Protection of AC Generators and Motors Question Bank with Answers:
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1. What are the abnormal conditions in a large alternator against which protection is necessary ? Discuss them briefly.
2. Explain the protection of a three-phase star-connected generator by means of a percentage differential relay.
3. Describe the general principle of operation of generator differential protection scheme. Give the 3-phase connection of the biased scheme.
4. Give a complete protection scheme for alternator.
5. Restricted earth-fault protection is provided to alternators though it does not protect complete wiring against earth fault. Justify this type of protection.
6. What are the abnormal conditions in a large alternator against which protection is necessary ? Discuss a protection scheme for an alternator against inter-turn faults.
7. Describe various protection schemes of an alternator for different faults and abnormal running conditions.
8. Describe with suitable diagrams how the stator windings of a 3-phase alternators are protected from
- Earth faults on the stator winding.
- Unbalanced current in the stator winding.
- Earth fault on the rotor winding.
- Overloading of alternator.
9. What types of protective device are used for the protection of an alternator against overheating of its (i) stator and (ii) rotor ? Discuss them in brief.
10. Discuss suitable protection schemes which are used for (i) rotor earth fault (ii) rotor open circuit of a synchronous generator.
11. Explain : A loss of excitation relay used for alternator protection.
12. What are the types of faults that are likely to occur in a 3-phase induction motor ? If the motor is not fully loaded, is it necessary to provide protection against single phasing ? Explain why ?
13. What are the types of faults that are likely to occur in a three-phase induction motor ? Distinguish between short-circuit protection, overload protection and earth-fault protection of motor.
14. Explain the term “single phasing”. In what form such protection is provided in case of a large 3-phase induction motor ? If the motor is not fully loaded, is it necessary to provide protection against single phasing ? Explain why.
15. Write short notes on the following :
- Generator faults.
- Protection of alternator.
- Overload protection of alternator.
- Self balance protection system.
- Inter-turn fault protection of alternators.
- Overspeed protection of alternators.
- Loss of excitation protection of alternators.
- Protection of small motors.