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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