Question

The neural point of a 3-phase, 25 mva, 11 kv alternator is earthed through a resistance of 5 . The relay is set to operate when there is an out-of-balance current of 1.5
a. The cts have a ratio of 1000/5. What percentage of winding is protected against an earth fault and what should be the minimum value of earthing resistance to protect the 90 percent of the winding. (

Answer 1

Answer:

The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What should be the minimum value of the earthing resistance to protect 90% of winding? R = 2.12 Ω.

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