# Grounding Transformer



## Rap2 (Dec 17, 2010)

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


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## bobelectric (Feb 24, 2007)

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?


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## HARRY304E (Sep 15, 2010)

Rap2 said:


> 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.
> 
> ...


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 (Dec 17, 2010)

bobelectric said:


> 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?


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....


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## bobelectric (Feb 24, 2007)

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


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## Rap2 (Dec 17, 2010)

HARRY304E said:


> Take a look at article 690 in the NEC.
> 
> What are trying to produce with the transformer?
> 
> What type of building?


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.


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## Big John (May 23, 2010)

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 (Nov 8, 2008)

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 (Dec 17, 2010)

Lone Crapshooter said:


> 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.
> ...


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.


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## RIVETER (Sep 26, 2009)

Rap2 said:


> 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.
> 
> ...


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.


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## Rap2 (Dec 17, 2010)

RIVETER said:


> 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.



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

Thanks so much for the info.


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## Bkessler (Feb 14, 2007)

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


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## Meadow (Jan 14, 2011)

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.


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## Rap2 (Dec 17, 2010)

meadow said:


> 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.


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.


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## Rap2 (Dec 17, 2010)

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


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## macmikeman (Jan 23, 2007)

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. :no:


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## mfripp (Sep 6, 2013)

*Transient overvoltages*

I know it's late to respond to this thread, but I just came across this today. 

The local utility is probably worried about transient overvoltages. 

Suppose you have a 3-phase distribution feeder where the only wye transformer with a solid ground is at the start of the feeder (the utility end). If there is a line-to-ground fault on this circuit, the utility's transformer will try to keep line-to-neutral voltages on all three lines at their normal level. This will induce a large current on the ground-faulted line, and the circuit breaker will open, isolating the circuit from the utility power supply.

Now all the lines on this circuit are isolated from the fixed voltage provided by the utility transformer, so they are free to float to new voltages. In this state, the ground-faulted line will move to zero volts (equal to the neutral). If a PV system continues to inject power into this circuit, the other two lines will maintain their normal line-to-line voltage, and since they're free to float relative to the neutral, they will end up with a line-to-neutral voltage equal to their line-to-line voltage (i.e., 1.73x normal line-to-neutral voltage). This overvoltage is bad for other equipment on the feeder, and it will persist until the PV system shuts down (which can take up to 2 seconds).

There is some more information about this at http://pterra.us/blog2/archives/599.


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## RIVETER (Sep 26, 2009)

Rap2 said:


> 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.
> 
> ...


Zig- zag auto transformers for grounding are not that expensive. They give all of the benefits of an un-grounded system and are not a 24/7 I2R loss as you would have with a hi-resistance ground..


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