Generators and Short Circuits

Which is more disastrous-shorting (phase to phase) & starting a generator or short circuit (phase to phase) happening on a generator running on load.

By mistake a LT DG was started with two phases shorted (at a short distance before the breaker) - resulting in damage of alternator rotor. However, high impedance differential protection etc did not operate to shutdown and save the m/c. Are the protections provided by controllers like DEIF, Easygen, Cummins PCC etc equivalent to the protection relays of Siemens/Alstom etc.

Any insights would be highly appreciated.
 
P
Mas, following are answers to your question/comments in the order given:

1) Theoretically short-circuit duty is higher. This occurs because the generator’s pre-fault air-gap voltage, Egp, is higher when "On-Load" than "Off-Load". So yes, is the answer. But, the real problem is fault-duration. If the fault persisted for longer than a few 10ths of a second, the fault-current magnitude could have been less than its Full-Load rating. This "protection deficit" is usually covered with a back-up, Over-Current, Voltage-Restaint relay.

2) Rotor damage resulted from the fact that the ph-ph fault is asymmetrical, resulting in unbalanced-currents in the stator winding. Thus, undesirable currents are induced in the iron-structure,causing severe damage to the rotor.

3) Your statement, "save the m/c" indicates the fault was probably not within the differential relay’s zone of protection. Or the fault-level was too small to be detected by the Diff'l-relay. Where are the Diff'l-relay CTs located, relative to the fault location? Also is the m/c low voltage, i.e., less than 600V?

4) Yes, the relays are comparable.

Can you provide additional detail?

Regards,
Phil Corso
 
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