Download - 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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