Utility required grounding transformer

[ElectricPersonality]

A utility is requiring we install a grounding transformer for a new rooftop solar install that will intertie with the existing building electrical distribution system. The existing building system is 480/277 volt, 3 phase 4 wire, with the neutral grounded, which is typical. The question is, if we install a zig zag grounding transformer at the building service, is it correct, then to say that the neutral reference for the building will now be at the zig zag transformer AND NOT at the utility transformer outside the building? Does this cause any voltage to ground reference issues for the existing line to neutral loads in the building? Although the NEC does not define what "high impedance" grounding means, I think that a zig zag without a resistor would not be high impedance grounded and therefore this connection "should" be acceptable.

second question, it seems to follow that since we are deriving a new neutral conductor, then the neutral wire from the utility transformer would have to be disconnected at the service switchboard or is it okay to keep it connected? The Code has some verbiage about zig zags not being installed on the load side of system ground connections, so perhaps its okay to leave the service neutral connected, AS LONG as the connection to earth at the service is through the zig zag transformer?

 

[ggunn]

A utility is requiring we install a grounding transformer for a new rooftop solar install that will intertie with the existing building electrical distribution system.

Why?

 

[jaggedben]

I agree with ggunn. The utility is screwing you. Plan A should be to get an EE to write them a letter attempting to talk them out of it. The basic line of argument is that the grounding requirements are already met by the utility's transformer.

Seems to me that every time someone posts about this, it's in PA. Somehow your utilities are out of step with the rest of the country.

 

[ElectricPersonality]

Unfortunately, I am the EE in this case; and already tried fighting it, and got shot down by the utility. Since we are connecting a generator which can back feed onto their circuit, they pretty much hold all the cards and can say do it our way or forget connecting your generator. It's required primarily for two reasons;

1) most importantly; it's in their written distributed generator standards, so they will make us do it no matter what.
2) they are concerned that in the event that the substation breaker opens, that their circuit will be back fed by an ungrounded generator, which can potentially cause damage to equipment on their circuit.

We have designed grounding transformers on MV connected systems many times with no issues. This is the first time it has been required at a LV intertie with the utility. This utility is not in PA, but is actually out West.

 

[jaggedben]

Well, that sucks.

As far as getting code interpretation answers regarding your original questions, you might try reposting them in the NEC section of this forum. I think a few guys ignore the PV forum if it's not their interest or expertise. It seems to me that your questions are really not so PV specific, although there might be some wrinkles.

Myself, I'm curious about your mention of zig-zags not being allowed on the load side of system grounding connections. Code section?

And finally, if you're not already consulting the inverter manufacturer on these questions, you probably should be.

 

[winnie]

Groovy, a pissing match between the NESC and the NEC.

A 'zig-zag' transformer is an autotransformer used to derive a neutral between the three phase legs of an three phase supply. On an _ungrounded_ three phase supply this derived neutral can be used for grounding. On a _grounded_ three phase supply this derived neutral can cause problems.

If you use one on a _grounded_ system, then the 'derived' neutral will almost certainly be at a slightly different voltage than the supply neutral. If the derived neutral and the system neutral are connected in any way, then large currents will likely flow between these neutrals.

Have you looked at your POCO interconnection manual for their requirements for 'effective grounding'?

I have no knowledge of these requirements other than what I've gleaned from a google search just now, so as a reader of these forums I'd appreciate it if you post a summary/follow-up as you learn more.

But as far as what I've gleaned:

From looking at other interconnection manuals it appears that the requirement is based on the need to limit distribution phase to ground voltage in the event of a ground fault. As with any system, a phase to ground fault will transiently raise the phase to ground voltage of the other phases, but since the utility has a multi-earthed-neutral, this increase in phase to ground voltage could be seen by other customers as an increase in phase to 'neutral' voltage, and an increase in the supply voltage to single phase customers.

For inverter based generators, the 'effective grounding' may be provided by the characteristics of the inverter.

See:
https://www.xcelenergy.com/staticfiles/xe/Regulatory/Transmission/SD-DG-Interconection-Manual.pdf
http://solectria.com//site/assets/files/1484/solectria_effective_grounding_for_pv_plants.pdf

-Jon

 

[Electric-Light]

Under just the right conditions... I can see how this is possible.
If a phase grounds and fuse on that phase is blown on the utility side, the inverter may impose L-L voltage on L-N circuits.

 

[jaggedben]

Under just the right conditions... I can see how this is possible.
If a phase grounds and fuse on that phase is blown on the utility side, the inverter may impose L-L voltage on L-N circuits.

...for a handful of cycles, if it's a three phase inverter.