Cable Sizing for Generator-Connected Excitation Transformer
Cable Sizing for Generator-Connected Excitation Transformer
(OP)
I'm looking for guidance on sizing of cables between the generator terminals and its associated excitation transformer. I'm specifically concerned with short-circuit withstand, as the fault currents at the generator terminals can be quite high in comparison to the size of the cable that would be used if ampacity were the only consideration. Some specific questions I have are:
Thanks in advance,
mgtrp
- Is short-circuit withstand normally a consideration? I know that it is omitted for short lengths of conductor such as jumpers from the busbar to VTs within switchgear, but I don't think that it should be for a longer cable that leaves the switchgear (in this specific case, approx 30-50m).
- Should the fault clearing time used in the cable heating calculation account for breaker fail, or assume that the protection operates correctly?
- Is any consideration given for generator run-down--i.e, the generator continuing to feed a fault even after the main and field breakers have tripped.
Thanks in advance,
mgtrp






RE: Cable Sizing for Generator-Connected Excitation Transformer
Bill
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"Why not the best?"
Jimmy Carter
RE: Cable Sizing for Generator-Connected Excitation Transformer
RE: Cable Sizing for Generator-Connected Excitation Transformer
A separate ground fault protection for high resistance or ungrounded systems has also be provided.
In any case, this connection cable has to withstand the expected short-circuit current for a duration equal-at least- to the generator shut-down time.
RE: Cable Sizing for Generator-Connected Excitation Transformer
That will make a difference to the intensity and duration of the run-down effects.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable Sizing for Generator-Connected Excitation Transformer
RE: Cable Sizing for Generator-Connected Excitation Transformer
@waross, excitation current source during generator rundown is not a concern for us as the excitation is disconnected.
Cheers,
mgtrp