Another high-leg delta problem

[Babu62]

In the past for 240v Delta (208v high-leg) systems we've used either AP Systems 208v microinverters set to 240 ph-to-ph (no neutral) or a Mattox 480Y to 240delta transformer (no neutral connecting to the grid).
The utility we're dealing with requires a neutral connection, they say they don't usually allow PV interconnections on this type grid because of imbalance.
Anyone know of a solution? I'm stumped.

 

[ggunn]

In the past for 240v Delta (208v high-leg) systems we've used either AP Systems 208v microinverters set to 240 ph-to-ph (no neutral) or a Mattox 480Y to 240delta transformer (no neutral connecting to the grid).
The utility we're dealing with requires a neutral connection, they say they don't usually allow PV interconnections on this type grid because of imbalance.
Anyone know of a solution? I'm stumped.

Most of the time I have just interconnected on the A and C phases with a neutral just like any other single phase service and ignored the B (high leg) phase.

 

[Babu62]

Most of the time I have just interconnected on the A and C phases with a neutral just like any other single phase service and ignored the B (high leg) phase.

Right, we just did that on another utility's grid, they have normal decent human engineers, so they just disabled the imbalance alarm it trips at HQ.
That customers usage was 98% single phase, just like the current customer's, but SWEPCO refuses to let that work (makes too much sense), so that's why we have the problem.
And they're d!cks.

 

[jaggedben]

Sounds like the problem isn't really technical and therefore doesn't have a technical solution.

 

[jaggedben]

How big a system are we talking about?

 

[Babu62]

2 meters on the same side of a building, +/- 28kw to each meter.
Trying to figure an interconnection means, inverter-based or transformer (or Hail Mary) to satisfy utility (i.e. w/neutral).

 

[tortuga]

Is it an open delta (two transformer bank)?
Or a full delta (three transformer bank)?

 

[Babu62]

Is it an open delta (two transformer bank)?
Or a full delta (three transformer bank)?

2 transformers.

 

[kwired]

2 transformers.

I was going to ask where's the imbalance until you mentioned this.

Full delta with delta input to it should be well balanced.

Open delta you can potentially overload their "stinger pot" if you have enough capacity feeding back to it, chances are there is little load on the high leg in normal use and one reason they went with that type of source and possibly with a smaller unit than the other transformer.

 

[Babu62]

If there's no straightforward solution, I think the owner might request the utility remove the stinger pot and revert to all single phase, as they only have one 3-ph compressor in the whole facility.
Is that too drastic, or realistic?

 

[kwired]

If there's no straightforward solution, I think the owner might request the utility remove the stinger pot and revert to all single phase, as they only have one 3-ph compressor in the whole facility.
Is that too drastic, or realistic?

You still doing the same thing with what you are feeding into grid if you only install single phase PV system. It is all on one phase to neutral of their primary distribution.

They have single phase PV scattered about the system that can potentially throw off system balancing. The imbalance issue they are concerned about is more likely associated with loading on that second transformer on the service in question.

 

[electrofelon]

I am a bit confused about the "with neutral" comment. It would seem what you need is a connection that uses all three phases, not a neutral yes? In which case it seems a delta to wye transformer would work. Ok, if the POCO is saying nonsensical things like you need a neutral then why not connect just to A and C? Or are they requiring BOTH that it be balanced AND have a neutral?

 

[Babu62]

POCO stated from the start that we couldn't interconnect to their grid if it was a high-leg delta, both solutions we've proposed a)240 delta ph-to-ph inverter, no neutral & b) 480Y to 240delta transformer, have been nixed.
They demand a neutral connection plus all three phases.
We just did an almost identical system on another POCO, A & C leg connection single phase leaving the high-leg untouched, it works perfectly.
But not all POCOs are created equal.

 

[jaggedben]

If there's no straightforward solution, I think the owner might request the utility remove the stinger pot and revert to all single phase, as they only have one 3-ph compressor in the whole facility.
Is that too drastic, or realistic?

No, not too drastic, especially if there are no three phase loads. I think that might be the best approach. And a good way of pointing out to the utility why they are being irrational.

 

[electrofelon]

POCO stated from the start that we couldn't interconnect to their grid if it was a high-leg delta

They demand a neutral connection plus all three phases.

So which one is it?

 

[synchro]

If there's no straightforward solution, I think the owner might request the utility remove the stinger pot and revert to all single phase, as they only have one 3-ph compressor in the whole facility.

It's possible that going to only single phase could save the owner money if the utility charges an extra monthly fee for providing a 3-phase service.

 

[wwhitney]

Right, we just did that on another utility's grid, they have normal decent human engineers, so they just disabled the imbalance alarm it trips at HQ.

Can you or someone elaborate on what the imbalance alarm refers to? Is there some extra monitoring that is done by some POCOs on a 240V high leg open delta service that isn't done on a 120/240V single phase service? Which monitoring would be confused by backfeed from single phase PV on the A and C legs?

Because otherwise, as far as I can see, there's zero reason that the presence or absence of an A-C stinger transformer would have any effect on the single phase PV interconnection behavior for the A-N-C transformer.

Thanks,
Wayne

 

[electrofelon]

If the POCO wants "all three phases and a neutral", why can't you connect 3 single phase 240 inverters with 2 set for "no neutral" and one set for "with neutral" ?

(didn't double check spec sheets but I'm pretty sure both of those are selectable options on most inverters these days).

 

[tortuga]

2 transformers.

I think the issue is the open delta secondary / open wye primary.
If they have all three phases on the pole I would propose adding a 3rd transformer to make it a full delta.
Then you could have 1 '240v' inverter per phase.

 

[jaggedben]

POCO stated from the start that we couldn't interconnect to their grid if it was a high-leg delta, both solutions we've proposed a)240 delta ph-to-ph inverter, no neutral & b) 480Y to 240delta transformer, have been nixed.
They demand a neutral connection plus all three phases.
We just did an almost identical system on another POCO, A & C leg connection single phase leaving the high-leg untouched, it works perfectly.
But not all POCOs are created equal.

If the POCO wants "all three phases and a neutral", why can't you connect 3 single phase 240 inverters with 2 set for "no neutral" and one set for "with neutral" ?

(didn't double check spec sheets but I'm pretty sure both of those are selectable options on most inverters these days).

It's a good question, given what they told him. And with a closed delta it would probably be the right thing to do. But the truth is, that won't work well because it's an open delta. The utility people he's talking to seem to understand this much, much less than they should. There should be no rule of thumb for backfeeding all high-leg deltas. Open or closed matters, as does how much load the high-leg was engineered for.