Test: Instrument Transformer - 1 - Electrical Engineering (EE) MCQ

A transformer used to extend the range of an A.C. voltmeter is known as a potential transformer. A potential transformer is also abbreviated as P.T.

The primary winding of a C.T. has a very few number of turns. It is connected in series with the load circuit through which the primary current flows.

The turns ratio for a C.T. is defined as the ratio of the number of turns in the secondary to the number of turns in the primary. It is given by the relation

Secondary winding of a P.T. has a few number of turns. A low range voltmeter is connected in parallel with the secondary winding.

In an instrument transformer, the low rated secondary windings provide electrical isolation from the high rated primary windings.

Current transformers are standardized at 5 A of secondary winding current, whereas Potential transformers are standardized at 110 V of secondary winding voltage.

An 110000/110V potential transformer can be used for measuring a voltage of upto 110000V by making use of a voltmeter with 110V voltage reading.

Instrument transformers can be used for extending the range of the A.C. instruments. In instrument transformers, readings are independent of the values of R, L, and C. But in the case of shunts and multipliers, the readings depend on the values of circuit constants.

A transformer used to extend the range of an A.C. ammeter is known as a current transformer. A current transformer is also abbreviated as C.T.

C.T. is basically used for the measurement of high currents. A P.T. is usually used for the measurement of high voltages. They are used with A.C. ammeters and voltmeters in order to extend their range.

Secondary winding of a C.T. has a large number of turns. It is connected in series to an ammeter through which a small portion of the current flows through.

The primary winding of a P.T. has a very large number of turns. It is connected in parallel with the load whose voltage is to be measured.

A 1000/5A current transformer can be used for measuring a current of upto 1000A by making use of an ammeter with 5A current reading.

Leads of the secondary winding transformer are brought to the switch board thus separating them from high voltage windings. In this way, the meter circuit is isolated from the high voltage power circuit.

Given that, impedance of secondary winding = 0.01 Ω
Rated current = 5 A
fault current = 20 times of rated current = 20 x 5 = 100 A
VA output = I²z = 100² x 0.01 = 100 VA