CHP Grounding

[nhcf]

I observed an existing installation - 480/277 VAC wye generator, 1KW, operating in parallel with facility distribution/utility. It appeared the neutral in the generator was bonded to ground. It also appeared (based on site drawings) that utility transformer N was also bonded. Generally only one bond permitted -is this different for a generating facility or should one of the two bonding jumpers be removed?

 

[ScottyUK]

If you have two neutral-earth bonds then neutral current will flow in the earth path. This is almost certainly a code violation, even if it doesn't actually trip the set. If the set has some form of unit protection then it may well end up tripping because of current flowing in the earths.

There are a few options: a single shared neutral-earth bond; four-pole changeover switches without paralleling capability; a motorised switch or normally-closed contactor in the generator neutral-earth bond.

 

[nhcf]

ScottyUK I agree - I just was unsure of any situations where this would be accepted in a generating facility (vs. a normal electric system with only one N-G bond). I was unfortunately not able to open xfmr or service gear to validate actual config, so it is possible the generator is the only bond location.