GSU, Small Industrial Application 1

[lyla1711]

I'm hoping to get some advice for integrating some parallel generators and a step-up transformer at an industrial plant.

We have (4) 1MW generators which will be paralleled and backfeed a single 480-13.2 step-up transformer. This will feed a service rated MV ATS and pick up the entire plant. I'm struggling with the transformer configuration and the grounding. Would one typically go with a 480V Delta - 13.2 Grounded Wye for this application? The last one I worked on was Grounded Wye - Grounded Wye, and I'm not sure why they did that.

The gens will each have (2) 800A breakers, so we won't be implementing ground fault and our MV transfer switch that will be a 3P, solid neutral type.

Can anyone point me in the right direction on this one?

 

[dpc]

If your 13.2 kV system connects direct to the utility, they will probably want a grounded wye on their side so that a 13.2 kV ground fault will trip off your generators. Assuming the 480 V generators are grounded wye, then a delta-wye step up transformer should be a good option.

A one-line might help. Why does each generator need two breakers?

 

[waross]

A wye delta. looking from the utility side will back feed into a lost incoming phase.
The utility may prefer a wye/wye for that reason.
That also gives you a ground reference for your generators.
I am of two minds on this.
If they are my generators, I prefer a delta facing the generators. It distributes line to neutral loads better.
If I was the utility operator, I would want a wye/wye arrangement to avoid back feeds.
You may not need ground fault protection for code compliance, but in the event of a failure in a generator winding, good ground fault protection may make the difference between a rewind and a complete replacement.
If you are using differential protection then disregard my previous sentence.
Running without a ground reference will leave you susceptible to over voltage damage in the event of an arcing ground fault.

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

 

[lyla1711]

One line is attached. I know it lists the transformer on there at grounded wye to grounded wye, but that will be updated.

I'm thinking it's wye configured gens with the neutrals grounded, and that feeds a step up transformer which is delta on the gen side and wye on the utility side. I'm just reading through code to make sure I have my head wrapped around this correctly.

I worked on a cogen recently that was a similar configuration but a grounded wye to grounded wye was used. That was feeding a paralleling gear though.

 

[dpc]

Bill,

My experience with utilities is that they want a ground source. This allows ground fault current detection on their system and gets your generator tripped off before they reclose. The only issue is that if there are not generators on line, that system is ungrounded. But that can be dealt with. YMMV

Dave

 

[waross]

Hi dpc. I have no problem with your post.
However after spending about 15 years in Wye/Delta land, I am concerned about the back feed from the healthy phases into a faulted phase in the event of a line to ground fault.
Another issue is that with a wye/delta transformer on the circuit, if the line crews have occasion to pull a cut-out, there will still be full voltage back fed to the load side of the cut-out.
I see problems with a grounded wye/delta transformer.
You see problems with an ungrounded wye/delta transformer.
We both have valid points.
The solution is a wye/wye transformer as shown on the prints.

 

[davidbeach]

DPC is talking 3-phase sensing, gang operated disconnecting while waross is talking single phase fuses. Both are right in their own context, the two don’t overlap. I think the OP is more in DPC’s world rather than in Bill’s.

Delta customer side-grounded wye on the utility side is a disaster (waross) with fuses on the utility side while it is the only viable solution (DPC) with relays and breakers on the utility side.

Both are completely right and both are completely wrong. Context. Context.

 

[waross]

Thank you David.
I defer to dpc.

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

 

[Kiribanda]

If your generators are connected to the Utlity, then the Utility will not allow you to
have your transformer DELTA on their Utility side. Because, the Utility has to make sure that after
connecting your generators, their system will still be effectively grounded. By using a WYE grounded
on the utility side it can be ensured. Therefore, the step-up transformer should be WYE(GROUNDED)/delta
one.
But from your SLD the generators are not rated for "PRIME POWER" so that they can operate continously.
They seem to be "STAND-BY POWER". It implies that your generators are not supposed to run continously
or for an extended period. Based on that logic Utility might allow you to have a WYEGND/wyegnd
transformer as the step-up.

 

[lyla1711]

That's correct, Kiribanda. The gens are standby - they will only operate when there is a utility outage.

The transfer switch that the owner would like to buy is closed transition type.

I'm thinking the correct configuration is to ground the gen neutral, run 3-wire to delta 480 side of step-up transformer with a grounded wye 13.2kV side. That will leave us with a small multi-grounded system on the 13.2 side, and since we'll have multiple ground current paths, I'm a little concerned about the potential of a fault backfeeding the utility neutral when the gens are running and the utility is down. Any ideas on how to mitigate this? I'm wondering if the utility will want a recloser they can control on their drop.

 

[dpc]

Bill,

I never considered possibility of only fuses on the primary. You're right - if the primary side is fuses only, then the delta on the low side is a big problem. My experience has been a recloser or a breaker on the primary for even small cogen sets. Smaller than this. But I've been out of the game for awhile and probably shouldn't be giving advice anyway. Maybe I'll have another beer.

Cheers,

Dave

 

[waross]

If it is standby only, you should never be connected to the utility.
You don't need the utility permission for an open transition standby.
With a closed transition, you must follow the utility guidelines.
It is so much simpler and cheaper to wait for the next break or shift change and do an open transition.

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