zig zag grounding TX sizing

[startlearner]

I have three 600V 500kW 3phase generators connected to a 3ph 4w bus and want to provide a zig zag transformer with resistor to get a grounded neutral at the bus. The max. ground fault current allowed is 5A. Please advise me on how to size the TX and resistor. Thank you.

 

[rbulsara]

Others with more knowldege of a transforerm design will pitch in, but I beleive if you give this spec to zig-zag transformer manufacurer sthey will meet the requirements.

Just curious: Are the generator windings delta connected?

If wye connected, then can you not ground the generator neutrals through a high resistance?

 

[dpc]

I'd recommend specifying the transformer and grounding resistor together and specifying the desired ground current and the desired maximum duration.

You also need to specify any continuous rating required due to 3rd harmonic currents.

You say you have a 3 phase, 4 wire system. I'm not sure how this is possible if you don't presently have a grounded system. I assume you're aware that if you ground through a resistor, you cannot have any line-to-neutral loads, and will essentially be limited to a 3-phase, 3-wire system.

 

[RAVWARD]

Hi Guys,
Sorry for pitching in here:
To DPC comments:

"What continous rating has to do with 3rd harmonic currents".

I would appreciate a word, looks like my concept is a little astry.
Thanks

 

[davidbeach]

3rd harmonics are zero sequence and will therefore flow continuously through the windings of a zig-zag transformer. All triplen harmonics are zero sequence, but you are unlikely to have any even harmonics to speak of, so the next harmonic to be of concern for the zig-zag is the ninth harmonic, and the magnitude of the ninth is going to be significantly lower than the third.