Synchronous Generator Protection Interview Questions Answers

What are the common types of generator faults?

Answer: The common types of faults occurring in synchronous generators are:

Stator winding faults (phase to phase faults, phase to earth faults, inter turn faults), Rotor winding faults (conductor to earth faults, open circuit faults, inter turn faults), failure of prime mover, failure of the field, unbalanced loading, over loading, over-voltage at generator terminals, over-speed, ventilation failure, loss of excitation.

For what type of faults does differential protection is provided?

Answer:  Differential protection responds to the phasor difference between two or more electrical quantities. It operates for the internal faults occur in generators or transformers. For external faults differential protection does not operate

What is the disadvantage of ordinary differential protection?

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Answer: When differential relaying is used for protection, the CTs at both sides of the generator winding must be of equal accuracy. Otherwise if the CT errors are excessive it will cause the mal operation of the relay. To safeguard against such disadvantages percentage differential protection is employed.

What is advantage of using percentage differential relay protection?

Answer: 

Advantages:

• It does not require CTs with air gaps or special balancing features
• It permits a low fault setting to be used and this ensures maximum protection of the windings
• It ensures complete stability under the most severe through fault conditions

Explain Differential Protection?

Answer: Differential protection is generally provided for the equipment or group of equipments which are to be protected against internal faults. They are the primary protection systems (operates faster) for any internal faults occurring within the protection zone.

Under healthy conditions the currents at both ends of the windings will be equal. EMFs induced in the secondaries of the CTs will be equal and so no current flows through the operating coil of the relay. When an earth fault or phase to phase fault occurs the condition no longer holds good and the differential current flows through the relay operating coils makes the relay to operate. Relay operates for the faults occurring within the zone of protection