(1) Ct Vt Parameters

CT & VT PARAMETERS

By SUDHIR PATHAK

Profile Passed B.Tech in Electrical Engg.(with honors) from IT-

BHU, Varanasi in 1994. Worked with Telco,Jamshedpur from July 1994 till

Dec’95. Joined NTPC in Dec’95. Worked in Electrical Erection and carried out erection of

Switchyard & Generator Protection, Switchyard Equipments, Excitation system, Generator Bus-Duct for two and half years from year Jan ’97 till July’99 at NTPC Unchahar.

Carried out commissioning & maintenance of Switchyard & Generator Protection, Switchyard Equipments, Excitation system for about four years from Aug’99 till April’03 at NTPC Unchahar.

Working in the Protection Group at NTPC,EOC since May’03

CT PARAMETERS

FUNDAMENTALS

WHAT IS CT? WHEN IS CT REQUIRED ? WHY IS CT REQUIRED? HOW IS CT CONNECTED? WHERE IS CT CONNECTED? WHICH CT IS CONNECTED?

WHAT IS CT?

IT’S AN INSTRUMENT TRANSFORMER WHICH TRANSFORMS CURRENT FROM ONE LEVEL TO ANOTHER LEVEL SUCH AS 1000/1 (CT RATIO) i.e. TRANSFORMS CURRENT OF THE LEVEL OF 1000 AMPS INTO CURRENT OF 1 AMP LEVEL.

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WHEN IS CT REQUIRED ?

ELECTRICAL SYSTEMS IN WHICH LARGE AMOUNT OF CURRENT ( TO THE TUNE OF 100AMPS OR MORE) FLOWS , DIRECT MEASUREMENT OF THAT CURRENT IS NOT POSSIBLE AS DEVICES USED FOR MEASUREMENT OF CURRENT ARE NOT DESIGNED TO HANDLE SUCH HUGE AMOUNT OF CURRENT.

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WHY IS CT REQUIRED? THE SYSTEM WHICH CARRIES CURRENT

MEANS THERE IS A SOURCE WHICH INJECTS THE CURRENT INTO THE SYSTEM AND THERE IS A LOAD WHICH CONSUMES THE CURRENT (OR POWER/ENERGY).SUCH A SYSTEM HAS TWO BASIC REQUIREMENTS: METERING OF ENERGY SOURCED OR

CONSUMED. PROTECTION OF THE ELECTRICAL

SYSTEM FROM FAULTS AND DISTURBANCES. back

HOW IS CT CONNECTED?

CT HAS A PRIMARY AND ONE OR MORE SECONDARY WINDINGS. SECONDARY WINDING IS WOUND AROUND THE MAGNETIC CORE. METERING AND PROTECTION DEVICES ARE CONNECTED TO THE SECONDARIES OF THE CT.

HOW IS CT CONNECTED?contd

M/R

PRIMARY

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P1

P2

S1

S2

WHERE IS CT CONNECTED?

FOR METERING AND PROTECTION OF A FEEDER, CT IS CONNECTED AT THE BEGINNING OF THE FEEDER.

WHERE IS CT CONNECTED?

contd

Power Station to be protected

back

Unit prot

Unitprot

Non-unitprot

meter meterprot meterDistprot

One prim one sec

One prim two or more sec

WHICH CT IS CONNECTED? METERING: IF WE WANT TO MEASURE

CURRENT FOR METERING PURPOSE, WE DESIRE THAT: WHATEVER CURRENT WE MEASURE, THAT

SHOULD BE VERY ACCURATE AS THE METERED DATA MAY BE USED FOR TARIFF PURPOSE i.e. MONEY EXCHANGE IS INVOLVED AMONG VARIOUS PARTIES. MOREOVER, THE DATA IS USED FOR DECISION MAKING SUCH AS DECISION ON RAISING/LOWERING OF POWER OUTPUT etc.

WHICH CT IS CONNECTED?Contd metering

WHAT IS INACCURACY? THE SECONDARY CURRENT WHICH WE

GET IS NOT TRUE REFLECTION OF ITS PRIMARY CURRENT. FOR EXAMPLE, FOR A CT WITH CT RATIO OF 1000/1AMPS, IF WE GET 0.99 AMPS IN SECONDARY LEADING PRIMARY CURRENT BY 15 MINUTES FOR PRIMARY CURRENT OF 1000AMPS, SO THE CT HAS RATIO ERROR OF (0.99-1)/1 x 100=-1% AND PHASE ERROR OF 15 MINUTES.


CURRENT OR RATIO ERROR AS PER IEC IS:


NOW SECOND QUERY WHICH COMES IN MIND IS WHY AT ALL CTS ARE INACCURATE?

THE CULPRIT IS CORE LOSS AND MAGNETISING CURRENT, WHICH INTRODUCES RATIO AS WELL AS PHASE ERROR.


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THE INTRUMENT CONNECTED TO THE SECONDARY ESPECIALLY ANALOG INDICATING METER SHOULD NOT GET DAMAGED DURING PRIMARY FAULT CONDITION. FOR THIS INSTRUMENT SECURITY FACTOR (FS) IS DEFINED WHICH IS RATIO OF MINIMUM PRIMARY CURRENT AT WHICH COMPOSITE ERROR OF THE CT (AT RATED BURDEN) IS EQUAL TO OR GREATER THAN 10% OF RATED PRIMARY CURRENT.

THE TYPICAL VALUES ARE 5,10 etc.

WHICH CT IS CONNECTED?contd

COMPOSITE ERROR:


IEC60044-1 HAS LAID DOWN STANDARDS ON THIS:

25 25 25


FOR ACHIEVING ABOVE, A CT IS CHOSEN HAVING VERY HIGH PERMEABILITY AND HIGH REMANENCE


HYSTERISIS CURVE OF METERING CORE

H

Ψs

Ψr∆Ψ


THE ABOVE CT PARAMETERS ARE THEREFORE SPECIFIED AS FOR EXAMPLE: CTR:1000/1,0.2FS5 ,30VA etc.

IN NTPC, WE TYPICALLY SPECIFY, CTR: 2000/1, 0.2,FS 5, 20VA


PROTECTION CLASS:

HERE, MAIN REQUIREMENT IS ABILITY OF CT TO FAITHFULLY TRANSFORM THE PRIMARY CURRENT DURING FAULT CONDITION.

WHICH CT IS CONNECTED?Contd protection PR

DURING FAULT CONDITION, VALUE OF PRIMARY CURRENT MAY BE 10 TO 20 TIMES THE RATED PRIMARY CURRENT. PROTECTIVE RELAY BURDEN IS CONNECTED TO THE CT SECONDARY. AT SUCH HIGH LEVEL OF PRIMARY CURRENT, IF CT IS NOT PROPERLY DESIGNED, IT MAY SATURATE AND RELAY WILL RECEIVE VERY LESS CURRENT AND, THEREFORE, DOES NOT TAKE RIGHT DECISION.


FOR NON-UNIT TYPE PROTECTION SUCH AS O/C TYPE OF PROTECTION, CLASS PR TYPE CTs ARE USED.

THE PARAMETERS THAT ARE DEFINED FOR THE CT ARE:

STANDARD ACCURACY LIMIT FACTOR(SALF):

= RATED ACC. LIMIT PRIMARY CURR. RATED CURRENT


WHEREAS, RATED ACCURACY LIMIT PRIMARY CURRENT IS THE VALUE OF THE PRIMARY CURRENT UPTO WHICH CT WILL COMPLY WITH THE REQUIREMENT OF COMPOSITE ERROR.

STANDARD VALUES OF SALF ARE 5,10,15,20,30


ACCURACY CLASS: TELLS ABOUT %AGE COMPOSITE ERROR LIMIT AT SALF ,RATED FREQUENCY AND RATED BURDEN CONNECTED (at 0.8 pf). STANDARD VALUES ARE 5,10.


RATED BURDEN: DEPENDING ON THE LOAD (VA) CONNECTED TO THE CT, RATED VA BURDEN (AT 0.8Pf) IS DEFINED.

REMANENCE FACTOR FIXED TO 10%.

PROTECTION CLASS CTs HAVE LOW REMANENCE. AS HIGH REMANENCE CTs WILL SATURATE FASTER.


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HYSTERISIS CURVE FOR PROTECTION CTs


THE ABOVE CT PARAMETERS ARE THEREFORE SPECIFIED AS FOR EXAMPLE: CTR:1000/1, 5PR20 @ 30VA etc.

IN NTPC, FOR GENERATOR PROTECTION WE TYPICALLY SPECIFIY CTR 10000A/5A,5PR20 @ 75VA

WHICH CT IS CONNECTED?Contd protection

FOR UNIT TYPE PROTECTION:

HERE , REQUIREMENTS ARE RATHER STRINGENT AS WE COMPARE CURRENT OF TWO OR MORE CTS AND RELY ON THE THEIR MUTUAL FAITHFULL-NESS. MOREOVER, OUR AIM IS THAT THE PROTECTION MUST BE STABLE FOR EVEN WORST THROUGH FAULT AND FAST ACTING FOR INTERNAL FAULT.

WHICH CT IS CONNECTED?Contd protection PX

Internal Fault

Substation to be protected

External/Through Fault


I1 I1

i1 i1’

i1 – i1’

Equipment to be protectedI2

i2i1 + i2Stabilising

Resistance

Operating relay

UNIT PROTECTION


FOR THIS PX CLASS OF CTs ARE NEEDED (THESE CTs ARE SIMILAR IN ALL RESPECTS TO CLASS PS OF IS-2705 AND CLASS-X OF BS-3938 WITH SOME ADDITIONAL FEATURES)


THE PARAMETERS WHICH ARE DEFINED IN THIS CT ARE: KNEE POINT VOLTAGE (KPV):

That minimum sinusoidal voltage (r.m.s.) at rated power frequency when applied to the secondary terminals of the transformer, all the terminals being open-circuited, which when increased by 10% causes the r.m.s. exciting current to increase by no more than 50%.


Ie

V


Ie

V

∆V

∆Ie

KPV


FOR WORST THROUGH FAULT, CTs SHOULD NOT GET SATURATED.WHEN ONE OF THE CTs GETS SATURATED, LARGE AMOUNT OF CURRENT MAY FLOW THROUGH DIFFERENTIAL CIRCUIT AND RESULT IN OPERATION OF RELAY.HOWEVER, IF RELAY IS SET ABOVE THIS VALUE

i.e. Iset= Ifx (Rct+2xRl), Rstab


I1 I1

i1 i1’

i1 – i1’

Equipment to be protectedI2

Stabilising Resistance

Operating relay

UNIT PROTECTION (HIGH IMPEDANCE)

SATURATED CT


THEN, RELAY MAY NOT OPERATE FOR INTERNAL FAULT IF ONE OF THE CT GETS SATURATED FOR INTERNAL FAULT. THEREFORE, KNEE POINT VOLTAGE IS DEFINED.FOR HIGH IMPEDANCE UNIT PROTECTION, THIS IS USUALLY TAKEN AS:

KPV=2x If (Rct+2xRl)

WHICH CT IS CONNECTED?contd protection PX

FOR LOW IMPEDANCE UNIT PROTECTION:

KPV=Kx In (Rct+2xRl), WHERE Kx IS THE DIMENSIONING FACTOR

WELL, THESE ARE RELAY APPLICATION AND SYSTEM X/R SPECIFIC.


Eqpt to be protected

UNIT PROTECTION (HIGH IMPEDANCE)


MAGNETISING CURRENT AT KPV OR %AGE THEREOF:

TWO OR MORE CTs USED FOR UNIT PROTECTION SHOULD WORK LIKE CLONE BROTHERS i.e. FOR THROUGH FAULT CONDITION, THE SPILL CURRENT SHOULD BE IDEALLY ZERO. BUT PRACTICALLY, THIS IS NOT THE CASE.


IF TWO CTs HAVE DIFFERENT

MAGNETISING CURRENT AND HIGH LEAKAGE REACTANCE, IT WILL INTRODUCE HIGH CURRENT AS WELL AS PHASE ERROR.THUS, RESULTANT CURRENT OF THESE CT SECONDARIES MAY LEAD TO HIGH SPILL(DIFFERENTIAL) CURRENT DURING THROUGH FAULT.THIS MAY LEAD TO PROTECTION OPERATION DURING THROUGH FAULT.


CT DC RESISTANCE AT 75 deg C(Rct): THIS VALUE IS VERY IMPORTANT FROM THE

POINT OF VIEW OF KPV CALCULATION AS IT IS ONE OF THE LIMITING FACTORS TO THIS. THEREFORE, IT IS DEFINED.

TURN RATIO ERROR: LIMITED TO 0.25%.


THE ABOVE CT PARAMETERS ARE THEREFORE SPECIFIED AS FOR EXAMPLE:

CTR: 1000/1 ,PX , KPV=1000V, Ie=30mAmp @ KPV/2, Rct< 5OHMS @75 deg C etc. (TYPICAL TO NTPC)

VT PARAMETERS

FUNDAMENTALS

WHAT IS VT? WHEN IS VT REQUIRED ? WHY IS VT REQUIRED? HOW IS VT CONNECTED? WHERE IS VT CONNECTED? WHICH VT IS CONNECTED?

WHAT IS VT?

IT’S AN INSTRUMENT TRANSFORMER WHICH TRANSFORMS VOLTAGE FROM ONE LEVEL TO ANOTHER LEVEL SUCH AS 400KV/√3:110V/√3 (VT RATIO) i.e. TRANSFORMS VOLTAGE OF THE LEVEL OF 400KV/√3 INTO VOLTAGE OF 110V/√3 LEVEL.

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WHEN IS VT REQUIRED ?

ELECTRICAL SYSTEM WHICH HAS HIGH LEVEL OF VOLTAGE ( TO THE TUNE OF 3.3KV OR MORE) , DIRECT MEASUREMENT OF THAT VOLTAGE IS NOT POSSIBLE AS DEVICES USED FOR MEASUREMENT OF VOLTAGE ARE NOT DESIGNED TO HANDLE SUCH HIGH LEVEL OF VOLTAGE.

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WHY IS VT REQUIRED?

THE SYSTEM WHICH CARRIES VOLTAGE AND CURRENT MEANS THERE IS A SOURCE WHICH INJECTS THE POWER INTO THE SYSTEM AND THERE IS A LOAD WHICH CONSUMES POWER/ENERGY.SUCH A SYSTEM HAS TWO BASIC REQUIREMENTS: METERING OF ENERGY SOURCED OR

CONSUMED. PROTECTION OF THE ELECTRICAL

SYSTEM FROM FAULTS AND DISTURBANCES.

WHY IS VT REQUIRED? Contd

FAULTS CAN BE MANY KINDS. SOME FAULTS SUCH AS O/C CAN BE DETECTED SOLELY ON CURRENT MEASUREMENT, BUT CURRENT DOES NOT PROVIDE DISCRETION ABOUT NATURE AND LOCATION OF THE FAULT. FOR HIGHLY INTERCONNECTED ELECTRICAL SYSTEM CARRYING HUGE AMOUNT OF POWER, SUB-SYSTEM ISOLATION SELECTIVITY IS IMMENSLY DESIRABLE.

WHY IS VT REQUIRED? Contd

• THEREFORE, WHEN VOLTAGE IS ALSO MEASURED ALONGWITH CURRENT DURING FAULT, WE CAN IN A WAY COMPUTE POWER OR IMPEDANCE OF SYSTEM ALONGWITH ITS DIRECTION.

• MOREOVER, O/V,U/V, O/F AND U/F PROTECTIONS ARE ALSO CONFIGURED FROM VTs.

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HOW IS VT CONNECTED?

VT HAS A PRIMARY AND ONE OR MORE SECONDARY WINDINGS.

METERING AND PROTECTION DEVICES ARE CONNECTED TO THE SECONDARIES OF THE VT.

HOW IS VT CONNECTED?contd

M

P

P

INDUCTIVE VOLTAGE TRANSFORMER

HOW IS VT CONNECTED? contd

back

M

P

P

CAPACITIVE VOLTAGE TRANSFORMER

WHERE IS VT CONNECTED?

FOR METERING AND PROTECTION OF A FEEDER, VT IS CONNECTED AT THE BEGINING OF THE FEEDER.

WHERE IS VT CONNECTED?

contd

Power Station to be protected

back

WHICH VT IS CONNECTED?

AS STATED FOR CT, WE NEED IT FOR

METERING: VOLTAGE MEASUREMENT, ENERGY, POWER MEASUREMENT.

PROTECTION: FOR DISTANCE PROTECTION, O/V,U/V, O/F AND U/F PROTECTIONS, FIELD FAILURE, OVER-FLUXING,etc.

WHICH VT IS CONNECTED?contd

FOR METERING VTs WE NEED HIGH ACCURACY IN THE VOLTAGE MEASUREMENT DURING STABLE CONDITIONS i.e 80% TO 120% OF NOMINAL SYSTEM VOLTAGE WITH BURDENS FROM 25% TO 100% OF RATED BURDEN AT POWER FACTOR OF 0.8. IN VTs ALSO AS IN CTs, RATIO AS WELL AS PHASE ERRORS ARE THERE.


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WHICH VT IS CONNECTED?Contd metering

IEC 60044-2 AND 60044-5 DEFINES THIS AS :

WHICH VT IS CONNECTED?Contd metering

THE ABOVE VT PARAMETERS ARE THEREFORE SPECIFIED AS FOR EXAMPLE: PTR: 400KV/√3:110V/√3,0.2, 50VA etc (TYPICAL TO NTPC)


FOR PROTECTION VTs WE NEED FAITHFULNESS OF VOLTAGE MEASUREMENT IN THE HIGHER RANGE OF VOLTAGE SUCH AS FROM VALUE AS LOW AS 2% OF NOMINAL VOLTAGE TO THE RATED VOLTAGE MULTIPLIED BY RATED VOLTAGE FACTORS SUCH AS 1.2,1.5,1.9 WITH BURDEN OF 25% TO 100% OF RATED BURDEN AT 0.8pf LAGGING.

WHICH VT IS CONNECTED?contd protection

IEC 60044-2 AND 60044-5 DEFINES THIS AS :

WHICH VT IS CONNECTED?Contd protetcion

THE ABOVE VT PARAMETERS ARE THEREFORE SPECIFIED AS FOR EXAMPLE: PTR: 400KV/√3:110V/√3,3P, 50VA etc (TYPICAL TO NTPC)

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(1) Ct Vt Parameters

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