Transformer Protection Interview Questions and Answers:

1. What are the faults that are likely to occur in a power transformer ?

Ans. The faults likely to occur in a power transformer are open-circuit faults (an open circuit in one phase of a 3-phase transformer), earth faults, phase-to-phase faults, inter-turn faults and overheating.

2. What are incipient faults ?

Ans. Incipient faults are initially minor faults causing gradual damage. These faults include loose connections in conducting path, sparking, small arcing etc., and grow into major faults.

3. What is through fault ?

Ans. A through fault in one which is beyond the protected zone of the transformer but fed through it.

4. What are the various protective schemes available for power transformers ?

Ans. Different protective schemes available for the protection of power transformers are Buchholz protection, percentage differential protection, earth fault protection, overcurrent protection, combined leakage and overload protection, restricted earth fault protection, overfluxing protection etc.

5. On what factors does the choice of a protective gear for transformer depend ?

Ans. The choice of a protective gear for a transformer depends upon several factors such as (i) type of transformer (distribution transformer or power transformer), (ii) size of the transformer, (iii) type of cooling used (iv) systems where used i.e., its electrical location in the network and (v) the importance of the service for which it is required.

6. In what respect core-balance leakage protection is inferior to combined leakage and overload protection ?

Ans. The core-balance leakage protection suffers from the drawback that it does not provide protection against phase-­to-phase faults.

7. Does circulating current protection system provide protection against inter-turn fault in case of power transformers ?

Ans. Yes.

8. Name the two basic requirements that are to be fulfilled by differential relay.

Ans. The two basic requirements to be satisfied by differential relay on the protection of power transformers are that the differential relay should not operate on overload or external faults and it must operate on severe internal faults.

9. If T is the turn-ratio of power transformer, what will be the turn ratio of CTs on the lv side ?

Ans. If T is the turn-ratio of power transformer, then turn-ratio of CTs on the lv side will be T times that of CTs on the hv side.

10. What are the shortcomings of differential protection scheme as applied to power transformer ?

Ans. Simple differential protection, as applied to a power transformer, has the following drawbacks.

(i) Unmatched characteristic of CTs (ii) Change in ratio due to tap changing and (iii) magnetizing inrush current.

11. What is magnetizing inrush current ?

Ans. When the transformer is energized initially there is no induced emf, the condition is similar to switching of an inductive circuit. The resistance being low, a transient rush of current takes place. This transient rush of current is known as magnetizing inrush current and it may be equal to 6 to 8 times the rated current.

12. What is the reason behind providing a second harmonic restraint relay ?

Ans. Second harmonic restraint relay is provided to avoid maloperation of the relays due to magnetizing inrush current.

13. What are the advantages of differential magnetic balance protection system, as applied to power transformers ?

Ans. The main advantages of differential magnetic balance protection system in its application to power transformers are increased stability and sensitivity and its application to transformers provided with load tap changers.

14. How is frame leakage protection provided in power trans­formers ?

Ans. The transformer tank is connected to earth through CT to which an instantaneous earth fault relay is connected.

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