electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
zig zag transformer
- Thread starter electrofelon
- Start date
- Status
ramsy
Roger Ruhle dba NoFixNoPay
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Used to create artificial neutral, mitigate harmonics, & more. Check it out on Wikipedia:
https://en.wikipedia.org/wiki/Zigzag_transformer
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Looks like it's being fed from a grounded wye secondary, so that's a good question.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The customer transformer is a grounded wye-grounded wye transformer, so I do not see any need for a grounding transformer.
The only thing I can think of is that someone has specified that the inverter output has to remain solidly grounded even when the disconnect is open.
The only thing I can think of is that someone has specified that the inverter output has to remain solidly grounded even when the disconnect is open.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I can't see how an open disconnect would unground the inverter output.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Here's a wild and possibly completely boneheaded guess, since I have no understanding of how/why it would work, but...
Having noticed this (yellow highlight is mine)...
...perhaps it is a way of complying with the exception to 705.32, when using a switchboard that was not designed for backfeed.
Having noticed this (yellow highlight is mine)...
...perhaps it is a way of complying with the exception to 705.32, when using a switchboard that was not designed for backfeed.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I was thinking (mistakenly) of the disconnect also opening the neutral.
If there is no neutral connection to the inverters (line to line operation) then opening the three line conductors opens all connections between inverter output and ground.
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
The customer takes service at, and has mv switchgear. Could that have anything to do with it?
kwired
Electron manager
- Location
- NE Nebraska
There is no neutral conductor running to it on the drawing though, just indicates a ground connection at the midpoint.
K8MHZ
Senior Member
- Occupation
- Electrician
To me, it looks like the midpoint is the artificial neutral, and the only purpose of it is to provide a balanced connection to the earth.
kwired
Electron manager
- Location
- NE Nebraska
Yet the source ahead of this appears to already be a grounded wye, which should already fulfill that purpose.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Looking hard at the drawing, I renew my comment that the source grounding is only seen when the disconnect is closed, since there is no neutral connection to the inverters. I do not see why floating the inverter output (when not operating, because of anti-islanding) should be harmful, but it still might be the motivation.
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jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
While it's a little hard to say for sure with the photo cutting things off, it does look to me like the neutral goes to the inverters. It seems like the drafter just omitted the neutrals inside the switchboard for clarity.
Not saying you are wrong about the motivation, we've heard about demands for source grounding (which to my knowledge is unnecessary). I also wonder what you think of my 705.32 comment though.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
Also notice the cutting off. It looks like the breaker is a normally open breaker that closes upon activation of the "S" on the right side of the page. Perhaps seeing the rest of the drawing would give some clue as to the intention. I don find it interesting that this is odd enough that no one here with the vast knowledge represented, has yet said, "Oh that is a ..."
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Here is a more complete picture. I am confused by the logic of the kirk key. Here is what the note 11 says:
So if the Zig Zag or its supply circuit fails, in such a way that causes the breaker to trip (or someone manually turns it off for that matter), they want the MV supply to also open. The 480 system is already grounded, so why is the zig zag needed, and not only that, why do they see it as so important that the MV supply opens if the zig zag goes offline? Is this just perhaps a silly utility requirement based on grounding myths that system grounding is of paramount importance and that "more" is better?
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
The inverters are SMA sunny tripower, 30KW and do have a neutral. But I do agree that the motivation is likely to do with some (irrational) fear that the inverters will lose their reference to ground and float. Not only do I see that is very imnprobably, but even if it did I dont see that as that big a deal really.
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
wbdvt
Senior Member
- Location
- Rutland, VT, USA
- Occupation
- Electrical Engineer, PE
Why? A 25MVA, 13.2kV Transformer would be about 1100A full load. The system is being supplied by National Grid and this may be a large manufacturing facility supplied off of a 115kV transmission line.
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