Missing neutral question

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
I am working on a PV system design to connect to a 480/277V service; the junction box where we will be interconnecting does not have a neutral and it is over 100' (underground run) from the disconnect. One of our field guys has verified that all three phases are 277V to ground, so it isn't ungrounded or corner grounded. The inverters do not require a neutral, so the missing neutral wouldn't be a problem, except that the system is in a jurisdiction that requires a PV meter and the meter needs a neutral to work (verified by the utility).

I know next to nothing about zigzag transformers, but I looked them up and what I am seeing is that they can be used to create a neutral for an ungrounded service, but this one isn't ungrounded. How should I create the neutral for the meter?
 
ggunn said:
I know next to nothing about zigzag transformers, but I looked them up and what I am seeing is that they can be used to create a neutral for an ungrounded service, but this one isn't ungrounded. How should I create the neutral for the meter?
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Instead of a zig-zag, you can use a delta-wye transformer where the wye side is rated for 480/277V. The voltage rating on the delta side would not matter because it would not be connected to anything. The delta winding will force the neutral voltage produced at center of the wye to be such that the three L-N voltages are balanced.
 
don_resqcapt19 said:
Is this a line side or a load side connection?
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If the existing installation is compliant, it would have to be a load side connection, as the service neutral would have to be run to the service disconnect, so it would be present at any line side connection point.

I assume the meter just needs a neutral to run its internal electronics? Unless you want to run all the PV power output through a transformer, you'd need something like a neutral-forming autotransformer (not sure if that exists). Except that 215.11 would prohibit that configuration.

So it seems like either you need to move your connection point, add a neutral to your existing feeder, or use a transformer sized to carry all the PV power. In which case you could just a delta-wye transformer as usual. Which would require a GEC instead of a primary neutral, which might or might not be a win (although the extra transformer would be a loss).

Cheers, Wayne
 
synchro said:
Instead of a zig-zag, you can use a delta-wye transformer where the wye side is rated for 480/277V. The voltage rating on the delta side would not matter because it would not be connected to anything. The delta winding will force the neutral voltage produced at center of the wye to be such that the three L-N voltages are balanced.
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A comment and a question:

(1) I would think that arrangement would constitute an autotransformer, and therefore violate 215.11.

(2) So on a delta-wye transformer, say you delete the delta windings and hook up the wye side as above. The voltages from X0 to X1, X2, and X3 won't necessarily be balanced? Will they be well-defined? And then how does adding the delta windings back in impose an additional constraint on the secondary voltages?

Thanks,
Wayne
 
I know I could do it with a delta-wye isolation transformer, or trenching over 100' to the disco, or pulling a neutral through 100'+ of ancient underground conduit, but I am looking for something easier and cheaper.
 
ggunn said:
I know I could do it with a delta-wye isolation transformer, or trenching over 100' to the disco, or pulling a neutral through 100'+ of ancient underground conduit, but I am looking for something easier and cheaper.
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I think that 215.11 rules out any other options, but perhaps I am overlooking something.

Cheers, Wayne
 
What I read about zigzag transformers said that one can be used to generate a neutral in an ungrounded system, but it did not explicitly exclude using one on a grounded service. Is that not allowed?
 
ggunn said:
What I read about zigzag transformers said that one can be used to generate a neutral in an ungrounded system, but it did not explicitly exclude using one on a grounded service. Is that not allowed?
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Not something I'm familiar with, but Isn't a zigzag a type of isolation transformer? How is that easier/cheaper than a delta-wye isolation transformer?

Cheers, Wayne
 
wwhitney said:
Not something I'm familiar with, but Isn't a zigzag a type of isolation transformer? How is that easier/cheaper than a delta-wye isolation transformer?

Cheers, Wayne
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As I said, I don't know much about them; if using one for this is not disallowed it will at least be something worth checking out.

The connection to the neutral in the PV meter is just a short piece of #14. I would have thought (hoped) there existed a small transformer I could connect to the phase conductors that would tell the meter where the center of the wye is, but that's just my fantasy.
 
wwhitney said:
... how does adding the delta windings back in impose an additional constraint on the secondary voltages?
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Except for a possible scaling factor and polarity, the X1-X0, X2-X0, X3-X0 voltages will each be duplicated on their corresponding secondary winding on the delta side. But voltages across each delta winding have to add up to zero because the delta windings form a closed circuit. This produces the constraint on what the X0 voltage must be.
If the currents into X1, X2, X3 do not sum to zero (i.e., a finite zero sequence current), then a corresponding current will circulate around the closed delta winding. As a result, it will present a very low impedance to zero sequence currents (limited by the impedance of the transformer).

wwhitney said:
(1) I would think that arrangement would constitute an autotransformer, and therefore violate 215.11.
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I think either a zig-zag or a "wye grounded delta" (the one I was mentioning) would be considered as an autotransformer because it does not produce an output that is isolated from the input.
In the application being considered by ggunn, it would not really be a "grounding autotransformer" as in 450.5 because it is not "providing a neutral point for grounding purposes", even though it is providing a neutral.
I agree, though, that 215.11 could be problematical in this case.
 
ggunn said:
The connection to the neutral in the PV meter is just a short piece of #14. I would have thought (hoped) there existed a small transformer I could connect to the phase conductors that would tell the meter where the center of the wye is, but that's just my fantasy.
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What does the PV meter look like? Similar to a normal POCO meter, or does it use separate CT's?
 
So how does a utility normally meter a 480V Delta service, and why can't they do that for the PV meter? I guess if this utility doesn't provide 480V Delta services, then they wouldn't want to support a one-off meter just for this project. Hopefully it's not just a bureaucratic rule that the PV meter type has to exactly match the service meter type.

Cheers, Wayne
 
wwhitney said:
So how does a utility normally meter a 480V Delta service, and why can't they do that for the PV meter? I guess if this utility doesn't provide 480V Delta services, then they wouldn't want to support a one-off meter just for this project. Hopefully it's not just a bureaucratic rule that the PV meter type has to exactly match the service meter type.

Cheers, Wayne
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I spoke to the utility; they just told me that the PV Meter has to have a neutral. I don't know whether they provide 480V delta or not.
 
synchro said:
What does the PV meter look like? Similar to a normal POCO meter, or does it use separate CT's?
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What it will be is just an inline U-400 400A meter. There is a length of #14 wire that runs from the meter itself to the neutral conductor. That's what all the fuss is about.
 
This is why I ussually suggest running a neutral even if planned loads will not require one, eventually soneone wants to add something that does.
OP, is conduit large enough to pull in a neutral? Might be easier than trying some creative way to sidestep the required neutral. In this circumstance I see this particular meter as just another device requiring a neutral. Is there a different meter not requiring a neutral?
 
...and just an interesting side note:. A delta wye would have a sqrt(3) voltage change and that isn't a rational number. How accurately can they make a1:1 transformer on this case? Perhaps the 1.732 could just be a multiplier for the meter.
 
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