bars. If they were not to be there, then signal loss is more and communication will be ineffective or probably impossible.
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CAPACITOR BANK A capacitor bank is a grouping of several identical capacitors interconnected in parallel or in series with one another. These groups of capacitors are typically used to correct or counteract undesirable characteristics, such as power factor lag or phase shifts inherent in alternating current (AC) electrical power supplies. The energy storing characteristic of capacitors is known as capacitance and is expressed or measured by the unit farads. This is usually a known, fixed value for each individual capacitor which allows for considerable flexibility in a wide range of uses such as restricting DC current while allowing AC current to pass, output smoothing in DC power supplies, and in the construction of resonant circuits used in radio tuning. These characteristics also allow capacitors to be used in a group or capacitor bank to absorb and correct AC power supply faults. The use of a capacitor bank to correct AC power supply anomalies is typically found in heavy industrial environments that feature working loads made up of electric motors and transformers. This type of working load is problematic from a power supply perspective as electric motors and transformers represent inductive loads, which cause a phenomenon known as phase shift or power factor lag in the power supply. The presence of this undesirable phenomenon can cause serious losses in terms of overall system efficiency with an associated increase in the cost of supplying the power. The use of a capacitor bank in the power supply system effectively cancels out or counteracts these phase shift issues, making the power supply far more efficient and cost effective. The installation of a capacitor bank is also one of the cheapest methods of correcting power lag problems and maintaining a power factor capacitor bank is simple and cost effective. In Punnapra Substation, capacitor bank is rated for 123kV, 25 MVR consisting of 42 units of 10.14kV, 596.23 kVAR internal fuse capacitor units arranged in double star configuration.
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ISOLATOR In order to disconnect a part of system for maintenance and repair, isolators are used. It is a knife switch designed to open a circuit under no load. If isolators are to be opened, the Circuit Breaker connected must be opened first. Otherwise there is a possibility of occurrence of a spark at the isolator contacts. After repair, first isolators are closed and then Circuit Breaker. There are two types of isolators-
Line isolators and Bus isolators. For bus isolators, there is no earth switch. During maintenance works the line isolator contacts are opened, so that the three phases trip simultaneously. For the ease of earthing, dead weights are provided at the end of earthing arms.
SPECIFICATION Current
800A
Max Design Voltage
125kV
Impulse Withstand Voltage
550kV
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EARTHING SYSTEMS
In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply, and regulations can vary considerably among countries. Most electrical systems connect one supply conductor to earth (ground). If a fault
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within an electrical device connects a "hot" (unearthed) supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth (e.g., by standing on it, or touching an earthed sink) will complete a circuit back to the earthed supply conductor and receive an electric shock. The sole purpose of substation grounding/earthing is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation. The substation earthing system is necessary for connecting neutral points of transformers and generators to ground and also for connecting the non current carrying metal parts such as structures, overhead shielding wires, tanks, frames, etc to earth. Earthing of surge arresters is through the earthing system. The function of substation earthing system is to provide a grounding mat below the earth surface in and around the substation which will have uniformly zero potential with respect to ground and lower earth resistance to ensure that
To provide discharge path for lightning over voltages coming via rod gaps, surge arresters, and shielding wires etc. . To ensure safety of the operating staff by limiting voltage gradient at ground level in the substation To provide low resistance path to the earthing switch earthed terminals, so as to discharge the trapped charge (Due to charging currents even the line is dead still charge remains which causes dangerous shocks) to earth prior to maintenance and repairs.
Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. To provide a sufficiently low resistance path to the earth to minimize the rise in earth potential with respect to a remote earth fault. Persons touching any of the non current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each structure,
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transformer tank, body of equipment, etc, should be connected to earthing mat by their own earth connection. Grounding system in a electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’. : Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the feet (one step), through the flow of earth fault current through the grounding system. : Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The person should not get a shock even if the grounded structure is carrying fault current, i.e, The Touch Potential should be very small. : 1. Un earthed Systems: It is used no more. The neutral is not connected to the earth, also called as insulated neutral system. 2. Solid grounding or effective grounding: The neutral is directly connected to the earth without any impedance between neutral and ground. 3. Resistance grounding:
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Resistance is connected between the neutral and the ground. 4. Reactance grounding: Reactance is connected between the neutral and ground. 5. Resonant Grounding: An adjustable reactor of correctly selected value to compensate the capacitive earth current is connected between the neutral and the earth. The coil is called Arc Suppression Coil or Earth Fault Neutralizer Different Grounding Equipment in Electrical Substation
Earthing Electrodes Earthing Mat Risers Overhead shielding wire (Earthed)
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CONVENTIONAL METHODS OF EARTHING
For Pipe type earthing normal practice is to use GI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75 mm diameter GI flange having 6 numbers of holes for the connection of earth wires and inserted in ground by auger method.
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These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound.
Generally for plate type earthing normal Practice is to use Cast iron plate of size 600 mm x600 mm x12 mm. OR Galvanized iron plate of size 600 mm x600 mm x6 mm. OR
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Copper plate of size 600 mm * 600 mm * 3.15 mm Plate burred at the depth depth of 8 feet in the vertical vertical position and GI strip of size 50 mmx6 mm bolted with the plate is brought up to the ground level. These types of earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet f eet from the bottom of the pit.
Before designing the earthmat, it is necessary to determine the soil resistivity of the area in which H.V.substation is to be located. Depending up On the types of soil. Further, their resistivity may also vary at different depth depending upon the type of soil, moisture content and temperature etc., at various depths which affects the flow of current due to the fact that the earth fault current is likely to take its path through various layers.
Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M. Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of Earth and reduce Earth Resistance. Make a mixture of Wood Coal Powder Salt & Sand all in equal part Wood Coal Powder use as good conductor of electricity, anti corrosive, rust proves for GI Plate for fo r long life. The purpose of coal and salt is to t o keep wet the soil permanently.
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The salt percolates and coal absorbs water keeping the soil wet. Care should always be taken by watering the earth pits in summer so that the pit soil will be wet. Coal is made of carbon which is good conductor minimizing the earth resistant. Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity. Sand has used to form porosity to cycle water & humidity around the mixture. Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the mid of mixture. Use Double GI Strip size 30 mm X 10 mm to connect GI Plate to System Earthling. It will be better to use GI Pipe of size 2.5″ diameter with a Flange on the top of GI Pipe to cover GI Strip from EARTH PLATE to Top Flange. Cover Top of GI pipe with a T joint to avoid jamming of pipe with dust & mud and also use water time to time through this pipe to bottom of earth plate. Maintain less than one Ohm Resistance from EARTH PIT conductor to a distance of 15 Meters around the EARTH PIT with another conductor dip on the Earth at least 500 mm deep. Check Voltage between Earth Pit conductors to Neutral of Mains Supply 220V AC 50 Hz it should be less than 2.0 Volts.
It is the resistance of soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pit depends on soil resistivity. It is the resistance of the soil to the passage of electric current. It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size and distribution, seasonal variation,
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current magnitude etc. In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude.
Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry. Soil resistivity is measured in ohm-meters or ohm-cm. Soil plays a significant role in determining the performance of Electrode. Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high. If soil resistivity is high, earth resistance of electrode will also be high.
Moisture has a great influence on resistivity value of soil. The resistivity of a soil can be determined by the quantity of water held by the soil and resistivity of the water itself. Conduction of electricity in soil is through water. The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And further increase of moisture level in soil will have little effect on soil resistivity. In many locations water table goes down in dry weather conditions. Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity
Pure water is poor conductor of electricity. Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.
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Small quantity of salts in water reduces soil resistivity by 80%. common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged.
Increase or decrease of moisture content determines the increase or decrease of soil resistivity. Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.
Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of 4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohmmeter.
The location also contributes to resistivity to a great extent. In a sloping landscape, or in a land with made up of soil, or areas which are hilly, rocky or sandy, water runs off and in dry weather conditions water table goes down very fast. In such situation Back fill Compound will not be able to attract moisture, as the soil around the pit would be dry. The earth pits located in such areas must be watered at frequent intervals, particularly during dry weather conditions. Though back fill compound retains moisture under normal conditions, it gives off moisture during dry weather to the dry soil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is naturally not well drained.
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Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil. Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease or increase of soil resistivity. Thus in dry weather resistivity will be very high and during rainy season the resistivity will be low.
Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into the surrounding soil. The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and duration will cause significant drying and thus increase the effect of soil resistivity
Single electrode rod or strip or plate will not achieve the desired resistance alone. If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distance between the electrodes must be equal to the driven depth to avoid overlapping of area of influence. Each electrode, therefore, must be outside the resistance area of the other.
The soil may look good on the surface but there may be obstructions below a few feet like virgin rock. In that event resistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity. If the earth pits are close by, the resistance value will be high.
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A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.
Apparatus
Parts to be Earthed
Method of Connection
Power transformer
Transformer tank
Connect the earthing bolt on transformer tank to the station earth
High Voltage circuit breakers
Operating mechanism, frame
Connect the earthing bolt on the frame and the operating mechanism of Circuit breaker to earthing system
Surge arrestor
Lower earth point
To be directly connected to earth mat
Support of bushing insulators, lightning arrestors, fuse, etc.
Device flange or base plate Connect the earthing bolt of the device to the station earthing system
Potential transformer
Potential Transformer tank, LV neutral
Connect the transformer earthing bolt to earthing system. Connect LV neutral of phase lead to case with flexible copper conductor
Isolator
Isolator frame, operating mechanism, bed plate
Weld the isolator base frame, connect it to the bolt on the operating mechanism, base plate and station earth.
Current transformer
Secondary winding and metal case
Connect secondary winding to earthing bolt on transformer case.
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RELAYS Relays are one of the most important parts of substation. The function of a relay is mainly incorporated in the control panel section of the substation. A protective relay is mainly incorporated in the control panel section of the substation. A protective relay is a device that detects the fault and initiates the operation of the CB to isolate the defective element from the rest of the system. The relay detects the abnormal condition such as voltage, current, frequency, phase angle and temperature. The substation has control panels for its incoming as well as outgoing feeders and each control panel has various relays. The different types of relays which are used here are
Distance protection relay
Auto reclose relay
Synchronizing relay
Differential relay
Over current relay
Earth fault relay
Auxiliary relay
It is a special type of relay used to know at which place the line has failed. The lines are divided into zones. The relay will indicate the rough distance between the station and the point at
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which the breaking has occurred. The connection to the main relay is made through an auxiliary relay. This relay is very helpful in remote areas. The control panel has 2 types of distance protection relays.
Auto reclose relay is of mechanical type. It helps in speedier tripping and making up of the connection after fault rectification.
Advantages of bus couplers can be obtained only if the voltage and frequency of the bus bars to be coupled are the same. Synchronized relay does the function of constantly comparing the two voltages and frequency and thereby initiating the tripping mechanism at time of fault. The synchroscope aids it.
The relay is activated at difference in current flowing through the relay. In case of equipments like CT the relay is connected in between the equipments. In normal conditions the current through the relay is the same as the equipment current but when any fault occurs in the line enclosed ten there is a rise in current through the relay at the fault side above that which is on the other side. This activates the relay, tripping occurs.
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The relay activates when current exceeds the permissible limits. It will be connected to the circuit breaker in case of any fault due to over current. The relay acts and activates the circuit to the breaker hence tripping the breaker. DC supply is always given to the relay as it should trip even if there is an interruption in the power supply.
It will be activated when there is any fault in the earthing of the equipment. It is also connected to circuit breaker to help tripping the circuit. The main applications of this relay are in control, alarm, indication and other auxiliary duties in AC or DC systems. CAA & VAA relays are current and voltage operated respectively. These relays are attracted in a armature units of compact design supplied with reset contacts. Standard contacts are of silver or copper alloys. When it is necessary to break heavy or highly inductive DC loads, heavy duty magnetic blow type contacts are used. They use the magnetic field of a small permanent magnet to force the arc onto the arcing horns away from the contact tips. The new control panel uses automatic semaphore (mimic) indicators for better control.
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BATTERY AND BATTERY CHARGER
The station DC source is facilitated through battery of 400 Ah capacities and 200 Ah capacities. The 400 Ah battery bank no 1 is fed through the battery charger from the main control room. This is of 110kV, 50 A capacities. The second 400Ah battery bank has the same capacity. 200 Ah bank is fed through the battery charger located in the old control room. This is of 110kV, 15 A capacities. This feeds only 11kV cubicles located in the old control room. 110 volt supply is always provided as a standby as there is possibility of power failure in station. At this time also the tripping in case of fault should continue; for this the 80V DC Supply is very essential. 55 batteries each of 2 volt are provided giving a total of 110 V. In some area the required voltage is less; in such cases the batteries used also should be less. The batteries are lead acid cells and have sulphuric acid as its electrolyte with lead electrode along with spongy lead in between. They have 400 Ah capacity i.e. they can supply a current of 400 A for a time of 1 hour. So it can be used to supply 200 A at intervals of 2 hours. This voltage always provided in parallel with the AC supply. It can be used in case the AC fails. The batteries can be charged in 2 modes, float charging and boost charging. Float charging is used when AC is present and Boost charging is used when the battery is in the back up
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mode. Battery is regularly checked in the substation to check the acidity of Electrolyte. A hydrometer is used to measure the same. To measure the voltage there is the centre zero voltmeter.
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BUSBAR The lines operating at the same voltage is directly connected to a common electrical component called busbar. Busbar is of Cu or Al and is rectangular in cross-section. Conductor used is moose. The incoming and outgoing lines in the substation are connected to the busbar.
In this, busbar is divided into sections and load is equally distributed on all sections. The advantage of this arrangement is as follows: 1. If fault occurs in any section of the bus bar, that section canbe isolated without affecting the supply from other sections. 2. Repair and maintenance of any section of the busbar can be carried out by de energizing that section only. Thus eliminating the possibility of complete shutdown.
Buses are coupled by means of two isolators and a coupler 1. Load division is better. 2. Even if one bus fails the other bus can supply the load.
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CIRCUIT BREAKERS Circuit breakers have an in built fixed electric current load capacity which when breached causes automatic circuit shutdown. It basically detects the fault condition like a short or over load in the circuit, interrupts the continuity, and immediately stops the current flow. This safety feature makes insulation of a circuit breaker and essential part in an electric circuit. Overloading in an electrical circuit occurs when the wires are forced to carry and conduct an electric charge more than their capacity. This causes the wires to heat up and results in insulation breakdown and an electric fire. Short circuit occurs when two points in the circuit having different potential accidentally come in contact. This causes unwanted current flow from one node to another which may result in excessive heating, circuit damage, explosion or even fire. Therefore, circuit breakers are used to protect the circuit from unwanted consequences of wire overloading and accidental short-circuiting. CIRCUIT BREAKING MECHANISM
Generally, a circuit breaker panel consists of a switch and a moving, conductive contact plate which moves with the switch. When the switch is on an ‘ON’ position, the contact plate touches a stationary plate which is connected to the circuit so that the electric current can flow. But when the switch is in the ‘OFF’ position, due to the overloading or short circuit, the contact plate moves away from the stationary plate and the circuit gets opened and he electric current ceases to flow. Though most circuit breaker has common features in their operation, the mechanism may vary substantially as per the voltage class, current rating and type. In low voltage circuit breakers, when a fault condition is detected, it is rectified within the breaker enclosure, whereas in those meant for large currents or high voltages, special pilot devices like relays are arranged to sense the fault current and rectify it by employing trip opening mechanism.
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TYPES OF CIRCUIT BREAKERS
Its closing is by spring action and tripping is in air. Each CB has an air tank in which pressure is maintained at 15kg/cm2. If pressure goes below this a rotary compressor is automatically activated. Pressure of SF6 is continuously monitored. SF6 being costly, is filled separately in each CB. The gas can be reconditioned after each operation. Operation mechanism is through air, which is being stored in a closed tank. Other CBs are interconnected through tubes.
SPECIFICATION OF SF 6 CB Rated voltage
145kV Normal current 3150A Frequency 50Hz Lightning impulse withstand voltage 650kV Duration of short circuit 3s First pole to clear factor 1.5 Short Circuit Breaker Current (Symmetrical) 40kA Short Circuit Breaker Current (Asymmetrical) 44.8kA Short circuit making current 100kAp Operating Sequence 0-0.3s-CO-3min-CO SF6 gas pressure at 20°C (abs) 0.74mpa Total mass of SF6 gas (Kg) 12
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As the volume of the oil in bulk oil circuit breaker is huge, the chances of fire hazard in bulk oil system are more. For avoiding unwanted fire hazard in the system, one important development in the design of oil circuit breaker has been introduced where use of oil in the circuit breaker is much less than that of bulk oil circuit breaker. It has been decided that the oil in the circuit breaker should be used only as arc quenching media not as an insulating media. Then the concept of minimum oil circuit breaker comes. In this type of circuit breaker the arc interrupting device is enclosed in a tank of insulating material which as a whole is at live potential of system. This chamber is called arcing chamber or interrupting pot. The gas pressure developed in the arcing chamber depends upon the current to be interrupted. Higher the current to be interrupted causes larger the gas pressure developed inside the chamber, hence better the arc quenching. But this put a limit on the design of the arc chamber for mechanical stresses. With use of better insulating materials for the arcing chambers such as glass fibre, reinforced synthetic resin etc, the minimum oil circuit breaker are able to meet easily the increased fault levels of the system.
In a minimum oil circuit breaker, the arc drawn across the current carrying contacts is contained inside the arcing chamber. Hence the hydrogen bubble
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formed by the vaporized oil is trapped inside the chamber. As the contacts continue to move, after its certain travel an exit vent becomes available for exhausting the trapped hydrogen gas. There are two different types of arcing chamber is available in terms of venting are provided in the arcing chambers. One is axial venting and other is radial venting. In axial venting, gases (mostly Hydrogen), produced due to vaporization of oil and decomposition of oil during arc, will sweep the arc in axial or longitudinal direction.
SPECIFICATION OF MOCB Rated voltage Normal current Frequency Lightning impulse withstand voltage Rated breaking capacity Short time current Operating duty Total weight of oil Quantity of oil
66kV 800A 50Hz 350kV 1500mVA @ 66kV 13.1kA for 3sec 0-0.38-CO standard BSS 1050kg 150ltr
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage Vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 – 6 bar.
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The material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. Vacuum interrupter technology was first introduced in the year of 1960. But still it is a developing technology. As time goes on, the size of the vacuum interrupter is being reducing from its early 1960’s size due to different technical developments in this field of engineering. The contact geometry is also improving with time, from butt contact of early days it gradually changes to spiral shape, cup shape and axial magnetic field contact. The vacuum circuit breaker is today recognized as most reliable current interruption technology for medium voltage system. It requires minimum maintenance compared to other circuit breaker technologies.
The main aim of any circuit breaker is to quench arc during current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next current zero. At current zero this vacuum arc is extinguished and the conducting metal vapour is re-condensed on the contact
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surface. At this point, the contacts are already separated hence there is no question of re-vaporization of contact surface, for next cycle of current. That means, the arc cannot be re-established again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero. There are two types of arc shapes. For interrupting current up to 10kA, the arc remains diffused and the form of vapour discharge and cover the entire contact surface. Above 10kA the diffused arc is constricted considerably by its own magnetic field and it contracts. The phenomenon gives rise over heating of contact at its centre. In order to prevent this, the design of the contacts should be such that the arc does not remain stationary but keeps travelling by its own magnetic field. Specially designed contact shape of vacuum circuit breaker make the constricted stationary arc travel along the surface of the contacts, thereby causing minimum and uniform contact erosion.
SPECIFICATION OF VCB Rated voltage
11kV
Rated current
400A
Breaking capacity Making capacity Short time current
26.24kA 65.6kA 26.24A, 3sec
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POWER LINE CARRIER COMMUNICATION Carrier communication is basically the communication between the substation head offices through power lines. Each substation has wave trap arrangement, which consists of capacitor and inductor coupling circuit, which is used to separate the speech from 50Hz frequencies. There are controls of the communication lines in a separate area. It has an amplifier, modulator, interface etc used for amplifying, modulating and making intermediate connections. It cannot be made for practical domestic distribution as this will make the signal very weak. Using PLCC it is possible to make remote controlled connections so to the CT, PT, and CB etc. which will make the job easier. In this type of communication, there will be no interference from outside. Microwave communication can be used to link the dispatched centre within the substation and also to the head office. 9505 power line carrier terminals are intended for the transmission of speed, telemetering, teleprinting, telecontrol, teleindication & teleprotections signals in the carrier frequency range between 50Hz to 500 kHz over the following communication media with suitable line equipment.
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Model 9505 PLCC
It provides single or twin channel voice grade for the transmission of speech or audio tones over high volume transmission lines. The transmitted audio tones can be used for telemetering supervisory control, protective relaying, data or other purposes. When used for data only, each channel carries onto typically 2450-based telegraphed channels or a small no of channels at high base rates. Features of PLCC are: Programming Efficiency Flexibility Voice grade connections Output power Thus they are used for 3 purposes. Station to station communication Data transmission Protection purpose
AF signals are converted into IF signals using IF carriers of 5.12MHz generated in the system using a crystal oscillator. The required IF signals are filtered out using IO pole crystal filter to a final mixer stage. The carrier required for final mixing is derived from a VCO. The section works on PLL principle and can be programmed to oscillate so as to give HF output in the range of 50 -500 kHz in steps of 0.5 kHz. Programming can be achieved by simple strapping.
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CONSTRUCTIONAL DETAILS OF TRANSFORMER5
In all types of transformers the magnetic circuit is made of a laminated iron core. The core is laminated in order to reduce eddy current loss. The
laminations are insulated from each other by a light coat of core plate varnish or an oxide layer on the surface. The thickness of lamination varies from .35mm for 50Hz to .5mm for 25Hz. In addition to eddy current loss hysteresis loss occurs in the core as it is subjected to alternate magnetization and demagnetization. Hysteresis loss depends on area of hysteresis loop of the core material. Special silicon steel
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having a steel content of 4-5% is used for the lamination. The core loss can be minimized by employing laminations of special steel sheet having high silicon content. CRGO silicon steel laminations are used for the construction of transformer core. Each lamination is insulated from its neighbors by a thin no conducting layer of insulation (paper insulation).
The conduction material used for the winding depends upon the application, but in all cases the individual turns must be electrically insulated from each other so that the current travels throughout every turns. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns are small, the coil are often wound from enamel magnet wire such as formvar wire. Larger power transformers operating at high voltages maybe wound with copper rectangular strip conductors insulated by oil impregnated paper and blocks of pressboard. The LV winding is placed on the inner side nearer to the core due to the advantages such as reducing the insulation between core and windings, easier in connecting tap changer to the HV winding.
The main source of heat generation in transformer is its copper loss or I2R loss. Although there are other factors that contribute heat in transformer such as hysteresis & eddy current losses but contribution of I2R loss dominate them. If this heat is not dissipated properly, the temperature of the transformer will rise continually which may cause damages in paper insulation and liquid insulation medium of transformer. So it is essential to control the temperature within permissible limit to ensure the long life of transformer by reducing
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thermal degradation of its insulation system. Electrical Power transformer we use external transformer cooling system to accelerate the dissipation rate of heat of transformer. There are different transformer cooling methods available for transformer:
AN- Air natural cooling
AB- Air blast cooling
ON - Oil immersed air natural cooling
OB - Oil immersed air blast cooling
OFN - Forced oil air natural cooling
OFB - Forced oil air blast
OFW - Forced oil air water cooled.
Very small transformers will have wire leads connected directly to the ends of the coils, and brought to the base of the unit for circuit connection. Larger transformers may have heavy terminals, bus bars or high insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of electric field gradient without letting the transformer leak oil.
Conservator tank consist of oil level which depends on the operation of the transformer. The oil expands in summer with increase in load and the oil level
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decreases with the load. Conservator is a small auxiliary oil tank that may be mounted above the transformer and connected to the main tank by a pipe. Its main function is to keep the main tank of the transformer completely filled with oil in all circumstances. When the oil in the tank contracts then the conservator gives the oil to maintain the oil level in the tank. When the oil in the tank expands then the conservator takes the oil.
Breather is connected to one side of the conservator. It consists of a drying agent CaCl2 or Silica gel which absorbs the moisture from air and allows dry air to enter to the conservator. Thus sludge formation is avoided. Explosion vent consists of large diameter steel pipe fitted on the transformer tank. It is usually at an angle to the vertical. The pipe has an elbow at its end. A thin glass relief diaphragm is placed at the top of the device. This diaphragm will rupture whenever the pressure in the transformer rises to a dangerous value.
The bushings consist of a current carrying element in the form of a conducting rod. Up to 33kV ordinary porcelain insulators can be used, above this voltage ratings oil filled or capacitor type bushings are used.
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Bushing is very important to the overall transformer because without it, conduction would not be possible. The bushings are necessary to complete the conductive energy of the walls that are transferred within the transformer so that they can the move through the medium such as air and gas, including the grounding barriers that each unit is designed with. These are some figures of bushings.
To enable transformers to supply a range of secondary voltages to different part of a circuit it is common for power transformers to have the tapped windings, i.e. windings split into various sections by using a number of connections brought out from a single winding, each one at a particular number of turns along the winding.
The gas and oil actuated (Buchholz) relay is designed to detect faults as well to minimize the propagation of any damage which might occur within oil-filled transformers, capacitors and reactors supplied with oil conservator. The relay is therefore particularly effective in case of: -circuited core laminations -down core bolt insulation
Overheating of some part of the windings -circuits between phases
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Furthermore the relay can prevent the development of conditions leading to a fault in the transformer, such as the falling of the oil level owing to leaks, or the ingress of air as a result of defects in the oil circulating system.
Slight faults: When a slight fault occurs in the transformer, the small
bubbles of gas, which pass upwards to the conservator, are trapped in the relay housing, thus causing its oil level to fall. As a result, the upper float rotates on its hub and operates the alarm switch, thus operating an external alarm device. Serious faults: When a serious fault occurs in the transformer, the gas
generation is violent and causes the oil to rush through the connecting pipe to the conservator. In the relay this oil surge hits the flap fitted on the lower float (located in front of the hole for the oil passage) and causes the rotation of the float itself, thus operating the tripping switch and disconnecting the transformer. The float remains in the trip position even if the oil flow comes to a stop (the reset is done by means of the push-button). The tripping device is regulated in such a way that in transformers having forced oil cooling, the surges resulting from the starting of the oil circulating pump will not cause mal-operation of the relay. An oil leak in the transformer causes the oil level in the relay to fall, thus operating first the alarm (upper) float and then the tripping (lower) float. The ingress of air into the transformer, arising from defects in the oil circulating system or from other causes, operates the alarm float.
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CLASSIFICATION OF TRANSFORMERS Constructionally, the transformers are of two types, distinguished merely from each other by the manner in which the primary and secondary coils are placed round the laminated core. The two types are known as 1. Core type transformers 2. Shell type transformers Another recent development is the Spiral core or wound core type
transformers. In the core type transformers, the windings surround a considerable part of the core whereas in shell type transformers, the core surrounds a considerable part of the windings.
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The coils used are form-wound and are of the cylindrical type. The general form of these coils may be circular or oval or rectangular. In small size coretype transformers, a simple rectangular core is used with cylindrical coils which are either circular or rectangular in form. But for large-size core-type transformers, round or circular cylindrical coils are used which are so wound as to fit over a cruciform core section. The circular cylindrical coils are used in
most of the core-type transformers because of their mechanical strength. Such Cylindrical coils are wound in helical layers with the different layers insulated from each other by paper, cloth, micarta board or cooling ducts. Insulating cylinders of fuller board are used to separate the cylindrical windings from the core and from each other. Since the low voltage (LV) winding is easiest to insulate, it is placed nearest to the core.
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The coils are form-would but are multi-layer disc type usually wound in the form of pancakes. The different layers of such multi-layer discs are insulated from each other by paper. The complete winding consists of stacked discs with insulation space between the coils – the spaces forming horizontal cooling and insulating ducts. A very commonly-used shell-type transformer is the one known as Berry Transformer – so called after the name of its designer and is cylindrical in form. The transformer core consists of laminations arranged in groups which radiate out from the centre. It may be pointed out that cores and coils of transformers must be provided with rigid mechanical bracing in order to prevent movement and possible insulation damage. Good bracing reduces vibration and the objectionable noise – a humming sound – during operation. The spiralcore transformer employs the newest development in core construction. The core is assembled of a continuous strip or ribbon of transformer steel wound in the form of a circular or elliptical cylinder. Such construction allows the core flux to follow the grain of the iron. Cold-rolled steel of high silicon content enables the designer to use considerably higher operating flux densities with lower loss per kg. Transformers are generally housed in tightly-fitted sheet-metal; tanks filled with special insulating oil. This oil has been highly developed and its function is two-fold. By circulation, it not only keeps the coils reasonably cool, but also provides the transformer with additional insulation not obtainable when the transformer is left in the air.
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In cases where a smooth tank surface does not provide sufficient cooling area, the sides of the tank are corrugated or provided with radiators mounted on the sides. Good transformer oil should be absolutely free from alkalies, sulphur and particularly from moisture. The presence of even an extremely small percentage of moisture in the oil is highly detrimental from the insulation viewpoint because it lowers the dielectric strength of the oil considerably. The importance of avoiding moisture in the transformer oil is clear from the fact that even an addition of 8 parts of water in 1,000,000 reduces the insulating quality of the oil to a value generally recognized as below standard. Hence, the tanks are sealed air-tight in smaller units. In the case of large-sized transformers where complete air-tight construction is impossible, chambers known as breathers are provided to permit the oil inside the tank to expand and contract as its temperature increases or decreases. The atmospheric moisture is entrapped in these breathers and is not allowed to pass on to the oil. Another thing to avoid in the oil is sledging which is simply the decomposition of oil with long and continued use. Sledging is caused principally by exposure to oxygen during heating and results in the formation of large deposits of dark and heavy matter that eventually clogs the cooling ducts in the transformer. No other feature in the construction of a transformer is given more attention and care than the insulating materials, because the life on the unit almost solely depends on the quality, durability and handling of these materials. All the insulating materials are selected on the basis of their high quality and ability to preserve high quality even after many years of normal use.
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SPECIFICATION OF TRANSFORMERS The specification of the transformers installed at the Punnapra substation is as follows
Manufacturer: Bharat Heavy Electricals Limited Parameters
When Installed When Installed with with ONAF
ONAN
Rating of HV & LV (MVA) Rating of tertiary winding (unloaded) No load voltage - HV (kV) No load voltage - LV (kV) No load voltage – TV (kV) Line current (HV) (A) Line current (LV) (A) Line current (TV) (A) No of phase Rated frequency Temperature rise in oil ( 0°C) Temperature rise in winding
Oil Natural Air Forced Cooling Oil Natural Air Natural Cooling
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Manufacturer: Transformers and Electricals Kerala (TELK) Parameters
When Installed When Installed with with ONAN
ONAF
No load voltage - HV (kV) No load voltage - LV (kV) Line current (HV) (A) Line current (LV) (A) No of phase Rated frequency Impedance voltage (working tap) Vector group Weight of core and winding (kg) Oil weight (kg) Total weight (kg) Oil volume (litres)
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Manufacturer: Transformers and Electricals Kerala (TELK) Parameters
No load voltage - HV (kV) No load voltage - LV (kV) Line current (HV) (A) Line current (LV) (A) No phase Rated frequency Impedance voltage Vector group Mass of core and winding (kg) Mass of oil (kg) Total mass (kg) Volume of oil (liters)
When installed with ONAN
66 11 87.6 525 3 50 Hz 9.929 % YNyn0 10500 5960 25000 6700
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INSTRUMENT TRANSFORMERS Instrument transformers means current transformer & voltage transformer are used in electrical power system for stepping down currents and voltages of the system for metering and protection purpose. Actually relays and meters used for protection and metering, are not designed for high currents and voltages. High currents or voltages of electrical power system cannot be directly fed to relays and meters. CT steps down rated system current to 1 Amp or 5 Amp similarly voltage transformer steps down system voltages to 110V. The relays and meters are generally designed for 1 Amp, 5 Amp and 110V.
POTENTIAL TRANSFORMER Potential Transformer or Voltage Transformer is used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Commercially available relays and meters used for protection and metering, are designed for low voltage. This is a simplest form of Potential Transformer Definition
A Voltage Transformer theory or Potential Transformer theory is just like theory of general purpose step down transformer. Primary of this transformer is connected across the phases or and ground depending upon the requirement. Just like the transformer, used for stepping down purpose, potential transformer i.e. PT has lowers turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the PT.
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The secondary voltage of the PT is generally 110V. In an ideal Potential Transformer or Voltage Transformer when rated burden connected across the secondary the ratio of primary and secondary voltages of transformer is equal to the turns ratio and furthermore the two terminal voltages are in precise phase opposite to each other. But in actual transformer there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages. The errors in Potential Transformer or Voltage Transformer can best be explained by phasor diagram, and this is the main part of Potential Transformer theory.
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Is – Secondary Current Es – Secondary induced emf Vs – Secondary terminal voltage R s – Secondary winding resistance Xs – Secondary winding reactance I p – Primary current E p – Primary induced emf V p – Primary terminal voltage R p – Primary winding resistance X p – Primary winding reactance K T – turns ratio = numbers of primary turns/number of secondary turns Io – Excitation Current Im – magnetizing component of I o Iw – core loss component of I o
Φm – main flux β – phase angle error As in the case of Current Transformer and other purpose Electrical Power Transformer, total primary current Ip is the vector sum of excitation current
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and the electric current equal to reversal of secondary current multiplied by the ratio 1/K T Hence, Ip = Io + Is/K T If Vp is the system voltage applied to the primary of the PT then voltage drops due to resistance and reactance of primary winding due to primary current Ip will comes into picture. After subtracting this voltage drop from Vp, Ep will appear across the primary terminals. This Ep is equal to primary induced e.m.f. This primary e.m.f will transform to the secondary winding by mutual induction and transformed e.m.f is Es. Again this E s will be dropped by secondary winding resistance and reactance, and resultant will actually appear across the burden terminals and it is denoted as Vs So if system voltage is V p, ideally Vp/K T should be the secondary voltage of PT, but in reality actual secondary voltage of PT is Vs.
The difference between the ideal value V p/K T and actual value Vs is the voltage error or ratio error in a potential transformer, it can be expressed as
The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors K T.Vs is the phase error
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The voltage applied to the primary of the potential transformer first drops due to internal impedance of primary. Then it appears across the primary winding and then transformed proportionally to its turns ratio, to secondary winding. This transformed voltage across secondary winding will again drops due to internal impedance of secondary, before appearing across burden terminals. This is the reason of errors in potential transformer.
SPECIFICATION OF 110kV FEEDER PT High test system voltage
123 Kv
Insulation level
230/550Kv
Oil Quantity
180ltr
Frequency
50Hz
Secondary winding number Output Accuracy class Primary terminal Voltage Factor Secondary terminal Voltage ratio
1 Protective 200VA 3P A 1.2 continuous 1a.1n
2 Measuring/Protective 200VA 0.5/3P N 1.5/30 sec 2a.2n
( 11000/√3)/110
( 11000/√3)/110//√3
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CURRENT TRANSFORMER A CT is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree. CT Accuracy Class or Current Transformer Class
A CT is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. For metering and indication purpose, accuracy of ratio, between primary and secondary currents are essential within normal working range. Normally accuracy of current transformer required up to 125% of rated current; as because allowable system current must be below 125% of rated current. Rather it is desirable the CT core to be saturated after this limit since the unnecessary electrical stresses due to system over current can be prevented from the metering instrument connected to the secondary of the CT as secondary current does not go above a desired limit even primary current of the CT rises to a very high value than its ratings. So accuracy within working range is main criteria of a CT used for metering purpose. The degree of accuracy of a Metering CT is expressed by CT Accuracy Class or simply Current Transformer Class or CT Class. But in the case of protection, the CT may not have the accuracy level as good as metering CT although it is desired not to be saturated during high fault
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current passes through primary. So core of protection CT is so designed that it would not be saturated for long range of currents. If saturation of the core comes at lower level of primary current the proper reflection of primary current will not come to secondary, hence relays connected to the secondary may not function properly and protection system losses its reliability. One CT with current ratio 400/1A and its protection core is situated at 500A. If the primary current of the CT becomes 1000A the secondary current will still be 1.25A as because the secondary current will not increase after 1.25A because of saturation. If actuating current of the relay connected the secondary circuit of the CT is 1.5A, it will not be operated at all even fault level of the power circuit is 1000A. The degree of accuracy of a Protection CT may not be as fine as Metering CT but it is also expressed by CT Accuracy Class or simply Current Transformer Class or CT Class as in the case of Metering Current Transformer but in little bit different manner.
A CT functions with the same basic working principle of electrical power transformer, as we discussed earlier, but here is some difference. If a electrical power transformer or other general purpose transformer, primary current varies with load or secondary current. In case of CT, primary current is the
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system current and this primary current or system current transforms to the CT secondary, hence secondary current or burden current depends upon primary current of the current transformer. In a power transformer, if load is disconnected, there will be only magnetizing current flows in the primary. The primary of the power transformer takes current from the source proportional to the load connected with secondary. But in case of CT, the primary is connected in series with power line. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally CT has very few turns in primary where as secondary turns are large in number. Say Np is number of turns in CT primary and Ip is the current through primary. Hence the primary AT is equal to NpIp AT. If number of turns in secondary and secondary current in that current transformer are Ns and Is respectively then Secondary AT is equal to N sIs AT. In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT. So from the above statement it is clear that if a CT has one turn in primary and 400 turns in secondary winding, if it has 400 A current in primary then it will have 1A in secondary burden. Thus the turn ratio of the CT is 400/1A
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In an actual CT, errors with which we are connected can best be considered,
Is – Secondary Current Es – Secondary induced emf I p – primary Current E p – primary induced emf K T – turns ratio = numbers of secondary turns/number of primary turns Io – Excitation Current Im – magnetizing component of I o Iw – core loss component of I o
Φm – main flux. Let us take flux as reference. EMF E s and Ep lags behind the flux by 90o. The magnitude of the passers Es and Ep are proportional to secondary and primary turns. The excitation current Io which is made up of two components Im and Iw . The secondary current Io lags behind the secondary induced emf Es by an angle Φ s. The secondary current is now transferred to the primary side by reversing Is and multiplied by the turns ratio K T. The total current flows through the primary Ip is then vector sum of K T Is and Io.
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From above phasor diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio, i.e. K TIs. This difference is due to the primary current is contributed by the core excitation current. The error in current transformer introduced due to this difference is called current error of CT or sometimes Ratio Error in Current Transformer.
For a ideal CT the angle between the primary and reversed secondary current vector is zero. But for an actual CT there is always a difference in phase between two due to the fact that primary current has to supply the component of the exiting current. The angle between the above two phases in termed asPhase Angle Error in Current Transformer or CT. Here in the pharos diagram it is β the phase angle error is usually expressed in minutes.
The total primary current is not actually transformed in CT. One part of the primary current is consumed for core excitation and remaining is actually transformers with turns ratio of CT so there is error in current transformer means there are both Ratio Error in Current Transformer as well as a Phase Angle Error in Current Transformer.
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It is desirable to reduce these errors, for better performance. For achieving minimum error in current transformer, one can follow the following, 1) Using a core of high permeability and low hysteresis loss magnetic materials. 2) Keeping the rated burden to the nearer value of the actual burden. 3) Ensuring minimum length of flux path and increasing cross – sectional area of the core, minimizing joint of the core. 4) Lowering the secondary internal impedance.
123kV 230/550kV 50Hz 80ltr 500kg
High test system voltage Insulation Level Frequency Oil Quantity Weight
Core No Volt Ampere Accuracy Class ALF/ISF Current Ratio Short time current
Primary Connection P1-C1 C2-P2 C2-C1
1 Protective 80 5P 10 600-300/1 25kA/1sc
2 Measuring 30 1 600-300/1 -
Secondary terminal 1S1-1S2 2S1-2S2 1S1-1S2 2S1-2S2
Current ratio 600/1 600/1 300/1 300/1
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CAPACITOR VOLTAGE TRANSFORM ER A capacitor voltage transformer (CVT) is a transformer used in power systems to step down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form, the device consists of three parts: two capacitors across which the voltage signal is split, and inductive element used to tune the device to the supply frequency and a transformer to isolate and further step down the voltage for the instrumentation or protective relay. Capacitor voltage transformers are typically single phase devices used for measuring voltages in excess of one hundred Kilo Volts where the use of voltage transformers would be uneconomical. In practice, the first capacitor C1 is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor C2 and hence the secondary terminals.
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CONCLUSION Working at the 110kV substation, Punnapra as summer training was a very nice experience. I learned a lot about electrical substation system and the importance of substations in electrical generation, transmission and distribution. This industrial visit provided an insight on how substations work and thus helps in efficient transmission of electricity. We also studied about different substation equipments in detail. It has given us useful information related to our course which cannot be visualized in lecture classes such as transformers which is as big as one-fourth of an average room, which we learnt about but never saw in the college labs. In the beginning of this visit I was not aware about the merits we were going to receive from the visit but at the end I realised it was a very good experience which i would have regretted if I missed. Also the training was an opportunity for me to increase my personal relations both socially and professionally.
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REFERENCE 1. Handbook of Kerala State Electricity Board 2. Safety manual of Kerala State Electricity Board 3. Daily report diary of Punnapra substation 4. Equipment register of Punnapra substation 5. Operator’s Diary (KSEB) 6. Maintenance Book (KSEB – Punnapra substation) 7. Wikipedia (www.wikipedia.com) 8. Website of KSEB (www.kseb.in) 9. Electrical power system – M Rajalingam 10. Principles of power system – V K Mehta 11. Switch gear and protection – Sunil S Rao 12. Electrical Technology Vol II – Theraja 13. Electrical power system – Uppal
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