CT and PT
Current Transformer (C.T)
Current Transformer is a instrument transformer used to reduce the current of
power system to a lower level to make it feasible to be measured by small rating
Ammeter.
A CT is basically a step up voltage type transformer.
Primary
of C.T. is having very few turns. Sometimes single bar primary is also used.
Primary is connected in series with the power circuit. The secondary is having
large no. of turns. Secondary is connected directly to an ammeter. As the
ammeter is having very small resistance.
Hence, the secondary of current
transformer operates almost in short circuited condition.
As the power remains
same on both sides, when the voltage on secondary side increases, cause the
secondary side current to decrease.
One terminal of secondary is earthed to avoid the large
voltage on secondary with respect to earth. Which in turns reduce the chances
of insulation breakdown and also protect the person working nearby it against the
high voltage.
Potential Transformer (P.T)
Potential transformer is a instrument transformer used to reduce the voltage of
power system to a lower level to make it feasible to be measured by small
rating voltmeter i.e. 110 – 120 V voltmeter.
Primary of P.T. is having large no. of turns. Primary
is connected across the line (generally between on line and earth). Secondary
of P.T. is having few turns and connected directly to a voltmeter. As the
voltmeter is having large resistance.
Hence the secondary of a P.T. operates
almost in open circuited condition. The low turns in secondary reduce the
output voltage.
Advantages of Instrument Transformers
1. The
large voltage and current of AC Power system can be measured by using small
rating measuring instrument i.e. 5 A, 110V.
2. By
using the instrument transformers, measuring instruments can be standardized. Which
results in reduction of cost of measuring instrument.
3. Instrument
transformers provide electrical isolation between high voltage power circuit
and measuring instruments which also assures the safety of operators.
4. Several
measuring instruments can be connected through a single transformer to
power system.
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