Protection of AC Generators and Motors Interview Questions and Answers:
1. What are the common types of generator faults ?
Ans. The common types of faults occurring in synchronous generators are enumerated below.
- Stator winding faults — phase-to-phase faults, phase-to-earth faults and inter-turn faults.
- Rotor winding faults — conductor-to-earth faults, open-circuit faults, inter-turn faults.
The other abnormal operating conditions are (i) failure of prime mover (ii) failure of field (iii) unbalanced loading (iv) overloading (v) overvoltage at generator terminals (vi) overspeed (vii) ventilation failure (viii) current leakage in the generator body.
2. The occurrence of short-circuiting between the stator windings is quite rare. Why so ?
Ans. The occurrence of short-circuiting between the stator windings is quite rare because the insulation in a slot between coils of different phases is at least twice as thick as the insulation between one coil and the iron core.
3. What is differential protection ?
Ans. The differential protection is that which responds to the phasor difference between two or more electrical quantities.
4. Which type of relays are used in Merz-Price protection system for alternator ?
Ans. The relays used in Merz-Price protection system of alternator are instantaneous electromagnetic type (attracted-armature type).
5. How does the inclusion of resistance in neutral grounding circuit of an alternator affect the performance of the differential protection of a 3-phase synchronous generator ?
Ans. With the inclusion of resistance in neutral grounding circuit of an alternator, it will not be possible to protect whole of the stator windings of a star-connected generator against earth faults. The percentage of winding protected depends on the value of neutral grounding resistor and earth fault relay setting.
6. Does the Merz-Price system of protection applied to alternators provide any protection for turn-to-turn faults on the same phase ? If not why ?
Ans. Merz-Price system of protection applied to alternators does not provide any protection against turn-to-turn faults on the same phase because the current produced by such a fault flows in a local circuit between the turns involved and does not create the difference between the currents entering and leaving the windings at its two ends where the high voltage CTs are mounted.
7. Why it is not necessary to provide protection for turn-to-turn fault in turbogenerators ?
Ans. The coils of modern large turbogenerators are usually single turn and, therefore, it is not necessary to provide protection for turn-to-turn faults.
8. What type of protective device is used for the protection of an alternator against overheating of its stator ?
Ans. Resistance temperature detector.
9. Why is overcurrent protection not necessary for modern alternators ?
Ans. Overcurrent protection is not considered necessary for modern alternators because these are capable of withstanding a complete short circuit at their terminals for sufficient time without much overheating and damage.
10. Why is it not necessary to provide overvoltage protection in turbogenerators ?
Ans. The overvoltage in a generator occurs when the prime mover speed increases due to sudden loss of the load on the generator and the speed control governors in case of turbogenerators are very sensitive to speed variations and, therefore, generated overvoltage of significant duration or magnitude does not generally occur. That is why it is not necessary to provide overvoltage protection in turbogenerators.
11. What types of protective device are used for the protection of an alternator against overheating of its rotor.
Ans. DC relay and resistance measuring circuit.
12. Why is it necessary to suppress the field immediately after disconnection of faulty alternator from the system?
Ans. In the event of a fault on a generator winding even though the generator circuit breaker is tripped, the fault continues to be fed as long as the excitation will exist because emf is induced in the generator itself. Hence it is necessary to suppress the field immediately after disconnection of faulty generator from the system.
13. What is field suppression ?
Ans. The opening of the field circuit breaker and discharge of stored energy in the field winding through a resistor is called the field suppression.
14. What type of relays are required for backup protection of alternators ?
Ans. High set, definite minimum time, induction type, inverse overcurrent, earth fault relays are required for backup protection of alternators.
15. What are the types of faults that are likely to occur in operation of a 3-phase induction motor ?
Ans. The faults which are likely to occur in operation of a 3-phase induction motor are : Stator faults (phase-to-phase, phase-to-ground and inter-turn faults), rotor faults (earth faults and inter-turn faults), prolonged overloading, stalling, unbalanced supply voltages, single phasing, undervoltage, reverse phase.