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When we started 800kW at 11 kV, just after closing the motor feeder breaker. The motor feeder as well as utility breaker tripped immediately. The relay MICOM p220 indicated that there were two faults. 1) unbalanced current, 2) Earth fault.

The values of currents are as follows: Ia =304.62A Ib=302.5 A Ic=314.2A and Neutral Current In= 49.47A
Further to this it is also pointed that Capacity of utility feeder is 2MWe.

What are the causes? how they can be rectified? and lastly can the said utility feeder 2MWe bear such load?

Responding to Anonymous’ 12-Jul-08 (00:44) queries... please provide additional information:

1. Is this a commissioning problem or an ongoing problem?

2. Are relays hardware or software based?

3. What are the trip settings of the 'unbalanced current' relay and the 'earth fault' relay?

4. What are the trip settings of the utility circuit breaker?

5. What is CT arrangement for earth-fault relay, that is, three-residually connected line CTs? Or one window-type that encircles all conductors?

6. What are current and power ratings of the neutral Earthing resistor?

Regarding the feeder capability question:

7. What are cable parameters: conductor size; length; is it shielded?

Regards, Phil Corso (cepsicon@aol.com)