What Instrument Transformer
Instrument Transformers are used in AC systems for the measurement of electrical quantities i.e., voltage, current, power, energy, power factor, and frequency. Instrument transformers are also used with protective relays for the protection of the power system.
The basic function of Instrument transformers is to step down the AC System voltage and current. The voltage and current level of the power system is very high. It is very difficult and costly to design the measuring instruments for the measurement of such high-level voltage and current directly. Generally measuring instruments are designed for 5 A or 1A (current Transformer) and 110 V (potential Transformer).
Instrument transformers are of two types –
Current Transformer (C.T.)
Potential Transformer (P.T.)
Current Transformer (C.T.)
The Current transformer is used to step down the current of the power system to a lower level to make it feasible to be measured by a small rating Ammeter (i.e. 5A ammeter).
The primary of the Current Transformer is having very few turns of thick wire which is carry full load current. Sometimes bar primary is also used. Primary is connected in series with the power circuit which current to be measured or protected. Therefore, sometimes it is also called a series transformer.
The secondary is having a large number of turns of thin wire. The secondary is connected directly to an ammeter. As the ammeter is having very small resistance. Hence, the secondary of the current transformer operates almost in short, circuited condition. One terminal of the secondary is earthed to avoid the large voltage on the secondary with respect to the earth.
Which in turn reduces the chances of insulation breakdown and also protects the operator against high voltage. More ever before disconnecting the ammeter, the secondary is short-circuited through a short link to avoid the high voltage build-up across the secondary.
Potential Transformer (P.T.)
The potential transformer is used to step down the voltage of the power system to a lower level to make it feasible to be measured by a small rating voltmeter i.e. 110 – 120 V voltmeter.
The primary of P.T. is to have a large number of turns. Primary is connected across the line (generally between online and earth). Hence, sometimes it is also called the parallel transformer. The secondary of P.T. is having few turns and is 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.
One terminal of the secondary of P.T. is earthed to maintain the secondary voltage with respect to the earth. Which assures the safety of operators.