Hi this is a project on a 220 kV Grid substation in Sarusajai, Guwahati, Assam

Most of the documents are taken from different search engines and I am not responsible for any wrong information here. All the data are from the substation itself.

-Preetom Parasar

1. Introduction:1.1. About the substation:

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The substation in Sarusajai, Guwahati-781034, Assam was completed by the year 1986 under AEGCL; it is one of the largest power grids in the state of Assam and the north eastern India. This substation has the capacity of 220kv and can step down to 132kv using three input lines through the incoming feeders. The input feeders are namely:

AGIA- i; AGIA-ii; LONGPI- i; LONGPI- ii; SAMUGURI-i; SAMUGURI- ii.

All these feeders come into the substation with 220kv.

The substation has another substation under it. The capacity of this is 132kv/33kv. This substation was completed by the year 1997 under AEGCL. The purpose of this station was to step down the 132kv to direct distribution to the 33kv/11kv substations in six different areas of the state.

The substation of 132kv/33kv has six outgoing feeders, namely:

JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, and Kahilipara STATION. These out going feeders are of 33kv line.

The most important of any substation is the grounding of the instruments, transformers etc. used in the substation. For grounding of the substation a metallic square or some poly shaped metal boxes are placed in the ground. These ground the extra high voltage to the ground. As it is dangerous to us to go near the instrument without proper earth. If the instruments are not ground properly they may give a huge shock to anyone who would stay near it and also it is dangerous for the costly instrument as they may get damaged by this high voltage.

1.2. Construction – Site Selection & Layout

E HV S U B STATION

EHV Sub-Station forms an important link between Transmission network and Distribution network. It has a vital influence of reliability of service. Apart from ensuring efficient transmission and Distribution of power, the sub-station configuration should be such that it enables easy maintenance of equipment and minimum interruptions in power supply. Flexibility for future expansion in terms of number of circuits and transformer MVA Capacity also needs to be considered while choosing the actual configuration of the sub-station. EHV Sub-Station is constructed as near as possible to the load center. The voltage level of power transmission is decided on the quantum of power to be transmitted to the load center.

Generally, the relation between EHV Voltage level and the power to be transmitted is as follows: S.NO. POWER TO BE TRANSMITTED VOLTAGE LEVEL

1) Upto 150 MVA 132 KV.2) From 150 MVA to 300 MVA 220 K.V.3) 300 MVA to 1000 MVA 400 K.V.

1.3. SELECTION OF SITE

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Main points to be considered while selecting the site for EHV Sub-Station are as follows:

i) The site chosen should be as near to the load center as possible.

ii) It should be easily approachable by road or rail for transportation of equipments.

iii) Land should be fairly leveled to minimize development cost.

iv) 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.

v) The sub-station site should be as near to the town / city but should be clear of public places, aerodromes, and Military / police installations.

vi) The land should be 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 expansion.

vii) Set back distances from various roads such as National Highways, State Highways should be observed as per the regulations in force.

viii) While selecting the land for the substation preference to be given to the Govt. land over private land.

ix) The land should not have water logging problem.

x) The site should permit easy and safe approach to outlets for EHV lines.

1.4. Process of Land Acquisition

After the selection of site of the proposed EHV Substation and finalization of the area required, proceedings for acquisition of land have to be initiated. The acquisition of land generally takes quite a long time. Forecasting and planning of substation and selection of substation site needs to be done much in advance taking into account the normal period of acquisition of land. The acquision of land should not in any way disturb the commissioning of programme of sub-staion. In MSETCL a land acquisition is carried out by the concerned Civil wing. The proposal for acquisition of land is submitted to the District Collector in case of Govt. land and to the land

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acquisition officer in case of private land through the PWD Authorities accompanied by following documents:

(i) 7/12 abstracts along with the resolution of local authority.

(ii) Village map with suitable land duly marked on it.

(iii) Sales statistics around the area.

(iv) No Objection Certificate from the Forest Deptt. if applicable.

(v) The certificate in case of private agricultural land that the owner of the land does not become landless if his land is acquired.

(vi) These papers should be submitted after their due scrutiny. The land selected should be marked on the village map by taking joint measurements with Revenue Authorities.

(vii) Once the proposal is submitted to land acquisition officer further legal proceedings are completed by him, and the award is given followed by allocation of land to the utility.

Possessions of land are taken by taking joint measurements.

1.5. STORAGE OF EQUIPMENTS FOR THE SUB STATION:

All the substation equipments / materials received on site should be stored properly, either in the outdoor yard or in the stores shade depending on the storage requirement of that particular equipment.

The material received should be properly counted and checked for any damages / breakages etc. The storage procedure for main equipment is as follows:

i) EHV C.T.s and P.T.s Normally, 132 KV C.Ts. and P.Ts are packed and transported in wooden crates vertically while those of 220 KV and 400 KV are packed in iron structures for extra supports with cross beams to avoid lateral movement. 132 KV C.Ts. and P.Ts. should be stored vertically and those of 220 KV and 400 KV should be stored in horizontal position. C.Ts and P.Ts. packed in wooden crates should not be stored for longer period as the packing would may deteriorate. The wooden packages should be stored on a cement platform or on M.S. Channels to avoid faster deterioration of the wooden crates. C.Ts and P.Ts packed in iron cases stored in horizontal position should be placed on stable ground. No C.Ts and P.Ts. should be unpacked in horizontal position.

ii) L.A. s. and B.P.I. These are packed in sturdy wooden case as the porcelain portion is very fragile. Care should be taken while unpacking, handling and storage due to this reason.

iii) Batteries, Acid, Battery charger C & R panel, A.C.D.Bs copper piping, clamp connectors, hardwares etc. should be stored indoor.

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iv) Circuit breakers: The mechanism boxes of 33 KV – V.C.Bs should be stored on raised ground and properly covered with tarpaulins or should be stored in door. The interrupter chambers should be stored on raised ground to avoid rain water in storage area.

v) E.H.V. C.B. Now-a-days SF6 circuit breaker are used at EHV rottages. The control and operating cabinets are covered in polythene bags and are packed in wooden and iron crates. These should be stored on raised ground and should be covered with tarpaulins. The arcing chambers and support insulators are packed in iron crates and transported horizontally. The +ve pressure of SF6 gas is maintained in these arcing chambers to avoid the ingress of moisture. It should be ensured that this pressure is maintained during the storage. Other accessories like pr. Switches, density monitor, Air Piping, control cables, wiring materials, SF6 gas pipes; SF6 cylinder should be stored in store shed.

vi) Power transformers: The main Tank - The transformer is transported on trailor to substation site and as far as possible directly unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally oil filled, and those of higher capacity are transported with N2 gas filled in them +ve pressure of N2 is maintained in transformer tank to avoid the ingress of moisture. This pressure should be maintained during storage; if necessary by filling N2 Bushings - generally transported in wooden cases in horizontal position and should be stored in that position. There being more of Fragile material, care should be taken while handling them. Rediators – These should be stored with ends duly blanked with gaskets and end plates to avoid ingross of moisture, dust, and any foreign materials inside. The care should be taken to protect the fins of radiators while unloading and storage to avoid further oil leakages. The radiators should be stored on raised ground keeping the fins intact. Oil Piping. The Oil piping should also be blanked at the ends with gasket and blanking plates to avoid ingross of moisture, dust, and foreign

All other accessories like temperature meters, oil flow indicators, PRVs, buchholtz relay; oil surge relays; gasket ‘ O ‘ rings etc. should be properly packed and stored indoor in store shed. Oil is received in sealed oil barrels . The oil barrels should be stored in horizontal position with the lids on either side in horizontal position to maintain oil pressure on them from inside and subsequently avoiding moisture and water ingress into oil. The transformers are received on site with loose accessories hence the materials should be checked as per bills of materials.

1.6. SUBSTATION STRUCTURES: It should be properly checked as per bills of materials of stacked and stored outside.

1.7. THE FIRE PROTECTION: The fire protection device should be kept in store yard for

safety of equipments during storage. 1.8. INSURANCE – Transport of equipments from one place to other is done, where they are

actually erected. During transport, erection and commissioning the equipments may get damaged resulting into loss, hence the equipments should be insured against storage, transportations, erections, testing and commissioning as the paying authorities feels suitable and can be insured with ( Govt. Insurance Fund / Insurance Authority ) the cost of equipments and the period of insurance should be mentioned.

1.9. Earthing design

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Guiding standards – IEEE 80, IS: 3043, CBIP-223. 400kV & 220kV system are designed for 40kA. Basic Objectives: Step potential within tolerable Touch Potential limit Ground Resistance Adequacy of Ground conductor for fault current (considering corrosion)

1.10. Various outdoor equipments used are as follows:

1.10.1.L.A. - To discharge the switching and lightening voltage surges to earth. Coupling capacitor with line matching units – These are high pass Filters ( carrier frequency 50 KHZ to 500 KHZ ) pass carrier. Frequency to carrier panels and power frequency parameters to switch yard.

1.10.2.Wave Traps. - Low pass filter when power frequency currents are passed to switch yard and high frequency signals are blocked. Line Isolator with E.B. – To isolate the line from Sub Station and earth, it under shut down.

1.10.3.Current transformers. – These are used for i) Measurement of current ii) Protection current circuits when currents are passed through protective relays.

1.10.4.Potential transformers – A) Measurement of voltage. B) Provide secondary voltage for protection purposes and measurements.

1.10.5.Isolators ( w.o. EB ) without earth blade. – To isolate the bay from the Bus.

1.10.6.Circuit Breakers - These are used to operate on the Fault either on line or transformer, depending upon where it is connected. This isolates the Faulty line or equipments from the live portion of the Sub Station by opening automatically through protective relays; control cables etc. in a definite time.

1.10.7.Battery sets – to provide adequate D.C. supply voltage for operation of protection system and circuit breakers. When A.C. supply fails as an emergency stand by. Battery Charges to provide appropriate D.C. Voltage for operation of protective systems and circuit brakes. To keep the battery set continuously is charged condition as it gets discharged to certain extent due to internal resistance of batteries.

1.11. CONTROL AND RELAY PARTS - These are used to control the operations of breakers, isolates, through protective relays installed on these panels various protection schemes for transformers, lines etc, are provided on these panels. AC & DC DB’S – These are used for extending A.C. & D.C. supplies whenever required through various circuits.

There are two main Buses in this arrangement connected by each diameter.

i) Through either of line breakers the line side Main Bus can be charged normally (Bus-I).

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ii) The line breaker, tie breaker and IInd Bus breaker if closed in series will charge the IInd Main Bus.

iii) Outage on anyone Bus can be availed without interruption on any Bus. The second Bus can feed all the loads.

iv) Breaker from any bay can be taken out for maintenance without interrupting the supply.

v) For efficient working two diameters are required having source in each diameter preferably connected diagonally opposite to two different buses.

vi) If both the sources are connected to same Bus (i.e. from one side only one tie breaker can be attended at a time).

vii) If all the four breakers connected to Bus are out the transformer can be charged through the breaker from remote substation source.

viii) Changing over as in case of 2 Bus or 3 Bus systems is not necessary as supply is not interrupted, in any case as said above.

ix) All the breakers in the diameters are in energized position including tie breakers to keep the system in tact in case of any fault.

x) On line or transformer fault the tie breaker with respective line or transformer breaker will trip.

xi) On Bus fault on any Bus only the two breakers (of two diameters) connected Bus will Trip.

xii) The Teed-point remains unprotected in any of line or transformer or bus faults hence the Teed point protection is given by differential relay. In case of this protection the breakers (2 Nos.) connected to Teed point (tie breaker + Bus breaker) will Trip.

1.12. CLEARANCES

Distinction should make between electrical clearances; necessary to ensure satisfactory performance in service and safety clearances which are required for safety of personnel in inspection; operation and maintenance work. Electrical clearances - This is minimum distance required between live parts and earth materials (earth clearance) or between live parts of different potentials (phase clearances) in order to prevent flashovers. Safety clearances to the conductor - (Live Part) – Minimum distance required but live conductor and the limits of work section for safety to personnel working.

1.13. Ground Clearances –

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The minimum distance required between any exposed insulator which supports or contains live conductor, and limits of work section where safety of personnel is ensured. Work Section - The space where the person may work safely provided he remains with in that space. 132 KV Single – Bus system

1) Minimum clearance to earth on air for : 132 KV level – 1070 mm 22 KV level – 280 mm 33 KV level – 380 mm 220 KV level – 1780 mm 400 KV level – 3500 mm

2) Minimum clearance between phases in air : 22 KV level – 330 mm 33 KV level – 430 mm 132 KV level – 1220 mm 220 KV level – 2060 mm 400 KV level – 4000 mm Sectional clearances : 22 KV level – 2745 mm 33 KV level - 2770 mm 132 KV level – 3505 mm 220 KV level – 4280 mm 400 KV level – 6500 mm

3) This is Minimum clearance from any point where a man may be required to stand to the nearest live conductor in air.

4) Ground clearance. 33 KV – 3.7 meters. 132 KV – 4.6 meters. 220 KV – 5.5 meters. 400 KV - 8.0 meters. 132 KV SYSTEM: Height of Bus from ground – 8 meters, Bay width - Single Bus – 11 meters. Double Bus – 12 meters. Distance of Earth wire from ground – 10.5 meters. Ph to Ph Distance – 3 meters. Between equipments At right angle to Bus – 3 meters

For 220 Kv System – (Single Bus) i) Height and Bus forms ground – 12.5 meter

ii) Width to Bay – 18 Meters.

iii) Distance bet formulations – 4.5 meters.

iv) Distance bet equipments – 4.5 meters. ( meter) ( Right angle to Bus)

v) Height of Earth wire from ground 15.5 meters 220 KV/ Bus two / three.

vi) Height of 1st Steels Bus above ground – 18.5 meters.

vii) Height of IInd Bus – 25 meters. From ground.

viii) Height of Earth wire – above ground – 28.5 meters. 400 KV Height of Main Bus from ground –15.6 meters. Height of stab Bus from ground – 22 meters. Height of Earth conductor – 30 meters above ground Bay width – 27 meters. Distance bet Equipments (Right angle to Bus) - > 6 Meters. Distance bet. Phases 7 meters.

2. Single line diagram (SLD) A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the concerned Electrical System which includes all the required ELECTRICAL EQUIPMENT connection

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sequence wise from the point of entrance of Power up to the end of the scope of the mentioned Work.

As in the case of 132KV Substation, the SLD shall show Lightening Arrestor, State Electricity Board's C.T/P.T Unit, Isolators, Protection and Metering P.T & C.T. Circuit Breakers, again Isolators and circuit Breakers, Main Power Transformer, all protective devices/relays and other special equipment like NGR, CVT, GUARD RINGS, SDR etc as per design criteria.

2.1. Fig: Single line diagram of substation.

As these feeders enter the station they are to pass through various instruments. The instruments have their usual functioning. They are as follows in the single line diagram.

1. Lightening arrestors,2. C V T3. Wave trap4. Current transformer5. Isolators with earth switch6. Circuit breaker7. Line isolator8. BUS9. Potential transformer in the bus with a bus isolator10. Isolator11. Current transformer12. Circuit breaker13. Lightening arrestors14. Transformer15. Lightening arrestors with earth switch

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16. Circuit breaker17. Current transformer18. Isolator19. Bus20. Potential transformer with a bus isolator21. A capacitor bank attached to the bus.

2.2. Brief descriptions of the instruments in the line diagram are-

1. Lightening arrestors

Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation the main important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a cone.

2. C V T

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, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts 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.

3. Wave trap

Wave trap is an instrument using for tripping of the wave. The function of this trap is that it traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected to the main incoming feeder so that it can trap the waves which may be dangerous to the instruments here in the substation.

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4. Current transformer

Current transformers are basically used to take the readings of the currents entering the substation. This transformer steps down the current from 800 amps to 1 amp. This is done because we have no instrument for measuring of such a large current. The main use of this transformer is (a) distance protection; (b) backup protection; (c) measurement.

5. Lightening arrestors with earth switch

Lightening arrestors after the current transformer are used so as to protect it from lightening i.e. from high voltage entering into it. This lightening arrestor has an earth switch, which can directly earth the lightening. The arrestor works at 30° to 45° angel of the lightening making a cone. The earth switch can be operated manually, by pulling the switch towards ground. This also helps in breaking the line entering the station. By doing so maintenance and repair of any instrument can b performed.

6. Circuit breaker

The circuit breakers are used to break the circuit if any fault occurs in any of the instrument. These circuit breaker breaks for a fault which can damage other instrument in the station. For any unwanted fault over the station we need to break the line current. This is only done automatically by the circuit breaker. There are mainly two types of circuit breakers used for any substations. They are (a) SF6 circuit breakers; (b) spring circuit breakers.

The use of SF6 circuit breaker is mainly in the substations which are having high input kv input, say above 220kv and more. The gas is put inside the circuit breaker by force ie under high pressure. When if the gas gets decreases there is a motor connected to the circuit breaker. The motor starts operating if the gas went lower than 20.8 bar. There is a meter connected to the breaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gas to reduce the torque produce in it due to any fault in the line. The circuit breaker has a direct link with the instruments in the station, when any fault occur alarm bell rings.

The spring type of circuit breakers is used for small kv stations. The spring here reduces the torque produced so that the breaker can function again. The spring type is used for step down side of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution side.

7. Line isolator

The line isolators are used to isolate the high voltage from flow through the line into the bus. This isolator prevents the instruments to get damaged. It also allows the only needed voltage and rest is earthed by itself.

8. BUS

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The bus is a line in which the incoming feeders come into and get into the instruments for further step up or step down. The first bus is used for putting the incoming feeders in la single line. There may be double line in the bus so that if any fault occurs in the one the other can still have the current and the supply will not stop. The two lines in the bus are separated by a little distance by a conductor having a connector between them. This is so that one can work at a time and the other works only if the first is having any fault.

9. Potential transformers with bus isolators

There are two potential transformers used in the bus connected both side of the bus. The potential transformer uses a bus isolator to protect itself. The main use of this transformer is to measure the voltage through the bus. This is done so as to get the detail information of the voltage passing through the bus to the instrument. There are two main parts in it (a) measurement; (b) protection.

10. Isolators

The use of this isolator is to protect the transformer and the other instrument in the line. The isolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line. Thus an isolator is used after the bus also for protection.

11. Current transformer

Current transformers are used after the bus for measurement of the current going out through the feeder and also for protection of the instruments.

12. Circuit breaker

The circuit breakers are used to break the circuit if any fault occurs in the circuit of the any feeders.

13. Lightening arrestors

The use of lightening arrestors after the bus is to protect the instrument in the station so that lightening would not affect the instruments in the station.

14. Transformer

There are three transformers in the incoming feeders so that the three lines are step down at the same time. In case of a 220kv or more kv line station auto transformers are used. While in case of lower kv line such as less than 132kv line double winding transformers are used.

15. Lightening arrestors with earth switch

The lightening arrestors are used with earth switch so that lightening would not pass through the instruments in the station.

16. Circuit breaker

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The circuit breakers are used to break the circuit for any fault.

17. Current transformer

Current transformers are used to measure the current passing through the transformer. Its main use is of protection and measurement.

18. Isolator

These are used to ground the extra voltage to the ground.

19. Bus

This bus is to carry the output stepped down voltage to the required place.

20. Potential transformer with a bus isolator

Two PT are always connected across the bus so that the voltage across the bus could be measured.

21. Capacitor bank attached to the bus.

The capacitor banks are used across the bus so that the voltage does not gets down till the require place.

3. The line diagram of the substation:

The Sarusajai 220kv/ 132kv substation, Guwahati, has two stations in it. Firstly the 220kV and next the 132kV/ 33kV substation.

The 220kV substation has the capacity of 220kv and can step down to 132kv using three input lines through the incoming feeders. The input feeders are namely:

AGIA- i; AGIA-ii; LONGPI- i; LONGPI- ii; SAMUGURI-i; SAMUGURI- ii.

All these feeders come into the substation with 220kv.

The purpose of the 132kV/ 33kV substation was to step down the 132kv to direct distribution to the 33kv/11kv substations in six different areas of the state.

The substation of 132kv/33kv has six outgoing feeders, namely:

JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, and Kahilipara STATION. These out going feeders are of 33kv line.

The complete line diagram of the station are shown in the figure below:

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Fig: Line diagram of the Sarusajai substation, Guwahati, Assam.

4. Instruments used in the Sarusajai, 220kV grid substation are:

Lightening arrestors:

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Firstly we can see lightening arresters. These lightening arrestors can resist or ground the lightening if falls on the incoming feeders. The lightening arrestors can work in a angle of 30 degrees around them. They are mostly used for protection of the instruments used in the substation. As the cost of the instrument in the station are very high to protect them from high voltage from lightening these lightening arrestors are used.

Fig. lightening arrestor.

It is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal oxide varistors (MOVs) have been used for power system protection since the mid 1970s. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth.

Landscape suited for purpose of explanation: (1) Represents Lord Kelvin's "reduced" area of the region, (2) Surface concentric with the Earth such that the quantities stored over it and under it are equal; (3) Building on a site of excessive electrostatic charge density; (4) Building on a site of low electrostatic charge density.

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In telegraphy and telephony, a lightning arrester is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in a power and communications systems and the Earth. These provide a short circuit to the ground that is interrupted by a non-conductor, over which lightning jumps. Its purpose is to limit the rise in voltage when a communications or power line is struck by lightning.

The non-conducting material may consist of a semi-conducting material such as silicon carbide or zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simply open to the air, but more modern varieties are filled with dry gas and have a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or myriad voltage-activated solid-state switches called varistors or MOVs. Lightning arresters built for substation use are impressive devices, consisting of a porcelain tube several feet long and several inches in diameter, filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder.

Electric power system lightning protection

High-tension power lines carry a lighter conductor (sometimes called a 'pilot' or 'shield') wire over the main power conductors. This conductor is grounded at various points along the link, or insulated from the tower structures by small insulators that are easily jumped by lightning voltages. The latter allows the pilot wire to be used for communications purposes, or to carry current for aircraft clearance lights. Electrical substations may have a web of grounded wires covering the whole plant.

Lightning protection system design

Considerable material is used to make up lightning protection systems, so it is prudent to consider carefully where a rod structure will have the greatest effect. Historical understanding of lightning, from statements made by Ben Franklin, assumed that each device protected a cone of 45 degrees. This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building. A better technique to determine the effect of a new arrester is called the "rolling sphere technique" and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning 'moves'. As the step leader of a lightning bolt jumps toward the ground, it steps toward the grounded objects nearest its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere's radius as 46 m near the ground.

Electricity travels mostly along the path of least resistance, so an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object solidly OR within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a sphere that travels from the cloud to the ground. For lightning protection, it suffices to consider all possible spheres as they touch potential strike points. To determine strike points, consider a sphere rolling over the terrain. At each point, we are simulating a potential leader position. Lightning is most likely to strike where the sphere touches the ground. Points that the sphere cannot roll across and touch are safest from lightning. Lightning protectors should be placed where they will prevent the sphere from touching a

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structure. A weak point in most lightning diversion systems is in transporting the captured discharge from the lightning rod to the ground, though. Lightning rods are typically installed around the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 6.1 m or 7.6 m, depending on the height of the rod. When a flat roof has dimensions greater than 15 m by 15 m, additional air terminals will be installed in the middle of the roof at intervals of 15 m or less in a rectangular grid pattern.

Evaluations and analysis

A controversy over the assortment of operation theories dates back to the 1700s, when Franklin himself stated that his lightning protectors protected buildings by dissipating electric charge. He later retracted the statement, stating that the device's exact mode of operation was something of a mystery at that point. Diversion is a misnomer; no modern systems are claimed to divert anything, but rather to intercept the charge that terminates on a structure and carry it to the ground. The energy in a lightning strike is measured in Joules. The reason that lightning does damage is that this energy is released in a matter of microseconds (typically 30 to 50 microseconds). If the same energy could be released slowly over a period of many seconds or minutes, the current flow would be in milliamperes or a few amperes at most. This is the intent of charge dissipation.

The dissipation theory states that a lightning strike to a structure can be prevented by altering the electrical potential between the structure and the thundercloud. This is done by transferring electric charge (such as from the nearby Earth to the sky or vice versa). Transferring electric charge from the Earth to the sky is done by erecting some sort of tower equipped with one or more sharply pointed protectors upon the structure. It is noted that sharply pointed objects will indeed transfer charge to the surrounding atmosphere and that a considerable electric current through the tower can be measured when thunderclouds are overhead.

Lightning strikes to a metallic structure can vary from leaving no evidence excepting perhaps a small pit in the metal to the complete destruction of the structure. When there is no evidence, analyzing the strikes is difficult. This means that a strike on an uninstrumented structure must be visually confirmed, and the random behavior of lightning renders such observations difficult. The research situation is improving somewhat, however. There are also inventors working on this problem, such as through a lightning rocket. While controlled experiments may be off in the future, very good data is being obtained through techniques which use radio receivers that watch for the characteristic electrical 'signature' of lightning strikes using fixed directional antennas. Through accurate timing and triangulation techniques, lightning strikes can be located with great precision, so strikes on specific objects often can be confirmed with confidence.

The introduction of lightning protection systems into standards allowed various manufactures to develop protector systems to a multitude of specifications and there are various lightning rod standards. The NFPA's independent third party panel found that "the [Early Streamer Emission] lightning protection technology appears to be technically sound" and that there was an "adequate theoretical basis for the [Early Streamer Emission] air terminal concept and design from a physical viewpoint". (Bryan, 1999) The same panel also concluded that "the recommended [NFPA 780 standard] lightning protection system has never been scientifically or technically validated and the Franklin rod air terminals have not been validated in field tests under thunderstorm conditions." In response, the American Geophysical Union concluded that "[t]he Bryan Panel reviewed essentially none of the studies and literature on the effectiveness and scientific basis of traditional lightning protection systems and was erroneous in its conclusion

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that there was no basis for the Standard." AGU did not attempt to assess the effectiveness of any proposed modifications to traditional systems in its report.

No major standards body, such as the NFPA or UL, has currently endorsed a device that can prevent or reduce lightning strikes. The NFPA Standards Council, following a request for a project to address Dissipation Array Systems and Charge Transfer Systems, denied the request to begin forming standards on such technology (though the Council did not foreclose on future standards development after reliable sources demonstrating the validity of the basic technology and science were submitted). Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating their opposition to dissipater technology.

Various investigators believe the natural downward lightning strokes to be unpreventable. Since most lightning protectors' ground potentials are elevated, the path distance from the source to the elevated ground point will be shorter, creating a stronger field (measured in volts per unit distance) and that structure will be more prone to ionization and breakdown. Scientists from the National Lightning Safety Institute claim that these dissipation devices are nothing more than expensive lightning protectors and that they, unlike traditional methods, are not based on "scientifically proven and indisputable technical arguments". William Rison states that in his opinion the underlying theory of dissipation is "scientific nonsense". According to these sources, there is no proof that the dissipation arrangement is at all effective. According to opponents of the dissipation technology, the various designs of dissipaters indirectly "eliminate" lightning via the alteration of a building's shape and only have a small effect (either intended or not) because there is no significant reduction to the susceptibility of a structure to the generation of upward lightning strokes. Some field investigations of dissipaters show that their performance is comparable to conventional terminals and possess no great enhancement of protection. According to these field studies, these devices have not shown that they totally eliminated lightning strikes.

CVT :

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, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts 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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CVT 220 kV rating

Type: WP-245 V

Operating voltage: 220/√3 kV

Voltage factor: 1.5 V for 30 sec.

Test voltage: 460 kV

Test impedance 1050 kv peak

Ellec cap: 4400±10% PF of 50 Hz

± 5%

Nominal intermediate voltage 20/√3 kv

Spark over voltage: 36 kv

Voltage divider ratio 220000/√3 /20000/√3

Total thermal burden: 1000 VA

Temperature categ: 10 to 55°C

Total weight: 900 Kg.

Wave tape:

A device used to exclude unwanted frequency components, such as noise or other interference, of a wave. A device used to exclude unwanted frequency components, such as noise or other interference, of a wave. Wave trap is an instrument using for tripping of the wave. The function of this trap is that it traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected to the main incoming feeder so that it can trap the waves which may be dangerous to the instruments here in the substation.

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Current transformer:

The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer. This is done by constructing the secondary coil consisting of many turns of wire, around the primary coil, which contains only a few turns of wire. In this manner, measurements of high values of current can be obtained. A current transformer should always be short-circuited when not connected to an external load. Because the magnetic circuit of a current transformer is designed for low magnetizing current when under load, this large increase in magnetizing current will build up a large flux in the magnetic circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load.

These transformers are basically used to get the incoming current on the incoming feeders. It steps down the incoming 800 amps to 1 amps.

Rating factor:

Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute maximum measurable primary current. Conversely, the minimum primary current a CT can accurately measure is "light load," or 10% of the nominal current (there are, however, special CTs designed to measure accurately currents as small as 2% of the nominal current). The rating factor of a CT is largely dependent upon ambient temperature. Most CTs have rating factors for 35 degrees Celsius and 55 degrees Celsius. It is important to be mindful of ambient temperatures and resultant rating factors when CTs are installed inside pad-mounted transformers or poorly ventilated mechanical rooms. Recently, manufacturers have been moving towards lower nominal primary currents with greater rating factors. This is made possible by the development of more efficient ferrites and their corresponding

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hysteresis curves. This is a distinct advantage over previous CTs because it increases their range of accuracy, since the CTs are most accurate between their rated current and rating factor

Current transformer

Type 132 kV CT

Core 1 core 2 core 3

Ratio (A/A) 800/1 400/1 800/1 400/1 800/1 400/1

Sec. Conn: 1S1-1S2 2S1-2S3 3S1-3S3

Accuracy class: 0.2 5P 10 PS

Burden (VA): 30 15 NA

Highest system

Voltage: 145 kV insulation burn 275 kV/ 65014 Vp

Isolator with earth switch (ES):

The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer. This is done by constructing the secondary coil consisting of many turns of wire, around the primary coil, which contains only a few turns of wire. In this manner, measurements of high values of current can be obtained. A current transformer should always be short-circuited when not connected to an external load. Because the magnetic circuit of a current transformer is designed for low magnetizing current when under load, this large increase in magnetizing current will build up a large flux in the magnetic circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load.

The main use of using the earth switch (E/S) is to ground the extra voltage which may b dangerous for any of the instrument in the substation.

Isolator ratings

Voltage rating: 145 kV

Basic insulation level: 650 kVp

Current rating: 1250 Amp.

Circuit breaker: using SF6 gas:Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishing

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properties. The dielectric strength of the gas increases with pressure and is more than of dielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in electrical equipment like high voltage metal enclosed cables; high voltage metal clad switchgear, capacitors, circuit breakers, current transformers, bushings, etc. The gas is liquefied at certain low temperature, liquefaction temperature increases with pressure.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10, SF4 and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas is low if manufactured in large scale.

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During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation. As a result, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during the current zero and the arc is extinguished. Due to its electronegativity, and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength is very high and the time constant is very small.

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24

Fig: SF6 circuit breaker.

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Gas circuit breaker: high voltage side

Type 220-SFM-20B

Voltage rating: 220kv

Rated lightening impulse withstand voltage: 1050 kVp

Rated short circuit breaker current: 40 kV

Rated operating pressure: 16.5 kg/ cm2g

First pole to clear factor 1.3

Rated duration of short circuit current is 40 kA for 30 sec.

Rated ling charging breaker breaking current 125 Amp

Rated voltage 245 kV

Rated frequency 50 Hz

Rated normal current 1600 Amp

Rated closing voltage: 220 V dc

Rated opening voltage 220 V dc

Main parts:

(a) Power circuit(b) Control circuit

Gas circuit breaker: low voltage side

Type 120-SFM-32A

Voltage rating: 220kv

Rated lightening impulse withstand voltage: 650 kVp

Rated short circuit breaker current: 31.5 kV

Rated operating pressure: 15.5 kg/ cm2g

First pole to clear factor 1.5

Rated duration of short circuit current is 31.5 kA for 30 sec.

Rated ling charging breaker breaking current 50 Amp

Rated voltage 245 kV

Rated frequency 50 Hz

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Rated normal current 1250 Amp

Rated closing voltage: 220 V dc

Rated opening voltage 200 V dc

Main parts:

(a) Power circuit(b) Control circuit

220kv BUS:

It is a incoming 220kv feeder BUS from which the line is taken to the transformer for further step down.

Double main bus & transfer bus systemMerits 1. Most flexible in operation 2. Highly reliable 3. Breaker failure on bus side breaker removes only one ckt. From service 4. All switching done with breakers 5. Simple operation, no isolator switching required 6. Either main bus can be taken out of service at any time for maintenance. 7. Bus fault does not remove any feeder from the service Demerits 1. High cost due to three buses Remarks 1. Preferred by some utilities for 400kV and 220kV important substations.

Mesh (Ring) busbar systemMerits 1. Busbars gave some operational flexibility Demerits 1. If fault occurs during bus maintenance, ring gets separated into two sections. 2.Auto-reclosing and protection complex. 3. Requires VT’s on all circuits because there is no definite voltage reference point. These VT’s may be required in all cases for synchronizing live line or voltage indication 4. Breaker failure during fault on one circuit causes loss of additional circuit because of breaker failure. Remarks 1. Most widely used for very large power stations having large no. of incoming and outgoing lines and high power transfer.

Potential transformers: with BUS isolator

The instrument potential transformer (PT) steps down voltage of a circuit to a low value that can be effectively and safely used for operation of instruments such as ammeters, voltmeters, watt meters, and relays used for various protective purposes.

There are two potential transformers used in the bus connected both side of the bus. The potential transformer uses a bus isolator to protect itself. The main use of this transformer is to measure the voltage through the bus. This is done so as to get the detail information of the voltage passing through the bus to the instrument. There are two main parts in it (a) measurement; (b) protection.

Potential transformer ratings:

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High voltage side: 245 V

Rated insulation voltage: 395/ 900

Voltage rating: 220/√3 kV/ 110/ √3 V

BUS Isolator:

These isolators are used to isolate the incoming high voltage or the high incoming current from the incoming feeder which enters the bus. The isolator prevents damage to the instruments by just isolating the line current or the voltage.

Current transformer:

The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer. This is done by constructing the secondary coil consisting of many turns of wire, around the primary coil, which contains only a few turns of wire. In this manner, measurements of high values of current can be obtained. A current transformer should always be short-circuited when not connected to an external load. Because the magnetic circuit of a current transformer is designed for low magnetizing current when under load, this large increase in magnetizing current will build up a large flux in the magnetic circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load.

Circuit breaker using SF6 gas:

Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishing properties. The dielectric strength of the gas increases with pressure and is more than of dielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in electrical equipment like high voltage metal enclosed cables; high voltage metal clad switchgear, capacitors, circuit breakers, current transformers, bushings, etc. The gas is liquefied at certain low temperature, liquefaction temperature increases with pressure.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10, SF4 and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas is low if manufactured in large scale.

During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation. As a result, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during the current zero and the arc is extinguished. Due to its electronegativity, and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength is very high and the time constant is very small.

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Lightening arrestor:

These lightening arrestors are used to prevent the lightening from damaging the instruments in the substation.

Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation the main important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a cone.

Auto transformer:

Transformer is static equipment which converts electrical energy from one voltage to another. As the system voltage goes up, the techniques to be used for the Design, Construction, Installation, Operation and Maintenance also become more and more critical.

If proper care is exercised in the installation, maintenance and condition monitoring of the transformer, it can give the user trouble free service throughout the expected life of equipment which of the order of 25-35 years. Hence, it is very essential that the personnel associated with the installation, operation or maintenance of the transformer is through with the instructions provided by the manufacture.

It is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils. Except for air-core transformers, the conductors are commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the first or "primary" winding creates a varying magnetic field in the core (or cores) of the transformer. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the "secondary" winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will flow from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary to the number of turns in the primary as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS

less than NP.

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Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.

Pole-mounted single-phase transformer with center-tapped secondary. Note use of the grounded conductor as one leg of the primary feeder.

An auto transformer 220kv/132kv, in Sub Station, AEGCL, Sarusaji, Guwahati

Basic principles

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The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

An ideal transformer.

An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both primary and secondary coils.

Induction law

The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that:

where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation

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of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage

Ideal power equation

The ideal transformer as a circuit element

If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power.

Pincoming = IPVP = Poutgoing = ISVS

giving the ideal transformer equation

Transformers are efficient so this formula is a reasonable approximation.

If the voltage is increased, then the current is decreased by the same factor. The impedance in one circuit is transformed by the square of the turns ratio. For example, if an impedance ZS is

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attached across the terminals of the secondary coil, it appears to the primary circuit to have an

impedance of . This relationship is reciprocal, so that the impedance ZP of the

primary circuit appears to the secondary to be .

Detailed operation

The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core. The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.

The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF".This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

Transformer rating

Type of cooling: capacity:

OFAF ONAF ONAN

100mV 80mV 60mV

No load voltage: 220kv/ 132kv

Line current: HV 262.43 Amp. 209.94 Amp. 157.46 Amp.

LV 437.38 Amps 349.91 Amp. 262.43 Amp.

Impedence voltage: 12.5V

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1u

2u 3u

3w

2w 2v 1v

1w

3v

Vector symbol: YNad1

Lightening arrestors:

Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall on the station. If some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation the main important is of protection which is firstly done by these lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a cone

Current transformers:

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Current transformers are basically used to take the readings of the currents entering the substation. This transformer steps down the current from 800 amps to 1 amp. This is done because we have no instrument for measuring of such a large current. The main use of this transformer is (a) distance protection; (b) backup protection; (c) measurement.

Current transformer LV side

Type 132 kV CT

Core 1 core 2 core 3

Ratio (A/A) 800/1 400/1 800/1 400/1 800/1 400/1

Sec. Conn: 1S1-1S2 2S1-2S3 3S1-3S3

Accuracy class: 0.2 5P 10 PS

Burden (VA): 30 15 NA

Highest system

Voltage: 145 kV insulation burn 275 kV/ 65014 Vp

Isolator:

The line isolators are used to isolate the high voltage from flow through the line into the bus. This isolator prevents the instruments to get damaged. It also allows the only needed voltage and rest is earthed by itself.

Circuit breaker:

The circuit breakers are used to break the circuit if any fault occurs in any of the instrument. These circuit breaker breaks for a fault which can damage other instrument in the station. For any unwanted fault over the station we need to break the line current. This is only done automatically by the circuit breaker.

132kv BUS:

This bus is to carry the output stepped down voltage to the required place.

Potential transformer: 2 with bus isolator

Two PT are always connected across the bus so that the voltage across the bus could be measured.

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BUS isolator:

This are used for the protection of the instruments.

Current transformer:

Current transformers are used to measure the current passing through the transformer. Its main use is of protection and measurement.

Lightening arrestors:

The use of lightening arrestors after the bus is to protect the instrument in the station so that lightening would not affect the instruments in the station.

Transformer: with two windings

The simplified description above neglects several practical factors, in particular the primary current required to establish a magnetic field in the core, and the contribution to the field due to current in the secondary circuit.

Models of an ideal transformer typically assume a core of negligible reluctance with two windings of zero resistance. When a voltage is applied to the primary winding, a small current flows, driving flux around the magnetic circuit of the core. The current required to create the flux is termed the magnetizing current; since the ideal core has been assumed to have near-zero reluctance, the magnetizing current is negligible, although still required to create the magnetic field.

The changing magnetic field induces an electromotive force (EMF) across each winding. Since the ideal windings have no impedance, they have no associated voltage drop, and so the voltages VP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs. The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF".This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

Lightening arrestors:

The use of lightening arrestors after the bus is to protect the instrument in the station so that lightening would not affect the instruments in the station.

Current transformer:

These transformers are used for measurements and protections.

Circuit breaker:

The use of the circuit breaker again is to break the circuit if there is any fault in the line.

Isolator:

Isolator ground the extra voltage to the earth.

33kv BUS:36

This bus is used for the 33kV line. This bus carries 33kV voltage.

Potential transformer on the bus:

The potential transformer is used in the bus only. This is because to measure the voltage in the bus. The use of the potential transformer is to measure and to protect the instruments.

BUS isolator:

The bus isolator is used to isolate the extra high voltage through the bus.

Current transformer:

The use of the CT here is to protect the instrument and for measurement purpose.

Circuit breaker:

The circuit breaker breaks the circuit whenever there is any fault in the line.

Line isolator with earth switch (E/S):

An isolator with switch is part of an electrical circuit and is often found in industrial applications, however they are commonly fitted to domestic extractor fans when used in bathrooms in the UK. Isolator switches may be fitted with the ability for the switch to padlock such that inadvertent operation is not possible . In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power distribution systems where both end of the circuit need to be isolated. Major difference between isolator and circuit breaker is that isolator is an off-load device, whereas circuit breaker is an on-load device.

Lightening arrestors:

Lightening arrestors are used to protect the instruments from lightening.

Capacitor bank:

A capacitor bank is used in the outgoing bus so that it can maintain the voltage level same in the outgoing feeder.

Capacitor Control

is usually done to achieve as many as possible of the following goals: Reduce losses due to reactive load current, reduce kVA demand, decrease customer energy consumption, improve voltage profile, and increase revenue. Indirectly capacitor control also results in longer equipment lifetimes because of reduced equipment stresses. Experience shows that switched feeder capacitors produce some of the fastest returns on equipment investment.

Sources of Energy Loss

Energy losses in transmission lines and transformers are of two kinds: resistive and reactive. The former are caused by resistive component of the load and cannot be avoided. The latter, coming from reactive component of the load, can be avoided (Fig. 1). Reactive losses come from circuit

37

capacitance (negative), and circuit inductance (positive). When a heavy inductive load is connected to the power grid, a large positive reactive power component is added, thereby increasing observed power load (Fig. 1). This increases losses due to reactive load current, increases kVA demand, increases customer energy consumption, usually degrades voltage profiles, and reduces revenue.

Reactive Compensation

When capacitors of appropriate size are added to the grid at appropriate locations, the above mentioned losses can be minimized by reducing the reactive power component in Fig. 1, thereby reducing the observed power demand. There are many aspects to this compensation and its effects, depending on where capacitors get to be located, their sizes, and details of the distribution circuit. Some are discussed below.

Energy Loss Reduction

More than one half of system energy loss is caused by the resistance of the feeders. To minimize energy losses it is, therefore, important to locate feeder capacitors as close to the loads as possible. Substation capacitors cannot do the job - the reactive load current has already heated feeder conductors downstream from the substation. Reducing reactive current at the substation can not recover energy losses in the feeders. Another way to minimize energy losses is to use capacitor banks that are not too large. This makes it possible to put the banks on-line early in the load cycle. Since energy saved is the product of power reduction and the time the banks are on-line, the overall energy reduction is usually greater than when using large banks which are turned on for shorter amounts of time (Fig. 2).

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Demand Reduction

When capacitors are on-line reactive current and, therefore, total line current is reduced. During heavy load periods this has several advantages: The peak load is increased when it is most needed (essentially releasing demand), the effective line current capacity is increased, and the operating line and transformer temperatures are reduced – prolonging equipment lifetimes. The latter makes it possible to upgrade lines and transformers less frequently. All of these contribute to reduced costs and higher revenues.

Voltage Profile

Distribution feeder demand capacity is usually limited by voltage drop along the line. The customer service entrance voltage must be stable, usually ±5% to ±10%. The feeder voltage profile can be ‘flattened’ by connecting large capacity banks to the grid. Several benefits become available: The kVA demand can be increased to arrive at the original voltage drop (this is equivalent to releasing feeder demand), the substation voltage can be lowered to reduce peak demand and save energy, or the service entrance voltage can be allowed to increase thereby increasing revenue (at the expense of less than optimum kVA demand).

System Considerations

Obviously properly switched capacitors located at appropriate locations along distribution feeders provide great financial benefits to the utility.

If there is to be only one capacitor bank on a uniformly loaded feeder, the usual two-thirds, two-thirds rule gives optimum loss and demand reduction. This means that the bank kVAr size should be two-thirds of the heavy load kVAr as measured at the substation, and the bank should be located two-thirds the length of the feeder from the substation. If the objective is voltage control the bank should be farther from the substation.

With several banks on a uniformly loaded feeder, the total capacitor kVAr can more closely match the total load kVAr. Depending on the type of the switching control used, multiple banks on a feeder can lead to ‘pumping’ as the controls affect the operating points of each other. Usually no more than three or four banks are used per feeder.

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In the case of concentrated industrial loads, there should be a bank, sized to almost equal the reactive load current, located as close to each load as possible (Fig. 3).

Types of Control

VAr control is the natural means to control capacitors because the latter adds a fixed amount of leading VArs to the line regardless of other conditions, and loss reduction depends only on reactive current. Since reactive current at any point along a feeder is affected by downstream capacitor banks, this kind of control is susceptible to interaction with downstream banks. Consequently, in multiple capacitor feeders, the furthest downstream banks should go on-line first, and off-line last. VAr controls require current sensors.

Current control is not as efficient as VAr control because it responds to total line current, and assumptions must be made about the load power factor. Current controls require current sensors.

Voltage control is used to regulate voltage profiles, however it may actually increase losses and cause instability from highly leading currents. Voltage control requires no current sensors.

Temperature control is based on assumptions about load characteristics. Control effectiveness depends on how well load characteristics are know. Not useful in cases where those characteristics change often. Temperature control does not require any current sensors.

Time control is based on assumptions about load characteristics. Control effectiveness depends on how well load characteristics are know. Not useful in cases where those characteristics change often. Time control does not require any current sensors.

Power factor control is not the best way to control capacitor banks because power factor by itself is not a measure of reactive current. Current sensors are needed.

Combination control using various above methods is usually the best choice. If enough current, and/or other sensors are available, a centrally managed computerized capacitor control system taking into account the variety of available input parameters can be most effective, though expensive to implement.

BUS isolator:

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The bus isolators are used to isolate high voltage entering the bus or entering the substation.

Line Current transformer:

The current transformer used in the line is known as the line current transformer. The main use of this current transformer is to measure and to protect the instruments.

Line circuit breaker:

The circuit breaker used in the line is known as the line circuit breaker. The use of the circuit breaker in the outgoing feeder is to break the circuit when the any fault occurs in the line i.e, any fault on the outgoing feeder.

Line isolators with earth switch:

The line isolator with earth switch is to isolate the extra high voltage through the feeders going out of the station. The isolator used in the line is known as the line isolator.

To output feeder:

The outgoing feeders are used to give the step down voltage to the required area. This feeders supply the voltage to the required place for further step down or its use in the place.

The Sarusajai substation has six output feeders it six different places namely:

JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, and Kahilipara STATION.

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