Parallel Generator Grounding Configuration

[nhcf]

We are looking at a project (conceptual design) which will include four 4160V, 2KW synch generators tied to grid and to in-house loads.

Regarding grounding, various references, including Blackburn's book, discuss options for grounding, including grounding each generator or installing a common grounding xfmr or NGR at the main paralleling switchgear.

I would be leaning towards an individual NGR for each generator. However, I'm not certain of grounding of bus when all generators are off-line, and utility is still supplying house-loads.

Is there a configuration that is more typical than any other?

 

[dpc]

How are you supplying utility power at 4.16 kV? If there is a transformer on site, presumably it can have a grounded wye secondary and its own NGR. If the generators will always be in parallel with the utility, There are numerous options - none are perfect. Do you need/require selective coordination of ground faults at 4.16 kV? Suggest the Dunki-Jacobs (et al) book on Grounding of Industrial Power Systems, if you can find it.

 

[nhcf]

Yes there will be a transformer to step-up the 4.16 KV to distribution voltages.

Are you suggesting an NGR at the transformer in addition to the individual NGRs? Or in place of - with a single ground at the transformer?

Thanks for the suggested reference.

 

[dpc]

If the transformer is always energized and connected to the generator bus, its ground could be sufficient although each generator would be ungrounded until it was synchronized to the bus. Other options would be to have grounding resistors for each generator. In some cases, a disconnect switch is used so that only one grounding resistor is in service at one time to limit total fault current. With a transformer ground and a generator ground circulating harmonic current might be possible, but generally manageable. But grounding the transformer wye would maintain system grounding when the generators are off line. The transformer ground should be resistance grounded as well.

 

[nhcf]

Thanks for the feedback. I'll take another look at xfmr resistance grounding in conjunction with generator grounding.