Question

Power Systems

a) Explain how a differential protection scheme operates.

b) The loss of a generator has significant impact on the Transmission and Distribution system to which it is connected. Protection methods using Automatic Disconnection of Supply (ADS) can only be used to detect faults when they occur. The 2 main categories of faults are Insulation Failure and Abnormal Running Conditions.

(i)List and briefly explain the main types of abnormal running condition.

(ii)Explain how these abnormal running conditions may be detected at an early stage to prevent major faults occurring. Give specific examples of techniques for specific failure modes.

A 4.5 MW, 10 MVA, 11 kV star connected alternator is protected by a differential protection scheme using 600/1A current transformers and unbiased relays set to operate at 17% of their rated current of 1 A. If the earthing resistor is 8x% based upon the machine's rating, estimate the percentage of the stator winding that is not protected against an earth fault.

) Explain how increasing and decreasing the percentage of the winding being protected on a differential protection scheme impacts on the relationship of the required earthing resistor.


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