This set of Electrical Measurements & Measuring Instruments Multiple Choice Questions & Answers (MCQs) focuses on “Reduction of Errors in Potential Transformers”.

1. Winding resistance of a P.T. can be reduced by _________

a) using thick conductors

b) decreasing the length of the winding

c) shorting the primary and secondary windings

d) using thin conductors

View Answer

Explanation: In a potential transformer, the winding resistance is usually minimised by using thick conductors and by making use of small length for the turns.

2. Leakage reactance is minimised by _________

a) using thin conductors

b) reducing leakage flux

c) increasing flux density

d) shorting the windings

View Answer

Explanation: By maintaining the primary and secondary windings together in a P.T. and also by reducing the leakage flux, we can minimise the leakage reactance.

3. High flux density is due to less turns.

a) True

b) False

View Answer

Explanation: In a P.T., a high flux density in the core, gives rise to a less number of turns. This in turn results in a lower leakage reactance.

4. Ratio error in a P.T. depends on _________

a) secondary current

b) primary voltage

c) secondary current

d) turns ratio

View Answer

Explanation: In a P.T., the difference between actual ratio and turns ratio is given by the relation,

where, R is the ratio error

n is the turns ratio

I

_{s}is the secondary winding current

I

_{e}is the iron loss component

I

_{m}is the magnetising component

It is seen from the above equation that the ratio error in a P.T. depends on the secondary current, magnetising and iron loss components of current.

5. In a P.T. values of components of currents are negligible.

a) True

b) False

View Answer

Explanation: In a C.T. the various components of current such as magnetising current, iron loss component of current are almost comparable in magnitude with the value of the load current.

6. Ratio error can be minimised by _________

a) reducing the turns

b) reducing the current

c) increasing the voltage

d) using a good core material

View Answer

Explanation: By making use of a good quality core material, low value of flux density and following required precautions in the core assembly we can minimise the value of the ratio error.

7. Another method of eliminating the ratio error is _________

a) by reducing secondary turns

b) by increasing the primary turns

c) by increasing secondary turns

d) by reducing the primary turns

View Answer

Explanation: In a P.T., at no load, we get

where, R is the ratio error

n is the turns ratio

I

_{s}is the secondary winding current

I

_{e}is the iron loss component

I

_{m}is the magnetising component

From the above equation it is seen that to reduce the ratio error, actual ratio and nominal ratio must be made equal. This can be done by reducing the secondary turns.

8. How is the voltage ratio dependent on the frequency?

a) they are independent of each other

b) they aid each other

c) they oppose each other

d) depends on the setup of the circuit

View Answer

Explanation: As the voltage ratio changes, the frequency also changes. It depends on the relative value of the core loss component Io as well as the leakage reactance. The effects due to the voltage ratio and the change in frequency oppose each other.

**Sanfoundry Global Education & Learning Series – Electrical Measurements.**

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