Report On Industrial Visit to Punnapra Substation



TABLE OF CONTENTSACKNOWLEDGEMENT INTRODUCTION COMPONENTS OF THE SUBSTATION SITE SELECTION AND LAYOUT OF 110KV SUBSTATION AN OVERVIEW OF PUNNAPRA SUBSTATION FUNCTIONING OF THE SUBSTATION OPERATIONS TO BE CARRIED OUT LIGHTNING ARRESTOR WAVE TRAP CAPACITOR BANK ISOLATOR EARTHING SYSTEMS RELAYS BATTERY AND BATTERY CHARGER BUSBAR 3 4 6 7 9 10 13 16 17 18 19 20 32 35 37 CIRCUIT BREAKERS POWER LINE CARRIER COMMUNICATION CONSTRUCTIONAL DETAILS OF TRANSFORMER5 CLASSIFICATION OF TRANSFORMERS SPECIFICATION OF TRANSFORMERS INSTRUMENT TRANSFORMERS CAPACITOR VOLTAGE TRANSFORMER REFERENCE 38 44 46 52 56 59 70 72 Page | 2 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 this industrial visit. Polytechnic College, Punnapra for providing me an opportunity to conduct 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 each of them. working of organization. I am highly thankful for the support and guidance of I am highly indebted to my project guide, Mr. ………………………………… ( Executive Engineer), Mr. ……………………………… (Assistant Executive engineer) , Mr ……………………….. (Assistant Engineer), Mr. …………………………… (Station Engineer) for giving me their valuable their control instruments and their testing. time and helping me to grasp various concepts of switchyard equipments and I would like to express my gratitude towards my parents, classmates & my the completion of this report. friends for their kind co-operation and encouragement which helped me in 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 Page | 3 Substations are of different types. Punnapra. Page | 4 . and transformers. generation voltage (11kV/6.g. generally quite away from the consumers. This is accomplished by suitable apparatus called substation up to high voltage of transmission of electric power. transmitted and distributed in the form of alternating current. or the reverse. and distribution system.e. electrical power is generated. the major substations include one 400 KV sub-station. transmission. AC to DC frequency.6kV) at the power station is stepped consumer’s localities.INTRODUCTION A substation is a part of an electrical generation. Voltage. A transmission substation connects two or more transmission system to the distribution system of an area. Substations generally have switching. Alappuzha gives a chance to their students to spend two weeks in industrial companies. It is delivered to the electric power is produced at the power station. for example. 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. power factor etc) of places in the line of power system.. seventeen 220 KV substations. it may be desirable and necessary to change electric supply. At many some characteristics (e. the voltage may have to step down to utilization level. Substations transform voltage from high to low. The places. protection and control equipments. Similarly near the This job is again accomplished by suitable apparatus called substation. and The department of Electrical and Electronics Engineering of Carmel Polytechnic College. or perform any of several other important functions. which are located at favorable consumer through a large network of transmission and distribution. transmission lines and a distribution substation transfers power from the In Kerala. The present day electrical power system is AC i. A substation may include transformers to include voltage levels between high transmission voltage levels and lower distribution voltages. and transformers. As central generation station became larger. protection and control equipments. receiving their energy supply from Page | 5 . or at the interconnection of two different transmissions. plants were converted distribution stations. where the generators were housed. Devices such as capacitors and voltage regulators may also be located at a substation. and were subsidiaries of the power station. smaller generation a larger plant instead of using their own generators. The word substation comes from the days before the distribution system became a grid. Substations generally have switching. The first substations were connected to only one power station. COMPONENTS OF THE SUBSTATION TRANSMISSION PATH INSIDE THE SUBSTATION A: Primary power line's side B: Secondary power line's side 1. Lightning arrestor 9. 3. 6.Secondary power lines Page | 6 . 4. Main transformer 10.Security fence 12. 5.Control building 11. Primary power lines Ground wire Overhead lines Potential or Voltage transformer Disconnect switch Circuit breaker 7. Current transformer 8. 2. such as store yards and store sheds etc. earthing and for drinking purposes etc. This is because water is required for various construction activities (especially civil works). buildings. the substation configuration should be such that it enables easy maintenance of equipment and minimum interruptions in power supply. space for storage of material. aerodromes and military/police installations. Apart from ensuring efficient transmission and distribution of power.  It should be easily approachable by road or rail for transportation of equipments. The voltage level of power transmission is decided on the quantum of power to be transmitted to the load centre. It has a vital influence of reliability of surface. with roads and space for future Page | 7 . Substation is constructed as near as possible as the load centre.  The substation site should be as near to the town/city but should be clear of public spaces.  Source of water should be as near to the site as possible. staff quarters.  The land should have sufficient ground area to accommodate substation equipments.SITE SELECTION AND LAYOUT OF 110kV SUBSTATION 110kV substation forms an important link between Transmission network and Distribution network. 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.  Land should be fairly leveled to minimize development cost.  Set back distances from various roads such as National highways.expansion. state highways should be observed as per regulations in force.  Far away from obstructions. preference is to be given to government land over private land. to permit easy and safe approach termination of high voltage overhead transmission lines Page | 8 .  The land should not have water logging problem.  While selecting the land for the substation. 5 traction} = 170. 66kV SIDE BUS I INCOMING LINE 63 MVA I BUS II 63 MVA II OUTGOING LINE ALP NO: I ALP NO: II 10 MVA I MVKA NO: I MVKA NO: II 10 MVA II Page | 9 .5 MVA 110kV SIDE BUS I INCOMING LINE PLPU NO I BUS II PLPU NO:II OUTGOING LINE PUED NO I PUED NO:II 63 MVA I 63 MVA II 16 MVA TRACTION .AN OVERVIEW OF PUNNAPRA SUBSTATION Connector MVA: {2*63 + 16 + 16(EDTA II) +12. Sub Engineer (Electrical/Maintenance) attends to all maintenance work connected with lines and equipments of substation including routine and breakdown maintenance. Operator on duty shall carry out all the operations required for normal functioning of the substation as per the directions followed. 2. 2. OPERATIONS IN GENERAL The following operating instructions may be strictly followed for the smooth operation of the substation: 1. He / She shall also record the name of shift assistant in the diary and log book. trouble noted in any of the equipments etc. Operating crew of substation comprises of one Assistant Engineer as operator and one Overseer as Shift Assistant. He will assist the AE in the preparation of monthly returns and allied Db works. Handing over time and dated signature Page | 10 . Handover the charge with clear explanation in brief regarding the substation and feeders such as PW/IC/NBC in force. 3. The operator. Station Engineer holds overall charge of the substation. taking over the shift charge shall record the time of taking over the duty with name and signature.FUNCTIONING OF THE SUBSTATION RESPONSIBILITIES AND DUTIES 1. Read carefully previous operations and make a thorough picture regarding the substation feeder positions. Phone message received and transmitted shall be recorded with date and time and confirm the authenticity of the person at the other end.with the name of the relieving operator should invariably be recorded. 5. Visit the yard frequently and watch the various equipments and their functions carefully. Then the overall inspection of the control room and yard equipments should be conducted. 3. Message book and phone call register are to be maintained by the operator on duty. Check and confirm the reliability of emergency lights and accessibility of fire fighting equipments. 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. 4. 6. An operator should primarily check protective and alarm circuits of the individual feeders and also the control supply system including the battery system. Record all entries with time and sequence of operations performed. 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. Page | 11 . Take suitable steps to avoid overloading of equipments and feeders. Maintain the system voltage within the statutory limits with appropriate tab selections as far as possible. Station clock timings should be checked and corrected if necessary at 3pm on every day. Promptly record hourly and half hourly readings with utmost care. 8. 11. 12. with 220kV substation Kalamassery. Carry out various routine operations symmetrically as scheduled below separately. 13. The operator on duty shall see that the substation equipments and panels in the control room are kept clean. Page | 12 .7. 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 1st shift assistant operator every day. 9. 10.  OUTGOING FEEDERS EXCEPT AUXILIARY In case an outgoing feeder is tripped. note the relay indication. switch off all the outgoing feeders from the transformers.OPERATIONS TO BE CARRIED OUT  FAULTS ON TRANSFORMERS If the circuit breaker of a transformer has tripped. The load on the transformer may closely be watched and if found exceeding the admissible limit. If the tripping is for any other reason other than the over current. reduce the load on the transformer by switching off outgoing feeders from the transformer. the alarms may be accepted. If the incomer is again tripped. reset the relay and accept the alarm and test charge the feeder. the distribution authorities may be directed to limit the current. Then charge the incoming feeder one by one. the outgoing feeder last charged may be kept open and other feeders charged suspecting fault on the particular feeder. the relay indications carefully checked and noted. accept the alarm. If the feeder trips instantly or any apparent Page | 13 . Reset the relays and test charge the transformer on no load. If the tripping is an Overload. Then charge the outgoing` feeders one by one and ensure that the load is not more than the capacity of the transformer. the transformer may be charge only after consulting the higher officials. Reset the relay and charge the incoming feeder.  INCOMING FEEDERS If the incoming feeders are tripped on over current relay. Page | 14 . But as the station supply is taken from the beach feeder.fault or heavy fluctuations in the supply system. Avoid further test charging until confirmation from distribution authority is received that the load on the feeder has been reduced. Inform the distribution section to rectify the fault. only three test charging may be attempted.  AUXILIARY FEEDER The method in the case of other outgoing feeder may be adopted in this case also. when the feeder is faulty. open the AB switch in the 11kV outdoor structure and charge the breaker for taking the auxiliary supply. flashing the cubicle are noted. inform the matter to distribution section. 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. If a feeder trips on OC relay. STUDY OF SUBSTATION EQUIPMENTS Page | 15 . The current passing through it. maximum current passed through ammeter in the arrester carrying the topmost conductor will have maximum 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. initially the line is connected lightning arrester seems like a set of insulators connected together through a lightning arrester. Generally a with a ring in the top. The ammeter is reset. This is for the protection of the station. When a lightning surge or the surge is diverted around the protected insulation in most cases to the earth. This ring is grading rings is that in case of called grading ring. switching surge travels down the power system to the arrestor.LIGHTNING ARRESTOR Whenever an incoming comes to a substation. The purpose of heavy voltage surges the charge is distributed uniformly through the ring and then the discharge occurs. An ammeter is connected with the it. the current from 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 Page | 16 . Its shape is like that of a drum.WAVE TRAP Wave trap is also known as line trap. Page | 17 . The signals are primarily teleprotection signals and in addition. 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. then signal loss is more and communication will be ineffective or probably impossible. The wave trap offers high impedance to the high frequency communication signals thus obstructs the flow of the signals to the substation bus bars. Line signals sent on the line from the remote substation and diverting them to the telecom / teleprotection panel in the substation control room. If they were not to be there. It is an instrument used for tripping of the wave. voice and data communication signals. 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. The function of this trap is that it traps the unwanted waves. This is relevant in Power Line Carrier Communication (PLCC) systems for the communication among various substations without dependence on the telecom company network. 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. 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. and in the construction of resonant circuits used in radio tuning. 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. such as power factor lag or phase shifts inherent in alternating current (AC) electrical power supplies. In Punnapra Substation. which cause a phenomenon known as phase shift or power factor lag in the power supply.14kV. These groups of capacitors are typically used to correct or counteract undesirable characteristics.CAPACITOR BANK A capacitor bank is a grouping of several identical capacitors interconnected in parallel or in series with one another. These characteristics also allow capacitors to be used in a group or capacitor bank to absorb and correct AC power supply faults. 25 MVR consisting of 42 units of 10. This type of working load is problematic from a power supply perspective as electric motors and transformers represent inductive loads. capacitor bank is rated for 123kV. This is usually a known. 596. output smoothing in DC power supplies.23 kVAR internal fuse capacitor units arranged in double star configuration. The use of a capacitor bank in the power supply system effectively cancels out or counteracts these phase shift issues. The energy storing characteristic of capacitors is known as capacitance and is expressed or measured by the unit farads. making the power supply far more efficient and cost effective. 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. Page | 18 . the Circuit Breaker connected must be opened first. For bus isolators. If isolators are is a possibility of occurrence of a spark at the isolator contacts. During maintenance works the line isolator contacts are opened. dead weights are provided at the end SPECIFICATION Current Max Design Voltage Impulse Withstand Voltage 800A 125kV 550kV Page | 19 . There are two types of isolators- to be opened. isolators are used. It is a knife switch designed to open a circuit under no load. Otherwise there Line isolators and Bus isolators. For the ease of earthing. trip simultaneously. so that the three phases of earthing arms. After repair. first isolators are closed and then Circuit Breaker. there is no earth switch.ISOLATOR In order to disconnect a part of system for maintenance and repair. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply. an earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface.EARTHING SYSTEMS In electricity supply systems. Most electrical systems connect one supply conductor to earth (ground). and regulations can vary considerably among countries. If a fault Page | 20 . . The substation earthing system is necessary for connecting neutral non current carrying metal parts such as structures. frames. fault. 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. tanks. and shielding wires etc. or touching an earthed sink) will complete a circuit back to the earthed supply conductor and receive an electric shock. etc to earth.g. anyone touching it while electrically connected to the earth (e. To provide a sufficiently low resistance path to the earth to minimize the rise in earth potential with respect to a remote earth not receive a dangerous shock during an earth fault. The function of substation earthing system is to provide a grounding mat below the earth surface in and around the substation which will have ensure that  uniformly zero potential with respect to ground and lower earth resistance to To provide discharge path for lightning over voltages coming via rodgaps. overhead shielding wires.within an electrical device connects a "hot" (unearthed) supply conductor to an exposed conductive surface. by standing on it. the line is dead still charge remains which causes dangerous shocks) to   Earth Resistance Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. so as to discharge the trapped charge (Due to charging currents even earth prior to maintenance and repairs. 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 points of transformers and generators to ground and also for connecting the substation. surge arresters. Earthing of surge arresters is through the earthing system. Persons touching any of the non current carrying grounded parts shall Page | 21 .. Each structure. The person should not get a shock even if the grounded small. structure is carrying fault current. 2. with 0. etc.5 m spacing between the feet (one step). Touch Potential: 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. through the flow of earth fault current through the grounding system.e. Grounding system in a electrical system is designed to achieve low earth resistance and also to achieve safe ‘Step Potential ‘and ‘Touch Potential’. Un earthed Systems: It is used no more. Solid grounding or effective grounding: The neutral is directly connected to the earth without any impedance between neutral and ground. also called as insulated neutral system.transformer tank. i. should be connected to earthing mat by their own earth connection. The neutral is not connected to the earth. 3. body of equipment. The Touch Potential should be very Types of Grounding: 1. Step Potential: Step potential is the potential difference between the feet of a person standing on the floor of the substation. Resistance grounding: Page | 22 . Reactance grounding: Reactance is connected between the neutral and ground.Resistance is connected between the neutral and the ground. Resonant Grounding: An adjustable reactor of correctly selected value to compensate the capacitive Arc Suppression Coil or Earth Fault Neutralizer earth current is connected between the neutral and the earth. The coil is called Different Grounding Equipment in Electrical Substation     Earthing Electrodes Earthing Mat Risers Overhead shielding wire (Earthed) Page | 23 . 4. 5. CONVENTIONAL METHODS OF EARTHING PIPE TYPE 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 auger method. numbers of holes for the connection of earth wires and inserted in ground by Page | 24 . OR Galvanized iron plate of size 600 mm x600 mm x6 mm.These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound. OR Page | 25 . PLATE TYPE EARTHING    Generally for plate type earthing normal Practice is to use Cast iron plate of size 600 mm x600 mm x12 mm.   Copper plate of size 600 mm * 600 mm * 3.15 mm Plate burred at the depth of 8 feet in the 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 from the bottom of the pit. MAT EARTHING Design of earth mat 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 depth depending upon the type of soil, moisture content and temperature etc., at affects the flow of various depths which current due to the fact that the earth fault current is likely to take its path through various layers. may also vary at different Method for Construction of Earthing Pit   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 rust proves for GI Plate for long life. Wood Coal Powder use as good conductor of electricity, anti corrosive, The purpose of coal and salt is to keep wet the soil permanently.  Page | 26   The salt percolates and coal absorbs water keeping the soil wet. that the pit soil will be wet. Care should always be taken by watering the earth pits in summer so 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. Factors affecting on Earth Resistivity (1) Soil Resistivity 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, Page | 27 current magnitude etc. In depends on the composition of soil, Moisture current magnitude. (2) Soil Condition content, Dissolved salts, grain size and its distribution, seasonal variation, 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. (3) Moisture Moisture has a great influence on resistivity value of soil. The resistivity of a of the water itself. Conduction of electricity in soil is through water. soil can be determined by the quantity of water held by the soil and resistivity 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 earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity (4) Dissolved salts Pure water is poor conductor of electricity. Resistivity of soil depends on dissolved in it. weather conditions. Therefore, it is essential to pour water in and around the resistivity of water which in turn depends on the amount and nature of salts Page | 28 the same may be well above 1000 ohmmeter. Therefore. Based on the type of soil. or areas which are hilly. whereas for rocky or gravel soils. The earth pits located in such areas must be watered at frequent intervals. (6) Physical Composition Different soil composition gives different average resistivity. Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low. (5) Climate Condition Increase or decrease of moisture content determines the increase or decrease of soil resistivity. rocky or sandy. choose a site that is (8) Effect of grain size and its distribution Page | 29 . the resistivity of clay soil may be in the range of 4 – 150 ohm-meter. the process loses moisture over a period of time. it gives off moisture during dry weather to the dry soil around the electrode. In a sloping landscape. Though back fill compound retains moisture under normal conditions.Small quantity of salts in water reduces soil resistivity by 80%. 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. common salt is most effective in improving conductivity of soil. as the soil around the pit would be dry. But it corrodes metal and hence discouraged. and in naturally not well drained. (7) Location of Earth Pit The location also contributes to resistivity to a great extent. or in a land with made up of soil. particularly during dry weather conditions. must be outside the resistance area of the other.Grain size. (11) Obstructions The soil may look good on the surface but there may be obstructions below a like concrete structure near about the pits will affect resistivity. In that event resistivity will be affected. 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 (10) Area Available Single electrode rod or strip or plate will not achieve the desired resistance alone. If the earth pits are close by. Each electrode. Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease or increase of soil resistivity. Obstructions Page | 30 . If a number of electrodes could be installed and interconnected the desired resistance could be achieved. (12) Current Magnitude few feet like virgin rock. The distance between the electrodes must be equal to the driven depth to avoid overlapping of area of influence. the resistance value will be high. its distribution and closeness of packing are also contributory factors. since they control the manner in which the moisture is held in the soil. therefore. Thus in dry weather resistivity will be very high and during rainy season the resistivity will be low. (9) Effect of current magnitude Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into the surrounding soil. LV neutral Connect the transformer earthing bolt to earthing system. Potential transformer Lower earth point To be directly connected to earth mat Device flange or base plate Connect the earthing bolt of the device to the station earthing system Potential Transformer tank. DIFFERENT EQUIPMENTS AND GROUND CONNECTIONS Apparatus Power transformer Parts to be Earthed Transformer tank Method of Connection Connect the earthing bolt on transformer tank to the station earth High Voltage circuit breakers Operating mechanism. etc. frame Connect the earthing bolt on the frame and the operating mechanism of Circuit breaker to earthing system Surge arrestor Support of bushing insulators. operating mechanism. fuse. Current transformer Secondary winding and metal case Connect secondary winding to earthing bolt on transformer case. Connect LV neutral of phase lead to case with flexible copper conductor Isolator Isolator frame. Page | 31 . lightning arrestors. connect it to the bolt on the operating mechanism.A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity. base plate and station earth. bed plate Weld the isolator base frame. 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 Distance Protection Relay It is a special type of relay used to know at which place the line has failed. frequency. The relay detects the abnormal condition such as voltage. The relay will indicate the rough The lines are divided into distance between the station and the point at Page | 32 . 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. phase angle and temperature. A protective relay is mainly incorporated in the control panel section of the substation. The substation has control panels for its incoming as well as outgoing feeders and each control panel has various relays. current.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. zones. tripping occurs. The connection to the main relay is made panel has 2 types of distance protection relays. Synchronized relay does the function of 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. Synchronizing Relay Advantages of bus couplers can be obtained only if the voltage and frequency of constantly comparing the two voltages and frequency and thereby initiating the tripping mechanism at time of fault. Over Current Relay Page | 33 . In normal conditions the is the same as the current through the relay equipment current but when any fault occurs in is a rise in current the line enclosed ten there through the relay at the fault side above that which is on the other side. Auto Reclose Relay through an auxiliary relay. It helps in speedier tripping and making up of the connection after fault rectification. Differential Relay the bus bars to be coupled are the same. This activates the relay. The synchroscope aids it. This relay is very helpful in remote areas.which the breaking has occurred. The control Auto reclose relay is of mechanical type. indication and other auxiliary duties in AC or DC systems.The relay activates when current exceeds the permissible limits. The main applications of this relay are in control. These relays are attracted in a armature units of compact design supplied with reset contacts. magnetic blow type contacts are used. The relay is always given to the relay as it should trip even if there is an interruption in the power supply. The new control panel uses automatic semaphore (mimic) indicators for better control. acts and activates the circuit to the breaker hence tripping the breaker. heavy duty permanent magnet to force the arc onto the arcing horns away from the contact tips. Page | 34 . It is also connected to circuit breaker to help tripping the circuit. DC supply Earth Fault Relay It will be activated when there is any fault in the earthing of the equipment. Standard contacts are of silver or copper alloys. It will be connected to the circuit breaker in case of any fault due to over current. CAA & VAA relays are current and voltage operated respectively. alarm. They use the magnetic field of a small When it is necessary to break heavy or highly inductive DC loads. BATTERY AND BATTERY CHARGER The station DC source is facilitated through battery of 400 Ah capacities and 200 Ah capacities. 50 A capacities. At this time also the tripping in case of fault should continue. in such cases the batteries used also should be less. The second 400Ah battery bank has the same capacity. for this the 80V DC Supply is very essential. they can supply a current of 400 A for a time of 1 hour. The 400 Ah battery bank no 1 is fed through the battery charger from the main control room.e. They have 400 Ah capacity i. It can be used in case the AC fails. 110 volt supply is always provided as a standby as there is possibility of power failure in station. So it can be used to supply 200 A at intervals of 2 hours. This voltage always provided in parallel with the AC supply. 55 batteries each of 2 volt are provided giving a total of 110 V. float charging and boost charging. The batteries can be charged in 2 modes. Float charging is used when AC is present and Boost charging is used when the battery is in the back up Page | 35 . The batteries are lead acid cells and have sulphuric acid as its electrolyte with lead electrode along with spongy lead in between. This feeds only 11kV cubicles located in the old control room. 200 Ah bank is fed through the battery charger located in the old control room. In some area the required voltage is less. 15 A capacities. This is of 110kV. This is of 110kV. Page | 36 .mode. Battery is regularly checked in the substation to check the acidity of Electrolyte. To measure the voltage there is the centre zero voltmeter. A hydrometer is used to measure the same. Load division is better. Busbar is of Cu or Al and is rectangular in substation are connected to the busbar. Single busbar with sectionalisation cross-section. If fault occurs in bar. Conductor used is moose. The incoming and outgoing lines in the In this. busbar is divided into sections and load is equally distributed on all sections. Thus can be carried out by eliminating the possibility of complete shutdown. The advantage of this arrangement is as follows: 1.BUSBAR The lines operating at the same voltage is directly connected to a common electrical component called busbar. Even if one bus fails the other bus can supply the load. Page | 37 . Double busbar with bus coupler Buses are coupled by means of two isolators and a coupler 1. Repair and maintenance of any section of the busbar de energizing that section only. that section canbe isolated any section of the bus without affecting the supply from other sections. 2. 2. circuit damage. and immediately stops the current flow. interrupts the continuity. results in insulation breakdown and an electric fire. Overloading in an electrical circuit occurs when the wires are forced to carry and conduct an electric charge more than their capacity. consequences of wire overloading and accidental short-circuiting. CIRCUIT BREAKING MECHANISM Generally. This causes unwanted current flow from one node to another which Therefore. explosion or even fire. when a fault condition is detected. When the switch is on an ‘ON’ position.CIRCUIT BREAKERS Circuit breakers have an in built fixed electric current load capacity which when breached causes automatic circuit shutdown. the and he electric current ceases to flow. the mechanism may vary substantially as per the voltage class. whereas in those meant for large currents or high voltages. the contact plate touches a stationary plate which is connected to the circuit so that the electric current can flow. It basically detects the fault condition like a short or over load in the circuit. current rating and type. This safety feature makes insulation of a circuit breaker and essential part in an electric circuit. Short circuit occurs when contact. it is rectified within the breaker enclosure. devices like relays are arranged to sense the fault current and rectify it by Page | 38 . circuit breakers are used to protect the circuit from unwanted conductive contact plate which moves with the switch. Though most circuit breaker has contact plate moves away from the stationary plate and the circuit gets opened common features in their operation. But when the switch is in the ‘OFF’ position. a circuit breaker panel consists of a switch and a moving. In low voltage circuit breakers. due to the overloading or short circuit. special pilot employing trip opening mechanism. This causes the wires to heat up and two points in the circuit having different potential accidentally come in may result in excessive heating. which is being stored in a closed tank. is filled separately in each after each operation. SPECIFICATION OF SF6 CB Rated voltage Normal current Frequency Lightning impulse withstand voltage Duration of short circuit First pole to clear factor Short Circuit Breaker Current (Symmetrical) Short Circuit Breaker Current (Asymmetrical) Short circuit making current Operating Sequence SF6 gas pressure at 20°C (abs) Total mass of SF6 gas (Kg) 145kV 3150A 50Hz 650kV 3s 1.3s-CO-3min-CO 0.5 40kA 44.74mpa 12 Page | 39 .8kA 100kAp 0-0. SF6 being costly. Operation CB. Each CB has an air tank in which pressure is maintained at 15kg/cm2. If SF6 CIRCUIT BREAKER pressure goes below this a rotary compressor is automatically activated. The gas can be reconditioned mechanism is through air. Other CBs are interconnected through tubes.TYPES OF CIRCUIT BREAKERS Its closing is by spring action and tripping is in air. Pressure of SF6 is continuously monitored. But this put a limit on the design of materials for the arcing chambers such as glass fibre. It has been decided that the oil in the circuit been introduced where use of oil in the circuit breaker is much less than that breaker should be used only as arc quenching media not as an insulating media. hence better the arc quenching. resin etc. In this type of circuit breaker enclosed in a tank of the arc interrupting device is insulating material which as system. Higher the pressure developed in the current to be interrupted causes larger the gas pressure developed inside the the arc chamber for mechanical stresses. Hence the hydrogen bubble Page | 40 . one important development in the design of oil circuit breaker has of bulk oil circuit breaker. the arc drawn across the current carrying contacts is contained inside the arcing chamber. reinforced synthetic fault levels of the system. the minimum oil circuit breaker are able to meet easily the increased Working Principle or arc quenching in minimum oil circuit breaker In a minimum oil circuit breaker. For avoiding unwanted fire hazard in the system.MINIMUM OIL CIRCUIT BREAKER (MOCB) As the volume of the oil in bulk oil circuit breaker is huge. the chances of fire hazard in bulk oil system are more. This chamber is a whole is at live potential of called arcing chamber or interrupting pot. With use of better insulating chamber. Then the concept of minimum oil circuit breaker comes. The gas arcing chamber depends upon the current to be interrupted. 38-CO standard BSS 1050kg 150ltr VACUUM CIRCUIT BREAKER A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. Hydrogen).formed by the vaporized oil is trapped inside the chamber.1kA for 3sec 0-0. produced due to vaporization of oil and decomposition of oil during arc. 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 technology is suitable for mainly medium voltage application. vacuum interrupter consists of a steel arc chamber in the centre symmetrically Page | 41 . will sweep the arc in axial or longitudinal direction. There are two different types of arcing chamber is available in terms of venting are provided in the arcing chambers. For higher voltage Vacuum technology has been developed but not commercially viable. gases (mostly 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. The arranged ceramic insulators. after its certain travel an exit vent becomes available for exhausting the trapped hydrogen gas. One is axial venting and other is radial venting. As the contacts continue to move. In axial venting. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 – 6 bar. interruption technology for medium voltage system. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. low contact mass and no compression of medium the drive energy contact areas are just being separated to each other. For short required in vacuum circuit breaker is minimum. the current through the contacts concentrated on that the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Vacuum interrupter technology was first introduced in the year of 1960. At this instant of de-touching of last contact point on the contact surface and makes a hot spot. by establishing high dielectric strength in between the contacts so dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. from butt contact of early days it performance of the vacuum circuit breaker. they do not be separated instantly. The contact geometry is also improving with time. the to different technical developments in this field of engineering. But still it is a developing technology. extinguished and the conducting metal vapour is re-condensed on the contact Page | 42 . As time goes on. As it is vacuum. cup shape and axial magnetic field contact. The crossing.The material used for current carrying contacts plays an important role in the make VCB contacts. When two face to face contact gap. Although this happens contacts in a vacuum. CuCr is the most ideal material to size of the vacuum interrupter is being reducing from its early 1960’s size due gradually changes to spiral shape. Then the arc will be initiated and continued until the next current zero. The vacuum circuit breaker is today recognized as most reliable current maintenance compared to other circuit breaker technologies. contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. At current zero this vacuum arc is in a fraction of micro second but it is the fact. It requires minimum WORKING OF VACUUM CIRCUIT BREAKER The main aim of any circuit breaker is to quench arc during current zero that reestablishment of arc after current zero becomes impossible. 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.24A. the arc remains diffused and the form of vapour discharge and cover the entire contact surface. At this point.surface. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero. For interrupting current up to 10kA. the design of the contacts should be such that the arc does not remain stationary but keeps travelling by its own magnetic field. SPECIFICATION OF VCB Rated voltage Rated current Breaking capacity Making capacity Short time current 11kV 400A 26. That means. for next cycle of current. In order to prevent this. the contacts are already separated hence there is no question of re-vaporization of contact surface.24kA 65. 3sec Page | 43 . The phenomenon gives rise over heating of contact at its centre. There are two types of arc shapes. Above 10kA the diffused arc is constricted considerably by its own magnetic field and it contracts. the arc cannot be re-established again.6kA 26. teleprinting. there will be no interference from outside. Page | 44 . It communication lines in a separate area. cannot be made for practical make the signal very weak. and CB etc. which will make the job easier. Microwave communication can centre within the substation and 9505 power line carrier terminals are intended for the transmission of speed. modulating and making intermediate connections. In this remote controlled connections so type of communication. modulator. telemetering. which consists of capacitor and inductor coupling circuit. telecontrol. There are controls of the interface etc used for amplifying. It has an amplifier. PT. be used to link the dispatched also to the head office. which is used to separate the speech from 50Hz frequencies. teleindication & teleprotections signals in the carrier frequency range between 50Hz to 500 kHz over the following communication media with suitable line equipment.POWER LINE CARRIER COMMUNICATION Carrier communication is basically the communication between the substation head offices through power lines. domestic distribution as this will Using PLCC it is possible to make to the CT. Each substation has wave trap arrangement. data or other purposes. The carrier required for final mixing is derived from a VCO. The section works on PLL principle and can be steps of 0. Programming can be achieved by simple strapping. protective relaying.5 kHz. programmed to oscillate so as to give HF output in the range of 50-500 kHz in Page | 45 . When used for data only.12MHz generated in the system using a crystal oscillator. each channel carries onto base rates.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.  Data transmission  Station to station communication  Protection purpose Principle AF signals are converted into IF signals using IF carriers of 5. The required IF signals are filtered out using IO pole crystal filter to a final mixer stage. typically 2450-based telegraphed channels or a small no of channels at high Features of PLCC are: Programming Efficiency Flexibility Voice grade connections Output power Thus they are used for 3 purposes. The thickness of lamination varies from .CONSTRUCTIONAL DETAILS OF TRANSFORMER5  Laminated Steel Cores In all types of transformers the magnetic circuit is made of a laminated iron core. Special silicon steel Page | 46 . Hysteresis loss depends on area of hysteresis loop of the core material. The laminations are insulated from each other by a light coat of core plate varnish or an oxide layer on the surface. The core is laminated in order to reduce eddy current loss.5mm for 25Hz.35mm for 50Hz to . In addition to eddy current loss hysteresis loss occurs in the core as it is subjected to alternate magnetization and demagnetization. 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. in which currents are low and the potential difference between adjacent turns are small. 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 Page | 47 . the coil are often wound from enamel magnet wire such as formvar wire.  Windings The conduction material used for the winding depends upon the application. Although there are other factors that contribute heat in transformer such as hysteresis & eddy current losses but contribution of I2R loss dominate them.  Cooling The main source of heat generation in transformer is its copper loss or I2R loss. Each lamination is insulated from its neighbors by a thin no conducting layer of insulation (paper insulation). CRGO silicon steel laminations are used for the construction of transformer core.having a steel content of 4-5% is used for the lamination. but in all cases the individual turns must be electrically insulated from each other so that the current travels throughout every turns. easier in connecting tap changer to the HV winding. For small power and signal transformers. The core loss can be minimized by employing laminations of special steel sheet having high silicon content. Larger power transformers operating at high voltages maybe wound with copper rectangular strip conductors insulated by oil impregnated paper and blocks of pressboard. If this heat is not dissipated properly.  Conservator tank Conservator tank consist of oil level which depends on the operation of the transformer.Forced oil air water cooled.Forced oil air natural cooling  OFB . Larger transformers may have heavy terminals.Oil immersed air natural cooling  OB . bus bars or high insulated bushings made of polymers or porcelain.Air blast cooling  ON . A large bushing can be a complex structure since it must provide careful control of electric field gradient without letting the transformer leak oil.Oil immersed air blast cooling  OFN .Air natural cooling  AB.  Terminals Very small transformers will have wire leads connected directly to the ends of the coils.Forced oil air blast  OFW . There are different transformer cooling methods available for transformer:  AN. and brought to the base of the unit for circuit connection. The oil expands in summer with increase in load and the oil level Page | 48 . Electrical Power transformer we use external transformer cooling system to accelerate the dissipation rate of heat of transformer.thermal degradation of its insulation system. This diaphragm will rupture whenever the pressure in the transformer rises to a dangerous value. When the oil in the tank expands then the conservator takes the oil. Page | 49 . above this voltage ratings oil filled or capacitor type bushings are used. A thin glass relief diaphragm is placed at the top of the device.  Bushings The bushings consist of a current carrying element in the form of a conducting rod. 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. Explosion vent consists of large diameter steel pipe fitted on the transformer tank. Conservator is a small auxiliary oil tank that may be mounted above the transformer and connected to the main tank by a pipe. Thus sludge formation is avoided. It is usually at an angle to the vertical. When the oil in the tank contracts then the conservator gives the oil to maintain the oil level in the tank.  Breather and Explosion Vent Breather is connected to one side of the conservator. Its main function is to keep the main tank of the transformer completely filled with oil in all circumstances. The pipe has an elbow at its end. Up to 33kV ordinary porcelain insulators can be used.decreases with the load. 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. i. The relay is therefore particularly effective in case of: -circuited core laminations -down core bolt insulation The gas and oil actuated (Buchholz) relay is designed to detect faults as well to Overheating of some part of the windings -circuits between phases Page | 50 . windings split into various sections by using a number of connections brought out from a single winding.  Tappings 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. These are some figures of bushings. capacitors and reactors supplied with oil conservator.e.  Buchholz Relay minimize the propagation of any damage which might occur within oil-filled transformers. each one at a particular number of turns along the winding. conduction would not be possible.Bushing is very important to the overall transformer because without it. arising from defects in the oil circulating system or from other causes. thus operating an external alarm device. The tripping device is regulated in such a way that in transformers having forced oil cooling. which pass upwards to the conservator. 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 surges resulting from the starting of the oil circulating pump will not cause mal-operation of the relay. the upper float rotates on its hub and operates the alarm switch. operates the alarm float. thus operating first the alarm (upper) float and then the tripping (lower) float. thus causing its oil level to fall. the small bubbles of gas. or the ingress of air as a result of defects in the oil circulating system. The ingress of air into the transformer. such as the falling of the oil level owing to leaks. thus operating the tripping switch and disconnecting the transformer. 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. An oil leak in the transformer causes the oil level in the relay to fall. As a result. are trapped in the relay housing. OPERATION OF A BUCHHOLZ RELAY Slight faults: When a slight fault occurs in the transformer. the gas generation is violent and causes the oil to rush through the connecting pipe to the conservator.Furthermore the relay can prevent the development of conditions leading to a fault in the transformer. Page | 51 . Serious faults: When a serious fault occurs in the transformer. Shell type transformers Another recent development is the Spiral core or wound core type transformers. the transformers are of two types. Core type transformers 2.CLASSIFICATION OF TRANSFORMERS Constructionally. In the core type transformers. the core surrounds a considerable part of the windings. CORE TYPE TRANSFORMERS Page | 52 . the windings surround a considerable part of the core whereas in shell type transformers. The two types are known as 1. distinguished merely from each other by the manner in which the primary and secondary coils are placed round the laminated core. The general type transformers. The circular cylindrical coils are used in most of the core-type transformers because of their mechanical strength. Such from each other by paper. cloth. micarta board or cooling ducts. Cylindrical coils are wound in helical layers with the different layers insulated Insulating cylinders of fuller board are used to separate the cylindrical windings from the core and from each other. 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. Since the low voltage (LV) winding is easiest to insulate. SHELL TYPE TRANSFORMERS Page | 53 . it is placed nearest to the core. In small size corewhich are either circular or rectangular in form. a simple rectangular core is used with cylindrical coils form of these coils may be circular or oval or rectangular.The coils used are form-wound and are of the cylindrical type. The core is assembled of a continuous strip or ribbon of transformer steel wound in the form of a circular or elliptical cylinder. 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. Page | 54 . Transformers are generally housed in tightly-fitted sheet-metal. Good bracing reduces vibration and the objectionable noise–a humming sound–during operation. 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. but also provides the transformer with additional insulation not obtainable when the transformer is left in the air. tanks filled with special insulating oil. This oil has been highly developed and its function is two-fold. 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.The coils are form-would but are multi-layer disc type usually wound in the form of pancakes. The spiralcore transformer employs the newest in core development construction. Cold-rolled steel of high silicon content enables the designer to use considerably higher operating flux densities with lower loss per kg. By circulation. Such construction allows the core flux to follow the grain of the iron. The transformer core consists of laminations arranged in groups which radiate out from the centre. it not only keeps the coils reasonably cool. Good transformer oil should be absolutely free from alkalies. The presence of even an extremely small percentage of moisture in the oil is highly detrimental from the insulation the sides of the tank are corrugated or provided with radiators mounted on the viewpoint because it lowers the dielectric strength of the oil considerably. durability and handling of these materials. the transformers where complete air-tight construction is impossible.000. 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. Another thing to avoid in the oil is sledging which is simply the moisture is entrapped in these breathers and is not allowed to pass on to the decomposition of oil with long and continued use.000 reduces the insulating tanks are sealed air-tight in smaller units.In cases where a smooth tank surface does not provide sufficient cooling area. chambers known as breathers are provided to permit the oil inside the tank to expand and contract as its temperature increases or decreases. No other feature in the construction of a transformer is given more attention and care than the insulating materials. In the case of large-sized quality of the oil to a value generally recognized as below standard. sulphur and particularly from moisture. All Page | 55 . Hence. solely depends on the quality. because the life on the unit almost 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. sides. The atmospheric oil. 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. 923 ONAF – Oil Natural Air Forced Cooling ONAN – Oil Natural Air Natural Cooling Page | 56 .664 551.405 293.40 110 66 11 330.443 367.107 440.60 110 66 11 220.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 63 8.886 3 50 Hz 45°C 55°C 42 5.SPECIFICATION OF TRANSFORMERS The specification of the transformers installed at the Punnapra substation is as follows 63 MVA 110/66KV AUTO-TRANSFORMER 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 .16 MVA 110/33KV TRANSFORMER Manufacturer: Transformers and Electricals Kerala (TELK) Parameters When Installed When Installed with with ONAN ONAF No load voltage .98 279.49 174.93 Page | 57 .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) 110 33 52.56 % YNyn0 14000 7700 31200 8800 110 33 83.95 3 50 Hz 10. HV (kV) No load voltage .10 MVA 66/11KV TRANSFORMER Manufacturer: Transformers and Electricals Kerala (TELK) Parameters When installed with ONAN No load voltage .929 % YNyn0 10500 5960 25000 6700 Page | 58 .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) 66 11 87.6 525 3 50 Hz 9. e. Page | 59 . Actually relays and meters used for protection and metering.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. 5 Amp and 110V. potential transformer i. The system voltage is applied across the terminals of primary winding of that transformer. CT steps down rated system current to 1 Amp or 5 Amp similarly voltage transformer steps down system voltages to 110V. Primary of this transformer is connected across the phases or and ground depending upon the requirement. High currents or voltages of electrical power system cannot be directly fed to relays and meters. This is a simplest form of Potential Transformer Definition  Voltage Transformer or Potential Transformer Theory A Voltage Transformer theory or Potential Transformer theory is just like theory of general purpose step down transformer. are designed for low voltage. The relays and meters are generally designed for 1 Amp. used for stepping down purpose. are not designed for high currents and voltages. 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. and then proportionate secondary voltage appears across the secondary terminals of the PT. Commercially available relays and meters used for protection and metering. PT has lowers turns winding at its secondary. Just like the transformer. 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. Page | 60 . 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 secondary voltage of the PT is generally 110V. total primary current Ip is the vector sum of excitation current Page | 61 .Error in PT or Potential Transformer or VT or Voltage transformer Is – Secondary Current Es – Secondary induced emf Vs – Secondary terminal voltage Rs – Secondary winding resistance Xs – Secondary winding reactance Ip – Primary current Ep – Primary induced emf Vp – Primary terminal voltage Rp – Primary winding resistance Xp – Primary winding reactance KT – turns ratio = numbers of primary turns/number of secondary turns Io – Excitation Current Im – magnetizing component of Io Iw – core loss component of Io Φm – main flux β – phase angle error As in the case of Current Transformer and other purpose Electrical Power Transformer. m.Vs is the phase error  Cause of error in Potential Transformer Page | 62 . and resultant will actually appear across the burden terminals and it is denoted as Vs So if system voltage is Vp.f. Again this Es will be dropped by secondary winding resistance and reactance.and the electric current equal to reversal of secondary current multiplied by the ratio 1/KT Hence.m.  Voltage Error or Ratio Error in Potential Transformer (PT) The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio error in a potential transformer. This Ep is equal to primary induced e. it can be expressed as  Phase error or phase angle error in potential transformer The angle ′β′ between the primary system voltage Vp and the reversed secondary voltage vectors KT. This primary e.f is Es. but in reality actual secondary voltage of PT is Vs. Ip = Io + Is/KT 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.f will transform to the secondary winding by mutual induction and transformed e.m. After subtracting this voltage drop from Vp. ideally Vp/KT should be the secondary voltage of PT. Ep will appear across the primary terminals. 5/30 sec 2a. SPECIFICATION OF 110kV FEEDER PT High test system voltage Insulation level Oil Quantity Frequency Secondary winding number Output Accuracy class Primary terminal Voltage Factor Secondary terminal Voltage ratio 123 Kv 230/550Kv 180ltr 50Hz 1 Protective 200VA 3P A 1.2 continuous 1a.5/3P N 1. Then it appears across the primary winding and then transformed proportionally to its turns ratio.1n ( 11000/√3)/110 2 Measuring/Protective 200VA 0.2n ( 11000/√3)/110//√3 Page | 63 . 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.The voltage applied to the primary of the potential transformer first drops due to internal impedance of primary. to secondary winding. Normally accuracy of current transformer required up to 125% of rated current.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. So accuracy within working range is main criteria of a CT used for metering purpose. For metering and indication purpose. as because allowable system current must be below 125% of rated current. The degree of accuracy of a Metering CT is expressed by CT Accuracy Class or simply Current Transformer Class or CT Class. between primary and secondary currents are essential within normal working range. accuracy of ratio.  CT Accuracy Class or Current Transformer Class A CT is similar to a electrical power transformer to some extent. But in the case of protection. 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. the CT may not have the accuracy level as good as metering CT although it is desired not to be saturated during high fault Page | 64 . but there are some difference in construction and operation principle. primary current is the Page | 65 . but here is some difference. 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. In case of CT.25A because of saturation. One CT with current ratio 400/1A and its protection core is situated at 500A. 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. If actuating current of the relay connected the secondary circuit of the CT is 1. it will not be operated at all even fault level of the power circuit is 1000A. If saturation of the core comes at lower level of primary current the proper reflection of primary current will not come to secondary. primary current varies with load or secondary current. So core of protection CT is so designed that it would not be saturated for long range of currents.current passes through primary. hence relays connected to the secondary may not function properly and protection system losses its reliability.  Theory of Current Transformer or CT A CT functions with the same basic working principle of electrical power transformer. If a electrical power transformer or other general purpose transformer. as we discussed earlier.5A. In a power transformer. So from the above statement it is clear that if a CT has one turn in primary and 400 turns in secondary winding. there will be only magnetizing current flows in the primary. the primary is connected in series with power line. Generally CT has very few turns in primary where as secondary turns are large in number. if load is disconnected. hence secondary current or burden current depends upon primary current of the current transformer. The primary of the power transformer takes But in case of CT. In an ideal CT the primary AT is exactly is equal in magnitude to secondary AT. So current from the source proportional to the load connected with secondary.system current and this primary current or system current transforms to the CT secondary. 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 Page | 66 . 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 NsIs AT. 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. current through its primary is nothing but the current flows through that power line. Let us take flux as reference. The secondary current is now transferred to the primary side by reversing Is and multiplied by the turns ratio KT. Φ s. The magnitude of the passers Es and Ep are proportional to secondary and primary turns. EMF Es and Ep lags behind the flux by 90o. Is – Secondary Current Es – Secondary induced emf Ip – primary Current Ep – primary induced emf KT – turns ratio = numbers of secondary turns/number of primary turns Io – Excitation Current Im – magnetizing component of Io Iw – core loss component of Io Φm – main flux. Error in Current Transformer or CT In an actual CT. errors with which we are connected can best be considered. The total current flows through the primary Ip is then vector sum of KT Is and Io. The secondary current Io lags behind the secondary induced emf Es by an angle  The Current Error or Ratio Error in Current Transformer or CT Page | 67 . The excitation current Io which is made up of two components Im and Iw. This difference is due to the primary current is contributed by the core excitation called current error of CT or sometimes Ratio Error in Current Transformer. The angle between the above two phases in termed asPhase Angle Error in Current Transformer or CT. 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 error in current transformer introduced due to this difference is  Phase Error or Phase Angle Error in Current Transformer For a ideal CT the angle between the primary and reversed secondary current vector is zero.e. 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. Here in the pharos diagram it is β the phase angle error is usually expressed in minutes. KTIs.  Cause of error in current transformer The total primary current is not actually transformed in CT. i. current.  Reduction of error in current transformer Page | 68 .From above phasor diagram it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio. It is desirable to reduce these errors. one can follow the following. 3) Ensuring minimum length of flux path and increasing cross – sectional area 4) Lowering the secondary internal impedance. for better performance. 2) Keeping the rated burden to the nearer value of the actual burden. SPECIFICATION OF 110KV FEEDER CURRENT TRANSFORMER High test system voltage Insulation Level Frequency Oil Quantity Weight 123kV 230/550kV 50Hz 80ltr 500kg Core No Volt Ampere Accuracy Class ALF/ISF Current Ratio Short time current 1 Protective 80 5P 10 600-300/1 25kA/1sc 2 Measuring 30 1 600-300/1 - Primary Connection P1-C1 C2-P2 C2-C1 Secondary terminal 1S1-1S2 2S1-2S2 1S1-1S2 2S1-2S2 Current ratio 600/1 600/1 300/1 300/1 Page | 69 . 1) Using a core of high permeability and low hysteresis loss magnetic materials. For achieving minimum error in current transformer. minimizing joint of the core. of the core. In its most basic form. 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. Page | 70 . 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 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 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. I learned a lot about electrical substation system and the importance of substations in electrical generation. transmission and distribution. It has given us useful work and thus helps in efficient transmission of electricity. which we learnt about but never saw in the college labs. Also the training was an opportunity for me to increase my personal relations both socially and professionally. Punnapra as summer training was a very nice experience. We also studied 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. In the beginning of this visit the end I realised it was a very good experience which i would have regretted if I missed.CONCLUSION Working at the 110kV substation. I was not aware about the merits we were going to receive from the visit but at Page | 71 . This industrial visit provided an insight on how substations about different substation equipments in detail. Wikipedia ( 8. Switch gear and protection – Sunil S Rao 12. Maintenance Book (KSEB – Punnapra substation) 7.kseb. Safety manual of Kerala State Electricity Board 3. Website of KSEB (www.wikipedia. Electrical Technology Vol II – Theraja 9. Operator’s Diary (KSEB) 6. Daily report diary of Punnapra substation 4. Principles of power system – V K Mehta 11. Equipment register of Punnapra substation 5. Handbook of Kerala State Electricity Board 2.REFERENCE 1. Electrical power system – M Rajalingam 10. Electrical power system – Uppal Page | 72 . Thank you Page | 73 . 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