Rap2

The utility is telling me I need to hook up a grounding transformer in order to hook up my 200KW PV inverter to their grid.

My 10 minute google education on what the heck grounding transformers do suggests they are for protecting circuits being fed by an ungrounded delta or in my case a high impedance ground.

From what I read a grounding transformer hooks in parallel with said ungrounded circuit to protect it.

since I have no clue how much a grounding transformer costs and one of the largest transformer distributors in the us had to go through several salesman before they found someone who knew what it was to try and get me a quote, I take it they are rare and could possibly cost big bucks.

What i was wondering is that rather than going the grounding transformer route and hooking in parallel to the circuit why not just hook up to an isolation transformer, I could hook my PV inverter to the delta side of a 480 to 480 volt isolation transformer and ground the crap out of the wye side which i hook up to the utility.

Does that sound like a legit solution?

Thanks for the read and any reply

bobelectric

I don't get it. 200kw is a lot of power to all of a sudden say "here I am"! Don't you need approvals and drawings?

HARRY304E

Take a look at article 690 in the NEC.

What are trying to produce with the transformer?

What type of building?

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Rap2

Yes, we have submitted drawings for approval, just like our many installations of the like in the next county over(different utility), except this is the first time they have been sent back saying we need a grounding transformer... I have a meeting on Monday to discuss the matter with the utility, but I want to have my ducks in a row....

bobelectric

Understood,rap2. PV here of that size is only a concept we read about.

Rap2

I am trying to isolate my not so grounded PV circuit from the utility and provide them with a conventionally grounded one through a 480 volt to 480 volt delta wye transformer, and grounding the neutral side of the wye in order to provide a more conventionally grounded circuit.

690 does not have much on grounding transformers.

This is a large refrigerated packing plant.

Big John

If the utility is specifying a grounding transformer, especially if you already have a high-resistance-ground, then I would bet money they want it for protective relaying in order to shut your system down properly in the event of a fault.

Not sure if they'd find a hard ground more acceptable or not.

-John

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Lone Crapshooter

Most of the high resistance grounding (HRG) units that I have seen have a resister bank in series between the center point of the Wye and ground.
the resister bank is taped and there is a thumping contactor around a small portion of the bank. This is used for ground fault location.

A grounding transformer is used with a Delta power transformer and they are a zig zag transformer and their center point is grounded giving the ungrounded Delta a ground reference. Those that I have seen are not that big and they will fit in a vertical section of switchgear. Now I must say that I have no idea how to size a zig zag transformer.

I would talk to Powell Electric in Houston TX or Controlled Power Corp North Canton Ohio for guidance.

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Rap2

Thanks for the contact I will give them a call on Monday.

I have read a little on zig zag transformers. How big were the delta circuits you were hooking up? It sounds promising the zig zag transformers where physically small in comparison. It may be the cheaper route to go.

RIVETER

I don't understand why the utility would demand the way you ground your system. However, if you choose to use a zig-zag auto transformer to ground, it would be less of an I2R loss to you than using a resistive grounding system. They have been around a long, long time.

Rap2

All good stuff to know, any idea how much they cost or how to size one?

Thanks so much for the info.

Bkessler

One guy whats it done his way. That's typical with neighboring utility's.

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meadow

Also keep in mind if using an isolation transformer with different winding connections there will be a 30 degree phase shift which can be a problem when parelleling the inverter to the utility.

Rap2

Well I'm hooking up an inverter that syncs to the utility (or whatever is coming into it) so if the utility is on one side of a transformer that is 30 degrees out of phase and my inverter syncs to the other side based on what it sees coming in I don't believe that would be a problem.

Rap2

I forgot to post.... the utility accepted my re-design using a standard delta wye transformer to isolate my PV inverter from their stuff.

I dont like putting in a 480 to 480 volt transformer it seems dumb. I started another thread to discuss any advantage there may be in running the PV at 600 volts just to be able to use a 600 to 480 transformer so I am actually transforming something

macmikeman

I have dealt with utility and govt. officials here for years. The secret is to take them out to club gitchi goomi for a hundred rounds of beers first, and possibly also go to your meeting carrying some manapua.

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