63367301 Current Transformer

7/30/2019 63367301 Current Transformer

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Electrical and electronics(e.x)

Shri ram institute of

technology

Current

Transformer


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Overview Introduction to Current transformer (C.T.)

Need of C.T.

Equivalent circuit of a C.T. Specification & accuracy.

Applications.


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Schematic diagram of a transformer


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Introduction Current transformer is a

device used to measure

large primary currents

to smaller easy to

measure secondary

currents.

It has a few turns of finewires on the primary

winding and large

number of turns on the

secondary winding.


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Introduction The primary winding is placed in the line and the

secondary is closed through an ammeter.

A C.T. can operate with its secondary under nearlyshort circuited condition.

If in case the ammeter is to be removed, then thesecondary winding is Short-circuited.

NOTE- If the secondary is not short-circuited, then due

to absence of Ampere-turn of the secondary, theunopposed primary mmf will set up and a high flux inthe core will produce high core loss along with aheating effect and damages the transformer.


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In power distribution system, high voltage andhigh sensitivity of current is always present.

NEED FOR A CURRENT

transformer:-

FOR THIS SPECIAL

INSTRUMENTS ARENEEDED.


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NEED FOR A CURRENT

TRANSFORMER:-

INSTRUMENtWI

TH HIGH

VOLTAGE &CURRENT

PROBLEMS

SoLuTiOn

To isolate instrumentsfrom power circuits

To reduce current to lower

level to be measured more

easily.


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Equivalent circuit of a current

transformer


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Specification of a C.T.

The specification covers manufacture, test, &supply of Current transformers & theiraccuracy:-

The CTs shall be of ring type or windowtype (bar type or bus-bar type CTs shall notbe accepted).

The secondary leads shall be terminated withTinned Cooper rose contact terminals witharrangements for sealing purposes.


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Polarity (both for primary and second leads)shall be marked.

The CTs shall be varnished, fiberglass tapeinsulated or cast resin, air-cooled type. Only

super enameled electrolytic grade copper wires

shall be used.

The CTs shall conform to IS 2705:Part-I &

II/IEC:185 with latest amendments.


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Technical details:-

1.Class of accuracy 0.5

2.Rate of burden 5 VA

3.Power frequency

withstand voltage

3 KV

4.Highest system voltage 433 V

5.Nominal system

voltage

400 V

6.Frequeny 50 Hz

7.Supply system 3-Phase solidly grounded

neutral system


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Ratio 50/5 150/5 300/5 400/5 1000/5

Transformation ratio

Accuracy

Accuracy

class

% Ratio error

at % Rated

current

Phase

displacement

in minutes at

% ratedcurrent

0.5

5 20 100 120

1.5 0.75 0.5 0.5

5 20 100 120

90 45 30 30


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ApplicAtions of c.ts

Current transformers have standardized

ratings so that they can be used to measure

large currents and high voltages.

Using the standardized ratings of C.Ts,other instruments can also be rated around

this. Thus a high reduction in cost of these

is possible.


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Applications:-

In power systems, current values are very large,

which cant be measured by ordinary meters. Thus a

step down Current transformer is used so that these

values can be metered easily by those instruments.

Low power consumption by using C.T.

Several instruments can be operated using a single

C.T.


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63367301 Current Transformer

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