Using one NGR for grounding of two generators 1

[olicg]

Can one NGR of adequate rating be utilized for grounding two parallel connected generator neutrals?
If yes, what be the adv and disadv for doing so?

Thanks in advance guys!,
olic.

 

[rbulsara]

Yes. You can use a single NGR for multiple parallel generator. Size (current rating) will be same as if it were for a single unit as it is not dependent on generator size.

One of the drawbacks would be now the NGR will be located away from one or more generators. In fact you will need to bring all neutral of the gens to a common switchboard where NGR will be connected between the neutral and the ground bus. The if a L-N fault were to occur between a generator and the paralleling switchboard, the fault current will not be limited by the NGR.

 

[olicg]

Thanks,

If an E/F fault occurs on one of the generators, we will have to trip both? or is there a way to distinguish between faults on any one of the gen?

 

[rbulsara]

Well yes, in theory. However it’s not as simple. It depends how the feeder breakers are set and where the fault is.

If the fault is downstream of the paralleling gear and feeder breakers, the feeder breaker shall be set so that it trips before any one of gen will trip. For the two gens, say the individual gen’s E/F is set at 20A, that is equivalent to 40A E/F current on the feeder fault (fault current will be shared by the two gens). I would set the feeder breaker E/F to say 30A, which is greater than any individual gen E/F pick up but less than the total of the two. This will minimize nuisance generator trips. If the fault is on the bus, all bets are off, especially with a common NGR. With individual NGRs, at least it will protect individual gens.

 

[rbulsara]

By the way you do not "have" to trip the generators on E/F. If the NGR is sized so that the E/F current is held less than 10A, usually you can choose to stay on for long time, alram the E/F and try to find the fault within reasonable time or do a manual orderly shut down. If its more than 10A (for each gen) will have to trip the generators.

Refer to IEEE green book. Consult generator mfr. also.