Multipoint grounded neutral, generator connection 3

[barrs79]

A facility as a 12.47kV ring bus supplying (20) wye-wye transformers, solidly grounded with a common neutral (multipoint grounded neutral). The facility purchased a 3MVA 12.47kV generator to be connected and synchronized with the ring bus and offset a portion of their utility usage. Per the generator manufacturer the generator must be wye-impedance grounded.

Are there any options other than adding a 12.47kV/12.47kV isolation transformer to develop a separately derived system to satisfy the NEC?

Thanks!

 

[davidbeach]

Probably not, but in the long run that transformer will be money well spent.

 

[cranky108]

It's much easer to protect a generator that has a wye-delta transformer than on with no GSU. The issue is locating a ground fault. Is it in the generator, or system.
We have to trip all our direct connect generators for system ground faults (they are small), where we can locate the ground faults as system faults on units connected through a wye-delta GSU.

3MVA at 12.47?? I would have expected 4160 for that size.

 

[mosesnbklyn]

Neutral Grounding configurations could come in many flavors. If the transformer neutrals are connected to a single point/bus, you can install a neutral switching system to select which source neutrals are grounded (transformers or generator, generally 1 at a time); this could be as few as 2-switches/contactors. You could remove all the transformer ground connections and add a grounding transformer (zig-zag or YgD) - this will probably be less expensive and smaller than the isolation transformer but requires another switch or breaker on the main bus (not sure if you were planning on installing a separate GCB for the transformer option). The grounding transformer can work in conjunction with a generator NGT/R.

M. Nissen,P.E.
Senior Electrical Engineer
Waldron Engineering & Construction

 

[davidbeach]

I've always thought that neutral switches were always a recipe for disaster. I've long said that in Protection our motto is "Sh!t happens" and that our patron saint is Murphy. After all, it's Murphy that pays all the bills around here, if it wasn't for him I'd be out of a job. Well, Murphy loves sh!t like neutral switches. Every failure mode you build into a system will be exercised at some point. There's enough failure modes that only get discovered when they get exercised that I can't see any reason to build in other failure modes deliberately.

 

[waross]

Well said David. I agree with you completely.

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