Maquette 1

Magnetic circuit (core)

Instrument Transformer

U1 U2

Secondary Windings

Primary Windings

Maquette 2

Instrument Transformer

I2

I1

Maquette 3

K : Transformation

ratio

PrimaryU1

Secondary

U2

Voltage Transformer :

Kn

n

U

U

2

1

2

1 1n

2n

Definition of an Instrument Transformer

Maquette 4

PrimaryI1

Secondary I2

Current Transformer :

Kn

n

I

I

I.nI.n

1

2

2

1

2211

Ampere-TurnsConservation

Law

1n

2n

Definition of an Instrument Transformer

K : Transformation

ratio

Maquette 5

K.Us

Instrument Transformer Error

Secondary

Primary

Up

Us

K

:Ratio error

Up (theoretically = k.US)

: Phase error

Voltage Transformer :Voltage Transformer :

s

p

U

Uk

Maquette 6

Secondary

PrimaryIp

Is

K

TC : K=Ip

Is

K.Is :Ratio error

Ip

: Phase error

Current transformer :Current transformer :

Instrument Transformer Error

Maquette 7

1. Oil filling plug

2. Dome

3. Nitrogen filling valve

4. Collar

5. Primary terminal

6. Porcelain insulator

7. Insulated primary

8. Cover plate for tank

9. Tank

10. Secondary cores

Internal details

Eye bolt design

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Manufacturing Process

IT range

Primary steel pipe

Paper insulation

Seconday cores

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Active Part Manufacturing

IT range

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1. Dome

2. Nitrogen filling valve

3. Primary terminal

4. Collar

5. Porcelain insulator

6. Primary conductor with insulation

7. Adaptor cylinder

8. Secondary cores

9. Base

10. Oil drain plug

Hair Pin Design

Hair-Pin design

IT 400 Cross section

Maquette 11

Class

e

Ratio error

at In

Phase

displacemen

t at In

Composite

error at

fL.IN

5 P 1 % 60 min. 5 %

10 P 3 % 10 %

Current TransformersProtection accuracy classes

CEI 600 44-1CEI 600 44-1

Accuracy classes (Protection)Accuracy classes (Protection)Maximum current error in % of IP

Accuracy limit primary current = fL.IN

Burden :SN

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Factors for Protection

Parameters

1. ALF ( accuracy limiting factor)

2. Composite error

Over Current and Earth Fault Protection

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Factors for Protection

1. Accuracy Limiting Factor

What is Accuracy Limiting factor ?

It is the factor of over current above the rated current which determines the capability of CT to maintain the error at such a condition.

2. Composite error

It is the error of the CT when this over current is applied.

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Composite error : Under steady-state conditions, the r.m.s. value of the difference between:a) the instantaneous values of the primary current, andb) the instantaneous values of the actual secondary current multiplied by the rated transformation ratio

CEI 600 44-1CEI 600 44-1

T

psnp

c dtiIKTI 0

2.1100

Current TransformersProtection accuracy classes

Kn is the rated transformation ratio;Ip is the r.m.s. value of the primary current;ip is the instantaneous value or the primary current;is is the instantaneous value of the secondary current;T is the duration of one cycle.

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Factors for Protection1. Accuracy Limiting Factor/composite error

For e.g if the class designation is 5P20

20 is the Accuracy limiting factor which signifies that when 20 times the rated primary current is applied the compositeerror of 5P( +/- 5%) is maintained.

Typical Class designations are

5P10, 5P20, 10P10, 10P20 etc.,

Maquette 16

Factors for Protection

Parameters

1. Knee Point Voltage requirement(Vk)

2. Exciting current ( Io)

3. Resistance of CT (Rct)

Transformer and Busbar Differential Protection

Maquette 17

Current TransformersSaturation curve

10 20 30 40

InductionB [T]

Primary currentIp/In

Protection CT

18500 gauss

Metering CT

8000 gauss

Maquette 18

Factors for Protection1. Knee Point Voltage(Vk):

Knee point voltage is point beyond which an application of 10% of voltage increases the exciting current by 50%.

The typical equations for Vk ( based on relay used)

Vk > 24 In (Rc t+2RL)- for Transformer Differential

Where In: Relay rated currentRL = Total lead ResistanceRc t = CT secondary resistance

Maquette 19

Current TransformersType Tests

IEC 600 44-1IEC 600 44-1

Type tests

a) short-time current tests

b) temperature rise test

c) lightning impulse test

d) switching impulse test

e) wet test for outdoor type transformers

f) determination of errors

Maquette 20

Current TransformerRoutine tests

CEI 600 44-1CEI 60-1

CEI 600 44-1CEI 60-1

Routine tests

The following tests apply to each individual transformers:

a) verification of terminal markings

b) power-frequency withstand test on primary winding

c) partial discharge measurement

d) power-frequency withstand test on secondary windings

e) power-frequency withstand tests, between sections

f) determination of errors

The order of the tests is not standardized, but determination of

errors shall be performed after the other tests.

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CT Failure and remedial action

Remedial actions in CT at site to avoid failures

• Tangent delta and Capacitance measurement from the C terminal at periodic intervals once in three years or during shutdown.

• Dissolved gas analysis of oil taken out from CT alteast once in five years.

• Thermo vision scanning of CTs of rating 400kV ( or above).

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Partial discharge test COMPARISON OF EXISTING AND NEW METHOD OF PARTIAL

DISCHARGE MEASUREMENTSL STANDARD PRE STRESS VOLTAGE PD MEASURING LIMITS NO VOLTAGE OF PD

1 EXISTING STANDARD 0.8*1.3*Um For One Minute 1.1Um/sqrt3 <10pCIS:2705 eg 151kV for 132kV system 92 kv for 132kV

2 NEW STANDARD 80% of Power frequency 1.2Um/Sqrt3 <5pCIEC 60044-1 voltage for one minute 100.5kVCl 5.1.2.2 & 8.2.2.2 220kV for 132kV system Um = 145kV <10pC

Where Um is the Highest system voltage of the CT.

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Multiple Chopped Impulse test

Application of 100 chopped impulses of negative polarity on CTs of ratings above 300kV. These impulses will be applied at the rate of one impulse per minute. The test Voltage shall be 60% of the rated lightning impulse voltage

Before the test and three days after the test the dissolvedgas analysis of oil taken from CT will be carried out.Analysisprocedure and fault diagnosis shall be as per IEC 60599.

As per IEC 60044-1 ( 2002)

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Special test

A. Thermal Stability test : This involves simultaneous application of rated voltage (1.1Um/Sqrt3) and rated simultaneous current (1.2,1.5 etc) by using a synthetic test circuit.Capacitance, tangent delta, secondary resistance and temperature of primary terminal are recorded until stable values are acquired.

This test demonstrates the insulation capacity ( healthiness)under energised conditions.

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Special test

B. Temperature coefficient test: The CT is heated in a oven to approximately 90Deg C. The tan delta is measured at ambient, 80 and 90 deg C at voltages of 0.3,0.7,1.0 and 1.1Um/Sqrt3.

This test demonstrates the healthiness at high extreme temperature conditions.

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Accurate Measurement of HV

Capacitor Voltage Transformers Coupling Capacitors CCV / CC 72.5 to 765 kV Line protection HF transmission Compliance with IEC, ANSI or equivalent

standards Reduce the slope of the RRRV (Rate of

Rise of Recovery Voltage (Ik >40 kA)

Maquette 27

Capacitor Voltage TransformerDefinitions

Element Pack(or pack)

Element Pack(or pack)

Pile of elements : ± 10 to 25 kV

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Capacitor Voltage TransformerDefinitions

Assembly of elements in an insulating container: ± 245 kV.Can be connected to a HV line

Capacitor Unit(or unit)

Capacitor Unit(or unit)

HV Power line

Ground

Maquette 29

Capacitor Voltage TransformerDefinitions

Capacitor STACK(or stack)

Capacitor STACK(or stack)

Assembly of elements to reach higher voltage levels : ± 800 kV

HV Power line

Ground

In general, the termCAPACITOR

stands for a capacitor elementas well as a capacitor stack.

Maquette 30

Capacitor Voltage Divider( or CVT )

Capacitor Voltage Divider( or CVT )

Connecting an MV inductive voltage transformer to a tap.

HV Power line

Ground

Maquette 31

IntermediateVoltage :

10 to 20 kV/3

S1

S2

P1

P2

C1

C2

Maquette 32

Lµ = Inductance equivalent to magnetic losses of the magnetic circuit.

Rw = Resistance equivalent to the watt losses of the magnetic circuit.

Lfs = Secondary leakage inductance of the magnetic VT.

Rs = Resistance of the secondary winding

Ce La LfsRa Rs

Lμ Rw ZcUsU’P =Vp . C1

k . (C1+C2)

Equivalent DiagramEquivalent Diagram

Maquette 33

Capacitor stack

Inductive VT

Maquette 34

CCV 72.5 to 765 kV

Capacitor elements

Capacitor column

Insulating oil

Insulator flange

Secondary terminal box

Inductance

MV Transformer

Oil expansion device

Damping circuit

Maquette 35

Capacitor Voltage Transformer Capacitor Elements

All Paper Dielectric design Mixed dielectric design

Paper PPR film + paper

Copper tabs forconnection

Aluminum foilfolded for contactwith next element

Maquette 36

ExcessiveFrequencyVariations

Nominal burdenOVER-ESTIMATEDReal load < 25% VAN

Nominal burdenUNDER-ESTIMATEDReal load > 100% VAN

Capacitor Voltage TransformerTypical Error Curve

Typical graph of variation of errors with VA & frequencyTypical graph of variation of errors with VA & frequency

Phase error

Ratio error

f

25% VA

f+f

f -f100% VA

+20’- 20’

+0.5

- 0.5

Maquette 37

Voltage transformersProtection accuracy classesIEC 600 44-2IEC 600 44-2

Accuracy classes (Protection)Maximum error in % of VP

- Voltage between 5 % and fT x VNP

- Burden between 25% and 100% of SN

- Maximum error doubled for VNP=2%

Accuracyclass

Voltage(ratio)error

Phasedisplacement[minutes]

Cl 3P 3 % 120

Cl 6P 6 % 24O