lecture3 ct & vt

8/10/2019 Lecture3 CT & VT

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CURRENT AND

VOLTAGE

TRANSFORMERS


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Voltage transformers:

The function of voltage transformers isgrouped as:

Measurement VTs- Limits well defined. Protection VTs.

- Operation over wide range of

voltages.

They have different characteristics.


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They may be single phase or three phases.VTs are necessary for voltage, directional,and distance protection.

The primary of VT is connected directly topower circuit between phase and grounddepending upon rated voltage and

application. The volt- ampere rating of voltage

transformer is lesser as compared with thatof power transformers.



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There are two types of constructions:

Electromagnetic potential transformer inwhich primary and secondary are

wound on magnetic core.

Capacitor potential transformer, inwhich the primary voltage is applied to

to series capacitor group.



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Specifications for VTs:

Rated primary voltage

Rated secondary voltage

Rated burden Supply frequency

Number of phases

Class of accuracy

Insulation level, power frequency andimpulse voltage withstand.

Limits of dimensions, types ofconstruction,


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Rated characteristics:

Rated voltage.

The voltage marked on the rating plate ofthe VT. There are several values of

standard primary voltages. (11 kV, 33 kV,132 kV, 400 kV)

Rated transformation ratio:

The ratio of rated primary voltage to ratedsecondary voltage.

Rated secondary voltage:It is the value of secondary voltage markedon name plate. For example: 110/3 V


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Residual voltage:

Vector sum of three lines to earth

voltages, i.e

Vres = Vrn + Vyn +Vbn

Phase error:

The phase angle between primary voltage

vector and the reversed secondary voltagevector (measured in minutes).


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Ratio error:

Percentage ratio error = 100(KnVs - Vp)/ Vp

Where Kn = normal ratio

Vs = secondary voltage

Vp= primary voltage

Rated burden:

The burden assigned by manufacturer at which theV.T performs with specified accuracy.


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Site testing of VTs

Ratio test Insulation test

Resistance test

On load test


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Sample of low voltage VT


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Current transformers:

The function of current transformers isgrouped as:

Measurement CTs- Limits well defined.

Protection CTs.- Operation over wide range of

currents.

They have different characteristics.


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Current transformers:

The current transformers(CT) step down AC current

to a low value (1 A, 5A).The volt- ampere rating of

current transformer is lesseras compared with that ofpower transformers.


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Specification for CTs:

Rated primary current

Rated secondary current

Rated burden

Supply frequency Number of phases

Class of accuracy

Insulation level, power frequency and

impulse voltage withstand. Limits of dimensions, types of

construction,


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Rated primary current.

The value of primary current on which the performanceof the current transformer is specified by themanufacturer. (rms value)

Rated short time current (Primary)

It is defined as r.m.s value of a.c component which theCT can carry for rated time without damage due tothermal or electrodynamics stresses.

Rated secondary current:

It is the value of secondary current marked on nameplate.


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Phase error:

The phase angle between primary current vectorand the reversed secondary current vector(measured in minutes).

Ratio error:

Percentage ratio error = 100(KnIs - Ip)/ Ip

Where Kn = normal ratio

Is = secondary current

Ip = primary current Rated burden:

The burden assigned by manufacturer at which theC.T performs with specified accuracy.


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Accuracy class

The class assigned to the current

transformer with the specified limitsof ratio error and phase angle error.

(5P20)

Rated exciting currentThe r.m.s value of current taken by

the secondary-winding of a C.T.


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Site testing of CTs

Ratio test

Insulation test

Resistance test

Magnetizing current test

Polarity test

On load test


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Sample of low voltage CT

lecture3 ct & vt

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