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The neutral point of a three-phase 20MVA, 11KV alternator is earthened through a resistance of 5Ω, the relay is set to operate when there is an out of balance current of 1.5A. The CTs have a ratio of. What is the percentage of winding protected? Also calculate the earthing resistance required to protect 90% of the winding.?
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The neutral point of a three-phase 20MVA, 11KV alternator is earthened...
Percentage of Winding Protected:
- Given out of balance current = 1.5A
- CT ratio is not provided
- Let's assume CT ratio is 100:5 (primary:secondary)
- Out of balance current in primary = 1.5A * (100/5) = 30A
- Relay set to operate at 30A
- Total current capacity of alternator = 20MVA / √3 / 11kV = 1041A
- Percentage of winding protected = (30A / 1041A) * 100 = 2.88%

Earthing Resistance for 90% Protection:
- To protect 90% of winding, out of balance current should be limited to 10% (104.1A)
- Earthing resistance required = Voltage across relay / Maximum out of balance current
- Voltage across relay = 11kV / √3 = 6352V
- Earthing resistance = 6352V / 104.1A = 61.05Ω

Explanation:
- The percentage of winding protected is calculated based on the out of balance current and total current capacity of the alternator.
- To protect 90% of the winding, the earthing resistance required is calculated using the voltage across relay and maximum out of balance current.
- Proper earthing and relay settings are essential to ensure the safety and efficient operation of the alternator.
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The neutral point of a three-phase 20MVA, 11KV alternator is earthened through a resistance of 5Ω, the relay is set to operate when there is an out of balance current of 1.5A. The CTs have a ratio of. What is the percentage of winding protected? Also calculate the earthing resistance required to protect 90% of the winding.?
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