Cable Sizing for Generator-Connected Excitation Transformer 1

[mgtrp]

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:
[ol 1][li]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).[/li]
[li]Should the fault clearing time used in the cable heating calculation account for breaker fail, or assume that the protection operates correctly?[/li]
[li]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.[/li]
[/ol]

If it makes a difference, this is for a utility application under IEEE standards so NEC etc do not strictly apply. I'd be particularly interested in any standards or written sources that I could refer to.

Thanks in advance,
mgtrp

 

[waross]

Is there any way that you can incorporate current limiting fuses in the circuit at the generator end?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mgtrp]

No, it's an existing installation with no convenient space to install fuses.

 

[7anoter4]

In my opinion, the generator differential [main] protection "see" also the excitation transformer. A differential backup protection may be provided as per IEEE 242-2001 ch.12.5.3.2.3 Differential backup protection.
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.

 

[waross]

Does this generator use current derived field forcing to hold up the current despite voltage collapse?
That will make a difference to the intensity and duration of the run-down effects.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Not aware of any standard covering this specifically. As 7anoter4 indicates, if there is a generators differential relay, this cable will be in the differential zone. But this cable will only see fault current if the fault is in the cable or the excitation transformer. If the excitation transformer fails, this 50 meters of cable will be a secondary issue.

 

[mgtrp]

Based on comments here and research elsewhere, the advice seems to range from "size for the worst case" to "don't worry about it, size for load", and various inbetween. We will likely go for something larger than just load current, perhaps allowing for 0.5-1.0 second withstand, on the basis that the protection should operate within this time, and fault contribution will be significantly reduced once the main and field breakers open. This gives a reasonable cable size.

@waross, excitation current source during generator rundown is not a concern for us as the excitation is disconnected.

Cheers,
mgtrp