Induction 11

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a. Earthing :

Type of Earthing : (a) solid earthing (b) Resistance earthing (c) Reactance earthing.

Equipment earthing is most essential to provide the safety to the personnel working on system.

During any fault, the potential of earthed body does not reach to dangerously high value.

Earth fault current flows through earthing and it causes operation of fuse or relay.

b. Earthing Transformer :

To limit the earth fault current, the neutral of star winding are grounded with the help

of resistor or reactance. The reactance connected between neutral and earth provides a

lagging current which neutralizes the capacitive current. For a transformer of given rated

output and given ratio of neutral current and line current, solid earthing may be adopted uptohigher line voltages with increasing value of earth resistance. Earthing transformer is a core

type transformer. It has three limbs, which is built up in same manner as that of power

transformer. Each limb accommodates two equally spaced winding. Current flowing through

winding is in opposite direction on each limb. Impedance of earthing transformer is quite low

and hence magnitude of fault current will be high, hence current resistance is added in series.

Earthing Transformer are used for if neutral point is not available in case of delta connection,

if neutral point is desired on bus bar or for distribution purpose, if required three phase four

wire system.

Earthing transformer are designed for carrying maximum fault current for upto 30 seconds.

The rating of earthing transformer is different from rating of power transformer. Power transformer

are designed to carry its total load continuously, while earthing transformer are supplied withiron losses, copper losses due to short circuit occurs for a fraction of minute. When system is

normal only current flowing through earthing transformer is required to provide necessary

magnetisation and to supply iron loss.

c. Neutral Grounding Transformer :

The subject of grounding covers the problems relating to the conduction of electric

current to the earth and through the ground. The earth rarely serves as a part of the return

circuit, being used mainly for fixing the potential of circuit neutrals. The ground connection

improves service continuity and protects lives and equipment. The electrical conductivity of

the materials constituting the earths surface is very low compared with the high conductivity

of metals, since the main constituents of the earth, silicon dioxide and aluminum oxide, areexcellent insulators. The conductivity of the ground is due largely to salts and moisture. Even

such a semiconductor may carry a considerable amount of current if the cross-section is large

enough. The resistivity of the soil depends on its type and dryness and varies with distance as

well as depth. Because of the high resistivity all currents flowing through the ground suffer a

considerable voltage drop.

Two types of grounding transformer are in general use: (1) The wye-delta transformer,

EARTHING, EARTHING TRANSFORMER ANDNEUTRAL GROUNDING TRANSFORMER

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and (2) the zigzag transformer. The neutral of either type may be grounded directly or through

current limiting impedance. It is assumed here that neither load nor a source of generation is

connected to the delta winding of the wye-delta transformer, and that the zigzag transformer

does not have another winding connected to load or generation; should either type have such

connections, it would be treated as an ordinary power transformer.Generally the winding of transformer shall be connected delta connection on primary

side and star connection on secondary side. The neutral of LT. winding shall be brought out to

a separate terminal. In three phases balanced load system, the generator neutral that is

connected to ground, usually does not vary any voltage. To restrict earth current flow,

Generator neutral is earthed through resistances. This offers an inductive load and restricts

flow of current during short circuit on the system.

Advantages of Neutral Grounding Transformer are:

(a) Arcing rounds are reduced or eliminated.

(b) The neutral grounding stabilises the neutral point.

(c) By employing resistance or reactance in earth connection, the earth fault current can be

controlled.

(d) The over voltage surge due to lightening are discharged to earth hence less damages to

the equipment. Useful amount of earth fault current is required to operate earth fault

relay.

(e) Improved service reliability due to limitation of arcing ground and prevention of unnecessary

tripping of circuit breakers.

(f) Life of eqipment, machines and installation is improved due to limitation of voltages.

(g) Greater safety to personnel and equipment due to operation of fuses or relay on earth

fault and limitation of voltages. Hence it is economical to ground neutral point.

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1. INTRODUCTION :

Most of the industrial units are characterised by their installed capacities. ElectricalPower-generating Stations fall into this category. In respect of newly proposed power stations,

capacity of each new Unit of power station is decided based on:

Planned growth of energy consumption and power demand.

Stiffness of the Grid, etc.

Extent of consumption of electrical energy (auxiliary power requirements) in the power

station depends on

1. the type of Station i.e. Nuclear or Thermal (oil coal or gas fired)

2. steam parameters, capacity and number of auxiliary equipment.

Number of these auxiliary equipment in each of the process system is decided based on.i) the need to ensure specified reliability

ii) conformity of one equipment to develop the required throughput capacity.

The auxiliary power requirement varies from 8 to 12% of the Generating capacity of the

power station i.e. single unit or multiple unit.

Fans and Pumps are the main auxiliary equipment, which consume substantial quantum

of electric energy at power station. Selection of fans and pumps is carried out in a sequential

manner in consideration to :

a) Required flow rate (throughput capacity) and Head are determined.

b) Suitable type, size and required number of machines are decided.

c) Type and Power of the drive,d) Voltage of power supply,

e) Methods of control and protection selected.

Most critical pumps in power station are Boiler Feed Pumps, Condensate pumps, Circulating

water pumps. Boilers feed pumps are the largest of the pumps in TPS.

The fans/ pumps- both larger and of medium size are of conventional design and mostly

have electric drives. Hence need for a Station Service Power Supply System or Auxiliary Power

Supply System arises to provide power supply to the drives of the auxiliaries.

Electrical Power System in respect of a Power Station consists of Main Plant Power

output system and auxiliary power supply system. The main power output system transfers

power produced by the Turbine/ Generator to the State/ Regional EHV/ HV Electricity Grid.

The System includes:

Steam Turbine Generators (with all their accessories), Generator Breaker, Isolated Phase

Bus duct, Generator Transformer, Unit transformer, Station (Startup) Transformer.

A single line diagram pertaining to 200/500 MW power station as enclosed (Diagram No 1)

explains in general the power output schemes and station auxiliary power distribution system

TYPICAL ELECTRICAL SUPPLY SYSTEM

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to supply power to various process system and instrumentation load with the plant.

Division 1 and Division 2, supply the system of the plant dedicated to normal power production

and plant safety related loads. (Diagram No 2).

2. ASPECTS TO BE COVERED ON PLANT AUXILIARY POWER SYSTEM :a) Availability

1. Estimation and compilation of auxiliary loads

2. Selection of Auxiliary system voltages.

3. Determination of number and sizes of Unit Transformers, station (Startup) Transformers

and Auxiliary Transformers.

4. Determinations of one-line diagram of auxiliary power supply system including number

and rating of Switchgear, Bus, and schemes of interconnection.

5. Selection of Optimum impedance value, type and range of tap changing gear for

Transformers.

6. Determination of short circuit levels, switchgear duties and short circuit rating of

cables.

7. Determination of voltage dip at the motor terminals during starting of the largest

capacity motor (e.g. at 6.6 kV level Boiler feed pump: 4000 kW).

8. Different areas of Power Plant and the H.T./ L.T. Auxiliaries/ Equipment related/ used in

that area.

b) Reliability

1. Schemes for manual & automatic transfer of Auxiliary loads at Auxiliary High Voltage

(6.6 kV/ 11 kV as case may be) level.

2. Selection of type of system earthing for 6.6 KV (H.V. Voltage) and 415 volts (Low

voltage).

3. Selection of Protection Schemes for 6.6 KV and 415 volts levels.

4. Selection of Metering Schemes for 6.6 KV level.

c) Design Approaches :

Objectives to be attained in deriving the power supply for the auxiliary power systems are:

Station Status/ Condition Requirements

i) During a Unit Startup Availability of reliable off-site startup power to

facilitate commissioning of auxiliary systems required

for Unit Startup.

ii) During Normal Provision of two independent sources of power

Operation of Unit supply to feed two independent auxiliary systems.

The two power supply sources to be independent

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of each other as far as possible to minimise

simultaneous outage of both.

Each supply source should be rated for 100% duty.

iii) Unit Shutdown Smooth operation of auxiliaries should be possibleduring unit shutdown.

3. AUXILIARY POWER DEMAND STATISTICS

Total demand for power in respect of auxiliary systems is estimated at 8% to 12%

under Load Schedules and capacities of various equipment.

a) Major electrical equipments of the power plant :

1. Main Generator 2. Generator Transformer 3. Station Startup Transformer

4. Unit Transformer 5. Auxiliary Transformer 6. Diesel Generators

7. UPS/ MG Sets (Power) 8. Batteries (Power) 9. Batteries (Control)

Ratings of Unit auxiliary transformers and Station Startup transformers are arrived at in

accordance with load requirements under various stipulated modes of operation. The above

two sources are connected at 6.6 KV level in such a way that in case of loss of power from

any of these two sources, an automatic fast bus transfer (FBT) scheme is initiated to derive

the power supply from the other source.

b) Categories of Supply voltage levels :

Voltage levels to be operated, Controlled,regulated and protected through switchgears

are categorized as :

i) Low Volts supply i.e. Voltage level upto 650 Volts.ii) Medium Volts supply i.e. Voltage level above 650 Volts upto 1000 Volts.

iii) High Volt Supply i.e. Voltage level above 1000 Volts upto 33000 Volts.

iv) Extra High Volt Supply i.e. All Voltage above 33000 Volts.

c) Classification of Power Supplies :

i) Class I Category ii) Class II Category iii) Class III Category iv) Class IV Category

d) Class IV Category Supply System (HT/LT) :

This derives the power supply from the grid substation, or from its own A.C. Generator

when it is on load. This has two voltage levels supplying power at :

1) 6.6 KV 3 Phase A.C. for motors for rating of 200 KW and above (as per clause 7.1.1 of

IS : 325-1978)

2) 415 V, 3 Phase, A.C. for motors below 200 KW rating.

Major loads connected to 6.6 KV system are :

I. D. Fan, F. D. Fan, P. A. Fan; Coal Mills, Boiler Feed Pumps, Condensate Extraction

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Pumps; G. S. Water Pumps; Condenser Cooling Water Pumps; River Water Pumps, etc., 415

Volts Class IV Systems.

These consists of 2; 4; 6 Nos of buses supplied through associated 11 KV/ 415 V or 6.6

kV / 415 V. transformers of capacity, 2000 KVA; 1600 KVA; 1000 KVA etc. To maintain

continuity of supply with minimum time of interruption when any of the servicing transformerfails, a hot stand by transformer is provided to supply the load of the affected bus, which will

be switched in manually.

e) Class III Category Supply System :

This derives the power supply from the Class IV, Supply Switch Board. It is also assisted

by the power from its own standby generator i.e. individual station M.G.Set or D.G.Set. Thus

Class III Power Supply can be resumed from its own M.G. Set/D.G. Set on putting it in service

under total grid supply failure.

This system supplies power loads to auxiliaries/eqipments, which can tolerate interruption

of supply say, upto 1 minute. (This is design intent, but, actually power supply is made

available in 10-15 seconds).

f) Class II Category Supply System :

This system provides uninterrupted A. C. Power to the loads connected to this system.

It is divided into 2 divisions as.

i) Class II 415 Volts, AC 3 Phase 3-wire power supply system.

ii) Class II 240 Volts, AC 1 Phase 2-wire control supply system.

System has one M. G. Set / Inverter. This receives power from 2 sources i.e.

i) From Class I power supply system through motor Generator/ inverter under normal operating

conditions.

ii) From 415 V Class III power system directly.

Class II control supply system is designed on similar lines as the class II power supply

system. It has its own inverter and receives supply from 2 sources.

g) Class I Supply System :

These systems supply uninterrupted DC power to the loads, on this system. 220 V DC

Class I power supply system derives its power from 6.6 KV Class III power system through

power Automatic Constant Voltage Rectifiers (ACVRs). In addition to feeding various loads. It

also supply trickling/ equalizing/ floating current to the control batteries in its division.

Similarly 220 V DC Class I control supply gets its power from 415 Volts AC Class III Power

System through control ACVRs. In addition to feeding the control loads, it also supplies trickle/equalizing/ floating current to the control batteries in its division.

4.0 Different Areas of Thermal Power Plant:

There are thermal power plants, where there may be only one generating unit or may

be more than one generating units.

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a) Recognition of different areas in the power plant :

1) Turbine Area. 2) Generator Area. 3) Boiler area.

4) Oil Handling plant Area. 5) Coal Handling plant 6) Water Treatment plant

7) River Water pumps Area. 8) Cooling Tower Area. 9) Ash Handling plant10) General Auxiliaries Area. etc.

b) Recognition of LT/HT Auxiliaries :

In each area as above there are the auxiliaries and equipments, power to these auxiliaries

and equipments is fed from L.V./H.V.Supply Board. Auxiliaries/equipments running at L.V. and

H.V. Supply are called L.T. auxiliaries & H.T. auxiliaries respectively.

c) Recognition of Main Supply Boards :

Usually there are two main Boards and are called as,

1) Station supply board. HV/LV. & 2) Unit supply board. HV.

H.V. Station Supply Board is fed from station transformer having its secondary voltage

level 6.6 KV and H.V. Unit Board supply is fed from unit transformer having its secondary

voltage of 6.6 KV. Unit transformer can only be put in service when the units own generator of

the unit is on load. In absence of availability of supply from unit transformer, H.V. Unit Board

gets the supply from H. V. Station Board.

d) Auxiliaries fed from HV Station Board :

H.V. Station Board extends the supply to the H.V. Boards of different areas to feed HT

auxiliaries lying in that area, which are common to the whole plant e.g.

1) River Water Pump Board for auxiliaries in RWP area.

2) W. T. Plant transformer/ D. M. Plant transformer for auxiliaries in WTP area

3) Ash handling plant board for auxiliaries in AHP area.

4) Coal handling plant board for auxiliaries in CHP area.

5) Fire fighting board transformer for auxiliaries on Fire fighting board.

6) Station lighting/welding supply board transformer for auxiliaries on lighting / welding

supply Board.

7) Oil handling plant transformer for auxiliaries on OHP Board.

8) Station service board transformer for auxiliaries LT Station service board.

e) Auxiliaries fed from HV Unit Board :

H.V.Unit Board extends the power supply to H.V.Auxiliaries/equipment and transformers,

which are made only for a particular unit e.g.

1) I. D. Fans 2) F. D. Fans

3) P. A. Fans 4) Coal Mills

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5) C. W. Pumps 6) Boiler feed pumps

7) Starting oil pump. 8) Condensate Extraction Pumps

9) Unit lighting transformer. 10) Boiler board transformer

11) Turbine Board transformer. 12) Ash handling transformer.

13) G. S. Pumps 14) Emergency transformer

15) E.S.P transformer. etc.

e) Auxiliaries fed from different HV Board : e.g.

H.V. River Water Pump Board

1) R. W. P Motors 2) R. W. P, L. T. Board transformer

H.V. AHP Board

1) AHP H. V. Pumps 2) AHP L. T. Board transformer & 3) Clearwater Pumps.

H.V. CHP Board.

1) CHP H.V. Coal rusher motor & 2) CHP Auxiliary transformer.

5.0 415 V LT switchgear distribution Boards and their Supply

a) Nomenclature of L.T. Boards

L.T.Boards are generally nominclated based on the name of area in which the various

L.T.auxiliaries are installed and are fed from the respective switchgear Board.e.g.

i) Turbine board extends the supply to the auxiliaries located in turbine Area.

ii) Boiler Board extends the supply to the auxiliaries located in boiler area.

iii) D.M.Board/ W.T.Board extend the supply to the auxiliaries located in D.M.Plant/ W.T.Plant.

iv) Station Service Board extends the supply to the auxiliaries mainly meant for the station

irrespective of the specific unit.

v) E.S.P. Board extends the supply to the auxiliaries in ESP area.

N .B . This is a general concept to locate the supply of any auxiliary from a respective board.

However the concept may change from station to station & place to place. It is therefore of

utmost important to get confirmed the location and supply point of any auxiliary and supply

interconnections etc to eliminate any chance of accident.

b) L.T.Boards Supply Scheme :

The general scheme of supply to L.T.Auxiliary boards is as below.

Station Service Board :

Two bus sections of station service feeder Board are fed from station board through

two Xmers of 6600/ 415 V. These bus sections are connected through the switchgear called

bus coupler.

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Standby Board :

Two sections of standby board are fed from station board feeders through two Xmers

of 6600/ 415 V.

Boiler Board :Two sections of Boiler Board are fed from unit board feeder through 6600/415 V

transformers and standby board respectively.

Other LT boards are supplied with power more or less as below :

Fuel oil handling board

It is fed from Station Service Board through two feeders.

Turbine Board

It is fed from Station Service Board and Boiler Board.

Emergency BoardIt is fed from Station Service Board or from Unit Board through an emergency transformer

of 6600/ 415 on Unit Board. In total AC failure, this emergency board is fed/charged

from 415 V, sufficient kW capacity Diesel Generator Set.

DM Plant Board

It is fed from Station Board feeder through 6600/ 415 V Xmer. It can also be fed

through Standby Board.

Coal Handling board

It is fed from 6.6 kV CHP Board through two number of 6600/ 415 V Xmers.

Ash Handling Plant Board

It is fed from 6.6 kV AHP Board through two numbers of 6600/ 415 V Xmers.

Electrostatic precipitator Board

It is fed from Unit Board feeder through 6600/ 415 V, Xmer. It can also be charged

from Standby board.

Main lighting Board

It is charged from two feeders on Station Board through two Xmers, of 6600/ 415 V.

Main welding Board

It is charged from two feeders on Station Board through two transformers of6600 / 415 V.

6.0 L.T. Auxiliaries / Equipments ingeneral fed from different L.T. switchgear Board :

a) Turbine Board

1. Dewatering pump. Fire fighting House. 2. Booster pump to Hydrogen cooler.

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3. Gen. Transformer oil pumps 4. Centrifugal oil pump for turbine

5. G. S. Hot water transfer pump (stationary).

6. Drip pump 7. Unit auxiliary transformer cooling fans.

8. Dewatering pump C. W. Suction pit 9. Dewatering pump. G. S. Pump House

10. DM/ GS cooling pump 11. Hydrogen dryer fan12. Oil vapour fan 13. Motor space heater transformer.

14. Hydrazine injector pump (L.P. dosing no.1) 15. Control P. T.

16. D. C. supervision module 17. Control P. T. station.

b) Emergency Board

1. Emergency lighting 2. Seal oil pump

3. Air heater lub, oil pump motor 4. Igniter air fan motor

5. Barring gear motor 6. I. D. Fan lub oil pump

7. Hydrogen exhaust fan. 8. Battery charger.

9. F. D. Fan lub oil pump 10. B. F. P. lub, oil pump

11. Air heater main drive motor 12. Supply for D. G. control

13. Excitation cubicle. 14. Air heater lub, oil pump

15. Scanner air fan motor 16. Standby lub, oil pump motor

17. Alarm cubicle 18. Motor space heater

19. Emergency service cubicle 20. Bus P. T. cubicle

21. Control transformer 22. Incoming 1 from station service board

23. Incoming 2 from Diesel generator board 24. Seal air fan

25. Jacking oil pump. 26. Excitation supply (Field flashing)

27. Stator water cooling pump 28. P. A. fan lub, oil pump motor

29. Spare fan seal air fan 30. Oil unit motor for auxiliary PRDS 1.

31. Oil unit motor for HP/ LP bypass.

c) M. C. C. For ash handling plant

1. Lighting 2. Spare clinker grinder motor

3. Clinker grinder Motor 4. Reserve

5. Spare 6. H. P. pump motor

7. Rotary feeder motor 8. Slurry pump motor

9. Spare slurry pump motor 10. Motor space heater P. T.

11. Control P. T. 12. Raw water transfer pump motor

13. Bus P. T. 14. Incomer

15. Incomer control circuit 16. Bus coupler

17. Bus coupler control circuit 18. Incomer 2.

d) Boiler Board

1. Temperature probe motor 2. Supply to soot blower pannel

3. P. A. fan lub, oil pump 4. Phosphate dosing pump.

5. Unit drain pump 6. B. F. P. oil pump

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7. Raw coal feeder variation 8. Feeder inst. Control supply

9. Oil unit motor for HP/ LP bypass station 10. F. D. Fan lub, oil pump

11. I. D. Fan lub, oil pump 12. Motor space heater module

13. Roof extraction fan panel 14. 24 V supply cubicle

15. Bus P. T. cubicle 1. 16. Control transformer.17. Incoming supply 18. Bus coupler

19. Outgoing to turbine

e) Fuel oil handling board (control & schematics)

1. Heavy fuel oil pump 2. Unloading pump feeder

3. Recovery pump motor feeder 4. Unloading pump feeder

5. Ventilation fan 6. Unloading pump house sump pump

7. HSD pump 8. Dewatering pump No. 2.

9. Fuel oil pump No. 2

f) Electro-static precipitator (ESP) board

1. H. V. rectifier Unit 2. Auxiliary control panel No. 1

3. Incomer A from ESP Xmer 4. Bus coupler

5. Incomer B. from standby board

g) Station service board

1. Fire fighting pump 2. Supply to Boiler & Turbine basement

3. Station air compressor 4. Dewatering pump

5. Control supply for station compressor 6. E. O. T. crane

7. Workshop feeder 8. Ventilation

9. Turbine Board 10. Instrument compressor

11. F. O. H. plant 12. Standby board.

13. A. C. alarm control 14. D. C. alarm control

15. Standby supply to emergency Board 16. Elevator

17. A. C- supply

h) D. M. Plant board

1. Caustic soda mixer 2. Alkali transfer pump

3. Degasser air blower 4. Acid transfer pump

5. Brine transfer pump 6. M. B. air blower

7. Degassed water pump (CWP-A) 8. Cooling water pump

9. Effluent water pump 10. C. T. fan

11. CWP 12. Lime mixer

13. Degasser air blower

i) Standby board

1. D. M. Plant board 2. Boiler Board

3. E. S. P. Board 4. Station service board supply

5. D. C. control and alarm supply

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j) RWP L/T board auxiliaries

1. Lub. Water pump 2. Cooling water pump

3. Raw water pump 4. Grease pump

5. C. W. Pump 6. Oil pump

7. Canal water pump 8. Lighting supply feeder.

k) C.H.P. Supply board

1. Belt F. D. R. 2. Belt F. D. R.

3. Conveyor 4. A. C. T. R. Flap gate

5. Control panel for tunnel ventilation 6. Magnetic pulley

system for conveyor 7. Actuator for flap gate

8. Coal sampling unit 9. Magnetic separator

10. Control panel for dust extraction 11. Primary screen

system for primary crusher. 12. Primary crusher

13. Sump pump 14. Vibrating feeder control panel

15. Metal detector 16. Belt weigher

17. Tripper conveyor 18. Telescopic Chute, panel

19. Tripled M. C. C. 20. Vibrating feeder control panel

21. Bunker level annunciation panel 22. Telescopic chute panel

23. Secondary screen. 24. Control panel for dust extraction system

25. Motor Space heater 415/ 240 V 5 kVA for Junction Tower

transformer SPH-1 26. Bus P. T.

27. 415/ 240 V 5 kVA, control transformer-1 28. Incomer No. I

29. Bus coupler 30. Incomer No. II31. Bus P. T. 32. Control panel for D. E. S. (dust

33. Control panel for tunnel ventilation extraction system) for Bunker

system for conveyor 11. 34. Supply for belt weigher

35. Mimic control panel

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324

Operational Strategies which lead to approach toDesign Capacity and other parameters of switchgear


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325

Schematic Diagram of Electrical System


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326

Class II 415 V System A.C.

Class I 220 V System D.C. (Power)Class I 220 V System D.C. (Control)

Diagram - 5


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327

Class IV 6.6 KV and 415 V System

Diagram - 4

Class III 6.6 KV and 415 V A.C. System

Diagram - 3


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328

High Capacity Electrical Motors, Generators, Switchgear etc are installed and used in

Power Systems and Power plants. Normal Working voltages are in the range of 3.3 kV to

400 kV. Capacities of individual Electrical Equipment range from 500 kW to 10 MW for the

motors used in Power Plants. Generator capacities range between 120 MW to 500 MW. In all

circuits where we have either high voltage or heavy currents, it is not practicable to connect

the measuring and indicating instruments or protective relays directly in the circuits carrying

heavy currents or working at High Voltages.

Instrument transformers are used to scale down primary currents and voltages to low

and safe level producing little hazards to a person and lot of saving in the cost. Current and

voltage transformer is thus input device for measuring instruments and protective relays.

Voltage transformer

Protective PT : A P.T. intended to supply protective devices viz. Relays or trip coils.

Measuring P.T. : A P.T. intended to supply indicating instruments, integrating meters and

other measuring apparatus.

Dual purpose P.T. : Often the same P.T. can be usued for both the purposes either is has one

secondary winding or two separate secondary windings or same different ratio to be used for

metering and protection circuits separately.

A voltage transformer is similar to a power transformer, the primary being excited by

nearly constant voltage. A P.T. is rated in terms of maximum burden (VA output) it will deliver

without exceeding specified limits of error, where as a power transformer is rated by thesecondary output it will deliver without exceeding specified temperature rise. The construction

of P.T. differs from a power transformer, as different emphasis is placed on cooling insulation

and mechanical problem.

a) The output of P.T. is maximum few hundred VA, so heat generated is not a problem.

b) As number of turns and insulation is proportional to primary voltage, size of the P.T. is

determined by system voltage.

c) Insulation presents special problem because of small conductor size, ventilation and

space restrictions, when it is to be accommodated in switchgear. Generally for system

upto 11 KV, P.T.s encapsulated in synthetic resin are used.

Technical and constructional details of PT :

The potential transformers (from 22KV to 220KV) are generally of single phase and are

oil immersed and self cooled. The design and construction of PTs is sufficient to withstand the

thermal and mechanical stress resulting from the rated normal and overvoltage.

INSTRUMENT TRANSFORMER (CTs PTs)


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The potential transformers consists of

i) Bushing

ii) Metal cap with terminal connector

iii) Core

iv) Primary and secondary windings.

Bushing : The bushing is provided for insulation between the H.V. terminal and the tank on

which it rests. It is made of homogeneous vitreous porcelain of high mechanical and dielectric

strength, any joints or coupling in between. The bushing is glazed to have uniform brown or

dark brown smooth surface arranged to shed away rain water.

Solid porcelain bushings are used upto 36KV class service. For services of 72.5 KV and

above oil filled condenser type bushing are used. The bushing are also fitted with a suitable oil

level gauge to indicate the level of oil inside the bushing. These oil filled bushing are hermetically

sealed to prevent ingress of moisture. The height of bushing of the PTs should be adequate to

avoid bird faults.

Metal Cap : The metal cap at the top, above the bushing is of high strength, hot dip galvanised

malleable iron.

Core : The core of a P.T. is housed inside the bushing and is made up of high grade non

electrical silicon laminated steel of low hysterics loss and high permeability to ensure high

accuracy.

Primary Winding : The primary winding wound on the core inside the bushing and has suitable

number of sections. It is insulated having good mechanical strength, high electrical withstand

properties and good aging qualities. The primary winding is connected between phase and

neutral with the neutral point solidly earthed.

Secondary Windings: The PTs are generally provided with at least two separate secondary

windings, which are connected in star and open delta respectively. The star connected

secondary winding is used for metering and relaying (e.g. distance relays, directional overcurrent

relays etc.) and has accuracy specified for particularly application. The rated burden of this

winding is typically 200VA. The open delta connected winding is used for polarizing directional

earth fault relays or driving a neutral displacement relay for detection of earth fault in non-

effectively earthed systems. The rated burden of the open delta winding is typically 100VA.

Suitable HRC fuses protect both these windings.

Terminal Connectors : P.T. Secondary terminals are brought out on a separate Bakelite boardwith flexible lugged copper connection between these terminals (on the back side) and the

outgoing terminals from the P.T. chamber. These secondary terminals are housed in a terminal

box, which is moisture and insect proof. Polarity markings are available both on primary and

secondary sides. The primary terminal has standard size of 30 mm dia x 80 mm length for all

PTs upto 220KV. This terminal is made of copper and heavily plated with silver or nickel. This

terminal is rigidly fixed on the side of the metal cap at the top. The potential transformers are

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hermetically sealed to eliminate breathing and to prevent air and moisture from entering the

tanks. Oil level gauge and pressure relieving device capable of releasing abnormal internal

pressures are provided. The grounding terminals, filling and draining plugs are also provided to

the PTs.

Voltage Error (Ratio error) & Phase Displacement Error of a PT

Ideally a PT should produce a secondary voltage which is exactly proportional to the

primary voltage and exactly opposite in phase. But this can never be achieved in practice.

Voltage drops in primary and secondary windings due to largely the magnitude and power

factor of the secondary burden results in ratio and phase angle errors.

Kn Vs Vp x 100

% of Ratio Erros =

Vp

Where Kn = rated transformation ratioVs = actual secondary voltage, under conditions of measurements.

Vp = Actual primary voltage.

If the error is +ve, secondary voltage exceeds nominal value.

IF the error is ve, secondary voltage is less than nominal value.

Phase angle error = The difference of phase between the primary and reverse secondary

voltage vectors.

Error is +ve when reverse secondary Voltage (-Vs) vector leads the primary voltage

vector vice-versa.

It is usually expressed in minute (Phase angle error is of importance, when the transformeris used with wattmeter and similar instruments where indication depends on voltage and phase

angle between voltage and line current).

Burden

The admittance of the secondary circuit expressed in Mho and P.F. (lagging or leading).

(The burden is usually expressed as apparent power in VA absorbed at the stated P.F. and at

the rated secondary voltage). In case of residual voltage transformer the burden is the valve

of the load connected across the appropriate secondary terminals expressed in VA at the rate

secondary voltage.

Rated Burden

A burden in VA assigned by the manufacturer as the burden at which a transformer will

comply with the specified limits of accuracy at the rated secondary voltage.

Accuracy Class

A classification assigned to PT, the errors of which remains within specified limits, under

prescribed conditions of use.

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Highest System Voltage

The highest r.m.s. Line to line voltage, which can be sustained under normal operations

at any time at any point on the system. It excludes temporary voltage variations due to fault

conditions and sudden disconnection of large loads.

Rated Voltage Factor

The M.F. to be applied to the rated primary voltage to determine the elevated voltage

which the transformer shall comply with the relevant requirements for a specified time and

with relevant accuracy requirements.

Rated Voltage Factor for 3 phase P.T.

The M.F. assigned to the transformer and to be applied to the rated primary line to

neutral voltage (either for all 3 phase windings or in the case of earthed transformers for any

2 out of the 3 windings), at which a transformer shall comply with the relevant thermal

requirements for a specified time and with the relevant accuracy requirements.

Recommended Voltage Factors

Rated Voltage Factor Rated Time System Earthing

1.2 Continuous 30 sec. Effective earthed system

1.5

1.2 Continuous 30 Sec. Non-effectively earthed system

1.9 (with automatic earth fault tripping)

1.2 30 seconds 8 Hours Isolated Neutral or Resonant Earthed System

19. (without automatic earth fault tripping)

Limits of temperature rise of winding

Class Of Insulation Temperature rise in 0C

All classes immersed in oil 55

All classes immersed in oil bituminous compound 45

Classes not immersed in oil or bituminous compound Y - 40

A - 55

E - 70

B - 80

F - 105

H - 130

(The reference ambient temp. for the purpose of temp. rise shall be 400C)

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Accuracy Classes (Selection of Measurement P.T.) as per I.S. 3156 (Part II)

Measuring P.T. (Limits of error I.S.-3156)

Class % voltage Phase displacement Remarksratio error (Minutes)

0.1 +0.1 +5 These values correspond to any

0.2 +0.2 +10 voltage between 80% to 120%

0.5 +0.5 +20 of rated voltage, with connected

1.0 +1.0 +40 burden between 25-100% at p.f.

3.0 +3.0 0.8 lag and rated frequency

1.6 Protective P.T. (Limits of Error I.S. 3156 Part III)

(Rated out-put 10, 25, 50, 75, 100, 150, 200, 500 VA)

Residual output for residual P.T. 25, 50. 100VA

Accuracy Ratio Error Phase displacement Remarks

Class (%) (Minutes)

3.0 +3 +120 1) Value are for a P.T. without

residual voltage winding.

5.0 +5 +300 2) Correspond to voltage 5 to 110%

of rated voltage.Burden 25 to 100%

of rated burden at 0.8 lag p.f.

Limits of Error for Residual P.T.Class Ratio Error Phase Displacement

5.0 +5% +200 minutes

10.0 +10% +600 minutes

Where a protective voltage transformer is used for measurement it shall comply with

the requirements corresponding to accuracy class 0.5 & 1.0.

Protection of Voltage - Transformer

a) H.V. Side Protection : On designs upto 60KV, fuses are provided on HV side, either

within the tank or separately mounted. For higher voltages a gas-actuated relay is usedas the current is very small and there are mechanical limitations to the size of the fuse

element.

b) L.V. Protection : As two windings of a transformer are conductors separated by an

insulation which constitutes a dielectric, they form a capacitor. It is possible for line to

ground voltage to feed through this capacitor and charge the terminals. This voltage will

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be built up between the terminals and may cause arcing to ground, which would damage

the equipments. These voltages may exceed 1000 V to ground and would be very harmful

to any one touching either the terminals o0r any wires connected to them.

If one side of the secondary circuit is connected directly to ground this voltage cannotbuild up. A very small amount of current flows through the capacitor directly to the ground

which does not harm. PTs are usually grounded directly at the transformer or at the marshalling

box. The ground lead must never be used. The so-called hot lead should be provided with fuse

to protect the transformer from being overloaded if a short circuit develops in the control

cable or a relay circuit. The short circuit also causes a temperature rise, which may rapidly

reach a dangerous value.

Choice of Connections of 3 phase P.T.

When metering or protection relays are used on circuits they must be supplied with 3

(usually 110 V between phase to phase). The secondary voltage must be in phase with and

proportional to primary voltages.

a) V Connection : Two single-phase transformers are connected in V both on the primary

and secondary sides (e.g. one across R-Y phases and the other across Y-B phases). As

there is no neutral on the primary winding, the zero sequence voltage cannot be obtained.

This connection is generally used for 3 phase 3 wire meters, which do not require phase-

neutral voltage. (This PT cannot be used where it is required to have zero sequence

voltage for protection or indication).

b) Star-Star Connection : Most common connection used in metering and relaying schemes,

when e3 phase 3 limb voltage transformers are used the zero sequence voltage will not

be transformed. The scheme requires 3 phase P.T. or 3x single phase P.Ts. with bothprimary and secondary connected in start, with neutrals solidly grounded (Typical limb

voltage rating 11 KV/3/110/3 volts).

c) Star Broken Delta Connection: (Residual Connection)

The connection is used when zero sequence voltage is required for earth fault relaying

scheme. The residual voltage is 3 times the zero sequence voltage i.e. 3 V). So long as

there is no residual voltage in the three phase system to which the transformer is

connected, there will be no displacement of the system neutral potential from earth and

hence the voltage in the open delta will be zero. This connection is therefore used in

neutral displacement schemes and for supplying the voltage circuit of directional earth

fault relays.

The core of such a transformer must be capable of carrying the residual flux brought

about by the imposed residual voltage. This residual flux cannot be contained within a normal

3-limbed core, hence either the phase of each primary winding must be on separate cores or

alternatively a 5-limbed core must be used. Such a P.T. is called as Residual Voltage Transformer

(RTV).

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Auxiliary P.T.

Small auxiliary PTs may be used where the main PT ratio and the instrument do not

match. When it is necessary to eliminate a direct metallic connection between two circuits, an

isolating transformer is used (e.g. when phase to phase voltage is used for synchronising. Here

the voltage source must not be grounded, but one side of the synchroscope must be grounded.The isolating transformer permits operation of synchroscope without connecting the source

directly to ground).

Phase Shifting Transformer

In certain metering equipment (viz. Trivectometer) it is necessary to have one pair of

voltage lagging another pair or voltages by 90 for recording reactive power or energy. The 3-

phase voltage of the main PTs is applied to 2 small auto-transformers (2-4 & 2-6) as shown.

Tap No. 1 : is marked to select 86.6% of full wdg. Between 2&6

Tap No. 2 : is similar to above (between 2&5)

Tap No. 5&7 : are centres taps of full windings 2-6 & 2-4

If the voltage is applied between 1&2 = 110 V, then voltage between 6&2 will be 127.

(If 86.6% windings is 110 V then 100% winding = 127 V0.

In the triangle, side 6-7 = 127 x Sin 60

= 110 V

and Side 7-2 = 127 x COS 60

= 63.5 V

Thus voltage V-4-5 = V1-2 & V4-5 lags by 90

V-6-7 = V3-2 & V6-7 lags by 90

Capacitive Voltage transformerA Capacitor presents a certain opposition to the flow of alternating current. This is

called Capacitive reactance.

1

Xc = (Xc is in ohms, C-capacity in farads, f-cycles/second)

2fc

If two capacitors are connected in series and an alternating voltage is applied across

the two, the voltage will divide according to the capacitive reactance of the two capacitors.

If capacitive reactance of C1 is twice the capacitive reactance of C2, the voltage drop

across C1 will be twice that of across C2. The electromagnetic voltage transformer is highly

efficient and reliable equipment, however the cost rises steeply as the system voltage increases.A more economic means of obtaining an accurate voltage reference is C.V.T., which is fairly

common on systems above 132 KV.

CVT is a transformer comprising a capacitor divider unit and an electromagnetic unit,

so designed and interconnected that secondary voltage of the electromagnetic unit is

substantially proportional to and in phase with the primary voltage applied to the capacitor

divider unit.

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Capacitor Divider Unit

It is a three terminal capacitive device having one terminal at earth potential

second at a H.V.(to be measured) and third at intermediate voltage.

Electromagnetic UnitIt is the component CVT connected between the intermediate terminal and theearth

terminal of the capacitor divider supplying the secondary voltage.The reactor coil is connected

in series with small electromagnetic P.T. The spark-gap(protective device) protects the built in

transformer. A sudden surge causes a break down of the gap. The voltage across the arc is

very low, so that the high voltage is never applied to the transformer. In commercial designs

the reactor and interposing transformer are combined into one unit and housed in a tank on

which is mounted HV capacitor divider. The capacitor divider may either be in the form of a

separately mounted capacitor or advantage may be taken of the condenser bushing of the

switchgear or Power transformer, by bringing out connection from one of its intermediate foils

(usually the earth based) rated output - 25, 50, 75, 100, 200 & 500VA.Standared accuracy

class 0.5, 1.0, 3.0, 5.0 & 10 (class 10 applies to only CVT with residual winding.The same device may be used to inject high a low frequency signal into the power line

for communication, telemetering, teleprotection etc. This signal in 100 to 175 Kilo Cycles

range is transmitted through the power line and is received by another capacitor device at the

other end of the line. The losses on account of secondary burden give rise to ratio and phase

angle errors which are some what larger than electromagnetic PT and variable with system

frequency and further more the load rating is also very much inferior.

Technical Specification

The technical specification for 220Kv and 132 PT are reproduced below as anexample.

No. Particulars 220 KV PT 132 KV PT

1) Nominal System voltage KV 220 KV 132 KV

2) Highest system voltage KV 245 KV 145 KV

3) Frequency 50 C/S 50 C/S

4) Earthing Effective Effective

5) No. of secondary windings 2 2

i) Rating of primary KV -220 / !3 KV 132 / !3 KV

ii) Rating of Sec. Winding II 110 / !3 V 132 / !3 V

iii) Rating of Sec. Winding I 110 / !3 V 110 / !3 V

6) i) Rated burden winding I 1000 VA 500 VA

Not less than

ii) Rates burden winding I 100 VA 100 VA

Not less than

7) i) Accuracy class winding I 0.5 0.5

ii) Accuracy class winding II 3 3

8) Basic insulation level KV 1050 KV 650 KV

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Current Transformer

A.C.T. is an instrument transformer in which the secondary current in normal conditions

of use, is substantially proportional to the primary current and differs in phase from it by an

angle which is approximately zero, for an appropriate direction of the connections.

The primary winding is connected in series with the load and carries the load current tobe measured. The secondary winding is connected to the measuring instrument or relay, which

together with the winding impedance of the transformer and the load resistance constitutes

the burden of the transformer.

Primary current Contains 2 Components

i) The secondary current which is transformed in the inverse ratio of the turns ratio.

ii) Exciting current, which supplies the eddy and hysterics losses and magnetizes the

core. This current is not transformed and therefor is a cause of error.

(Kn. Is Ip)Current Error =

Ip

Kn = Rated transformer Ratio

Is = Actual secondary current when Ip is flowing

Ip = Actual primary current

Phase Displacement

The difference in phase between the primary and secondary current vectors, the

direction of vectors being so chosen that the angle is zero for perfect transformer. The phase

displacement is said to be positive when secondary current leads the primary current vector

and vise versa.

The amount of exciting current drawn by current transformer depends material and the

amount of flux which must be deveploed in the core to satisfy the burden requirements of the

C.T. This is obtained from the excitation characteristics of the C.T. as secondary emf and

therefor flux developed is proportional to the product of secondary current and burden impedance.

The requirements of a protective CT differ radically from those for a measuring CT. A

measuring unit has to be accurate within the specified working range of rated current. Accuracy

is not required at high over currents. A Protective CT on the other hand, is not usually required

to be accurate below rated current, but it has to be accurate within the approximate limit at

all higher values of current up to the rated accuracy limit of primary current.

The question of using a single CT for the dual purpose of protection and measurement

an be decided by considering all relevant factors, such as design, cost, space and capability

of the instrument of withstanding high currents.

High permeability core material with a low saturation level (e.g. nickel-iron alloys) is

most suitable for measuring CT. Principle requirement of protective CT is high saturation level

(Grain oriented steel are used because they offer the advantage of very much higher knee

point flux density).

Knee Point

On the excitation characteristic is defined as the point at which a 10% increase in

secondary Emf; produces 50% increase in exciting current.

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Knee point voltage is defined as (I.S. 4201-1967) that sinusoidal voltage of rated

frequency applied to the secondary terminals of the transformer all other winding being open

circuited which when increase by 10% causes the exciting current to increase by 50%. (This

is a practical limit beyond which a specific ratio may not be maintained). Beyond knee point

the CT is said to enter saturation. In this region almost all the primary current is unlisted tomaintain the core flux and since the shunt admittance is non liner both the exciting and

secondary current depart from a sine wave.

Guide Lines for construction :

i) Core : Rectangular form built up of L shape punching. Windings are placed on one of the

shorter limbs with primary usually wound over secondary the advantage being there is

ample space for insulation space for insulation so that this form is suitable for HV work.

(a) Shell form : It gives considerable protection to winding as windings are placed on

center limb. But this form is difficult to built than other forms.

(b) Ring form : It is very commonly used when primary current is large. The secondary

winding is uniformly distributed round the ring and primary is a single bar. This is a veryrobust construction and has a further advantage of a joint-less core(Giving low reluctance)

and of very small leakage reactance.

ii) Windings : Windings should be closed together in order to produce the secondary

leakage reactance, as this increase the ratio error. The windings must be designed with a

view to withstanding without damage, the very large forces which are developed

when hort circuit occurs in the system.(Primary ring core construction is most satisfactory

from this point of view).

iii) Insulation : The windings are separately wound and are insulated by tape and varnish

for lower line voltage. For 7KV and above CTs oil immersed or compound filled. Thecompound introduces difficulty cooling is poor.

iv) Turn compensation : It is used in most CTs in order to obtain transformation ratio more

nearly equal to the nominal ratio. Usually the best number of secondary turns is on or

two less than that number, which would be making equal the nominal ratio of CT (e.g.1000/

5A bar primary, number of secondary turns would be 199 or 198 instead of 200) Phase

angle error is very little effected due to this.

Protective CTs and Measuring CTs (Common Terms)

a) Rated Primary Current: Ranges from 0.5 to 10,000 A (Unless otherwise specified, the

rated continues thermal current)

b) Thermal Rating : A rated short time thermal current (1th) for a rated time. The time

values will be 0.5,1.,2 and 3 seconds.

c) Dynamic Rating: The peak value of dynamic current (Idyn)shall not be less than 2.5

times the rated short time thermal current 1th

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d) Rated Secondary Current: The value of the rated secondary current shall be 5A. The

secondary current rating of 2A & 1A may also be used in some cases when

No. of secondary term is so low that the ratio can not be adjusted within the requisite

limits by addition or removal of one turn OR

If the length of secondary connecting lead is such that the burden due to them at thehigher secondary current would be excessive.

Relay burden = 10 VA Lead Resistance = 0.1 ohm

CT Secondary current = 5A

Total VA burden = 10+12R

= 10+(5)2 x 0.1

= 12.5

If CT secondary is 1A, then

Total VA burden = 10+(1)2 x 0.1 = 10.1

(Auxiliary CTs are some times used to reduce the current in high resistance leads butaux. CT itself imposes an additional burden of several VA on main CT and may some times

defeat the vary purpose).

e) Rated Output : The values of rated output shall be 2.5,5,7.5,10,15 and 30 VA.

Since the performance of the C.T. depends to some extent on the connected burden, it

should not be less than 25% of the rated VA as otherwise, accuracy will be effected. It

is desirable that the rated output should be as near to in value, but not less than the

actual output at which the transformer is to operate A.C.T. with a rated output considerably

in excess of the required output may result in increased error under operating conditions.

The burden comprises of individual connected load, for measuring C.T. (Ammeters

3VA;current coils of watt meters, p.f. meter 5VA).

As against this the determination of the rating of a protective CT depends on its

application, rated burden, Rated accuracy limit factor and accuracy class.

f) Rated Burden :

The burden on the protective CT is comprised of the individual burdens of the associated

trip coils and relays, the connecting leads. When the individual burden are expressed in

ohmic values, the total burden may be computed by addition. This total ohmic burden

should then be converted to a VA burden at the rated secondary current.

If the individual burdens are expressed in VA, it is essential to refer the VA burdens to acommon base (i.e. rated secondary current of a C.T.) before total burden may be computed

by addition.

Normally the standard VA rating nearest to the burden computed should be used, but

special attention should be given to a device having impedance characteristics varying

i) Constant impedance regardless of current setting (untapped relay coils).

ii) Impedance change with the current setting (Relays with tapped coils)

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iii) Impedance decreases when current passing through the coil exceeds the current setting

iv) Impedance changes with the change position of the armature of the relay or trip coil.

For (1) & (ii) above

Effective VA burden=(Ohmic burden of device )x(Rated Sec. Current of CT)2

The burden is always expressed per core of the CT depending upon requirements, CT

selected may have 2,3 or even 5 cores for EHV.

Typical Core Allocation for a 5 Core C.T.

No. of core Purpose Class of Accuracy

Core1 Metering 0.5

2 Backup Protection 5P

3 Main Protection PS

4 Bus Differential PS

5 Bus differential (check) Zone PS

g) Rated Accuracy Limit Factor

It is the ratio of highest primary current at which the CT will comply with the appropriate

limits of composite error under specified condition to the rated primary current. The

capabilities of a protective CT are determined by primary Amp. Turns, the core dimensions

and material and they are roughly proportional to the product of the rated burden and

the rated accuracy limit factor (with present day materials and normal dimensions this

product has a maximum value of 150).

h) Effect of Internal Burden

In determining the rated accuracy of the limit factor, the effect of internal burden, which

is mainly resistive should not be overlooked, particularly when the connected burden is

low, say less than 3 VA.

i) Co-relation of burden and accuracy limit factor

A.L.F. higher than 10 and rated burden higher than 15 VA are not recommended for

general purpose use. When product of these two exceeds 150, the resulting C.T. may be

uneconomical or of unduly large dimension of both. It is important to not that for a given

protective C.T. the accuracy limit factor and the burden as interrelated, that is the

decrease in the burden will automatically increase its accuracy limit factor and vise-

versa.

e.g. 15 VA, class 5P 10 CT means,

Error will be within prescribed limits, up to 10 times primary rated current when secondary

burden is 15 VA. If accuracy is required for 20 times primary current VA burden should be

7.5 VA only.

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30/31

Measuring CT-application Class

1) Precision (Laboratory CT) 0.1

2) Substandard for testing of industrial CT 0.2

3) Precision industrial metering 0.5

4) Commercial & Industrial metering 0.5 or 1

5) Indicating & graphic watt meters & ammeters 1 or 3

6) Purpose where ratio is of less importance 3 or 5

(in ammeters where approx. values are required)

Factors affecting Choice of protective C.T.

1) CT saturation may cause harmonics, which may increase the time of operation of IDMTL

relays. So where fault current is adequate, A.L.F. may be chosen 20. Alternatively a

relay with low VA burden or a CT with higher ratio may be chosen.

2) If CTs with higher A.L.F. & VA output than required is selected, under fault condition CTsmay be able to produce higher secondary current, resulting in heating of relay coil. This

factor is important, when current operated relays with time delay are used, as the relay

coil may burn before the fault is cleared.

3) For distance protection if CTs are not of adequate knee point voltage, it may result in

producing higher operating time. To allow for transient saturation, a transient saturation

factor (X/R) of primary system should be considered. While calculating knee point voltage.

4) For balanced protective system (e.g. differential and restricted E.F protection), it should

be confirmed that under external faults, CT saturation and mismatch does not produce

imbalance in the relay operating circuit. At the same time under internal faults, CTs

should produce adequate output to ensure that operating time of the relay is not exceeded.

5) CTs should not consume excessive magnetising current. If this is high, it may result inhigher primary fault settings in case of current operated relays and may cause under

reaching in case of distance relays. However, a very low value of magnetising current

should not be specified which may result in larger and costly C.T.

Over Current Rating of a C.T.

The mechanical stresses produced in a CT under o/c conditions depend upon maximum

peak value of the o/c, the number of turns in the primary winding and configuration of the coil

structure. For minimum stress, other things being equal the primary winding of the transformer

should have the minimum number of turns and minimum mean perimeter. The lower the product

of rated A.I.F. and rated burden, the stronger can the transformer be made mechanically and

higher may be its O.C.F. (over current factor). In general, CT may have an O.C.F. of 50 to 100based on the rated time of 0.5 second. CTs may be designed of OCF 200 to 400 (bar primary is

a must). The value of the over current should preferably be determined by short circuit study

of a system, in which the CT is to be installed. Alternatively the O/C rating of CT should be

determined from a consideration of the breaking capacity of the ci4cuit breaker with which it is

associated. Failure to observe these precautions may lead to destruction of CT under short

circuit conditions.

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e.g. CTR-50/5, 11 KV, Circuit Breaking cap. Of C.B. 150,000 KVA

150,000

RMS Value of short current = = 7900 A

11 x 3

7900O.C.B. = = 158

50

The transformer should be also capable of with standing peak value of current equal to 2.5 x

ay also correspond to

Duration of short circuit Permissible O/C r.m.s. in amps.

0.5 sec. 7900 A

1.0 5600

2.0 40005.0 2500

CTs 3 phase circuits : Secondary may be connected in star of delta.

The secondary may be connected in star or delta. In case of star connection, there is

no neutral current under balanced load condition. The neutral current appears only in the

event of an earth fault. Delta connected C.T.s are primarily used in case of differential protection

for shunting zero sequence currents on grounded star windings of transformers.

Auxiliary CTs: The use of auxiliary CTs may be made in watt meters and relay circuits. It is

recorded that auxiliary CTs be used to step up currents from main CTs (Burden on the main CTs

in table may be excessive).

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