WTG system grounding

[alehman]

Well it's been a long time since I've posted here. Maybe some of you guys and gals can give me some advice.

I'm involved in a small wind turbine project (two 2 MW units). We are connecting to an existing 12.47 kV campus system (the utility substation transf serving the campus is Dyn). The turbines have been ordered with 34.5 KV Dyn5 transformers internal to the nacelles. This happened before I was involved! We have about 3 miles of 34.5 kV cable from the turbines to the campus, where we have a transformer to drop back down to the 12.47kV Grd-Y system. The question is what connection to use for that tranny.

I see three options:
A. Dyn
B. Dyn with an added grounding transformer at the turbines
C. Ynyn

Protection for the high side of the transformer yet to be determined. I have an opinion, but I'd be interested to hear some others.

Thanks,


Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[waross]

How about an auto-transformer??

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

 

[alehman]

Good thought Bill. In terms of system grounding, that would be roughly equivalent to Ynyn. Do you think it would be less expensive given the ratio?

Here are my thoughts on the options:
An early proposal had been to use Dyn without a grounding transformer, leaving the feeder ungrounded. That seemed like a bad choice to me. An autotransformer or Ynyn provides ground source at substation end of the feeder. The feeder ground is lost though, if high-side transformer protection trips. These are doubly-fed induction generators and can produce output for a short time after being disconnected. This could result in overvoltage on the system. That led to the idea of Dyn at the substation and adding a grounding transformer at the WTG end of the feeder. This arrangement seems to be fairly common for wind farms.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[prc]

Ynyn connected transformer. Auto-connection will have no benefit as the co-ratio is pretty high.

 

[davidbeach]

Ynynd with the delta unconnected, possibly even buried. Transformer is then a ground source to both sides without reference to the other side; don't even have to ground both sides the same.

 

[alehman]

Thanks David. Your suggestion may be the best compromise. I've asked the turbine manufacturer for system grounding recommendations as well.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.