Another high-leg delta problem

[synchro]

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.

The document at the link below from AEP (the parent company of SWEPCO) says on pg. 26, 4.3.4:
"DER are not permitted to connect to an open-delta service transformer configuration, regardless of the DER being a single-phase or multi-phase system"

https://www.aep.com/assets/docs/req...ionandInteroperabilityRequirements_Rev0-1.pdf

Perhaps they would support a closed delta like tortuga mentioned.

 

[wwhitney]

The document at the link below from AEP (the parent company of SWEPCO) says on pg. 26, 4.3.4:
"DER are not permitted to connect to an open-delta service transformer configuration, regardless of the DER being a single-phase or multi-phase system"

So that's a clearly documented policy choice which is causing the OP's problem.

But is there any technical justification for such a policy choice? I.e. if on an open delta service we just ignore the stinger transformer, and then apply the single phase DER interconnection rules, what possible problems could occur for the utility that wouldn't occur if the stinger transformer were absent?

Thanks, Wayne

 

[tortuga]

So that's a clearly documented policy choice which is causing the OP's problem.

But is there any technical justification for such a policy choice? I.e. if on an open delta service we just ignore the stinger transformer, and then apply the single phase DER interconnection rules, what possible problems could occur for the utility that wouldn't occur if the stinger transformer were absent?

Thanks, Wayne

Either a net metering issue and or utility policies probably have to take in account that the customer side is out of their control.

FWIW around here the utilities have similar policies, but I can always convert open to closed delta for a PV.
If the customer has any plans for expansion a closed delta is a good thing to consider, as some single phase 240 could be moved to the B-C leg. Or if they are the only customer on the transformer bank perhaps that compressor can run off 208? and if so convert to a 208/120?
I think 3 phase adds a lot of value to a property and is worth keeping.

 

[Babu62]

Thanks for all the input.
This facility is a head office for a construction co. with a shop on the back half, they don't use 3-ph 99% of the time (one air compressor that craps out a lot). They have no other use for 3-ph.

In this part of the country, we see a few of these high-leg 3-ph hook ups, but I've never seen a "real" (480Y, etc) 3-ph system on small commercial.

They've already been funded for a $60k+ REAP grant, so they're motivated to make this happen, which is why I suggested pulling the stinger pot on the pole as a last resort, and thanks to all your input, I think that may be our only real answer. And I have no idea if that's really feasible, but I have a lot of reasons to find out.

 

[jaggedben]

So that's a clearly documented policy choice which is causing the OP's problem.

But is there any technical justification for such a policy choice? I.e. if on an open delta service we just ignore the stinger transformer, and then apply the single phase DER interconnection rules, what possible problems could occur for the utility that wouldn't occur if the stinger transformer were absent?

Thanks, Wayne

A semi-educated guess...

When you have an open delta, the additional transformer that provides the high leg is usually smaller and only engineered to provide a limited load. If it's overdrawn, I gather you can get a really big voltage drop on the high leg and your three phase motor may have issues with that.

Conversely, DERs tend to raise voltage locally. So if you backfeed the split-phase portion only, you get a similar voltage imbalance; higher on the split phase, and the high leg isn't as high, relatively. Basically makes it more difficult for the utility to maintain the voltage balance they are probably contractually obligated to maintain. (But by how much really? I don't know.)

I suppose worst case is a combination of the two.

So a wild guess is that the 'other utility' which turned off their imbalance alarm was like "OK, you would rather backfeed solar than have us guarantee voltage balance on your high leg. Fine, customer is always right." Whereas the SWEPCO personell have forgotten the technical reason for not allowing the backfeed, or never knew it, and are just going by a book (probably without actually doing any engineering calcs to see what the maximum likely imbalance would be.)

 

[wwhitney]

Conversely, DERs tend to raise voltage locally. So if you backfeed the split-phase portion only, you get a similar voltage imbalance; higher on the split phase, and the high leg isn't as high, relatively. Basically makes it more difficult for the utility to maintain the voltage balance they are probably contractually obligated to maintain. (But by how much really? I don't know.)

OK, we can quantify this. Let's take the idealized voltages as N=(0,0) ; A=(-120,0) ; B=(0,208); and C=(120,0).

So if we get voltage rise on A-C of 2V, that will push A and B to the left one (since B is referenced via the stinger transformer off A), and C to the right one. That makes A=(-121,0) ; B=(-1,208); and C=(121,0).

The A-B voltage is still 240V; the A-C voltage is now 242V; and the B-C voltage becomes sqrt(122² + 208²)=241V. So while the B-C voltage gets worse, the A-C voltage is still the controlling worst case for complying with a +/- 5% type rule.

I guess this means I don't buy this explanation. As I don't see how adding a simultaneous 3-phase load to the picture would make things worse.

Cheers, Wayne

 

[kwired]

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.

I think that is the root of the issue though someone at POCO maybe sees "one size fits all" when there may be options that do work.

 

[tortuga]

I think that is the root of the issue though someone at POCO maybe sees "one size fits all" when there may be options that do work.

Could be deltas are uncommon and hard for people to really understand. Around here its just open delta they have an issue with PV.
When bidding service upgrades I always look for open delta's (two pigs on the pole) and see if the utility can close them, it helps with lots of things besides solar.
Those specs syncro posted in post 21 are almost verbatim what they are in my area, they only prohibit an open delta:

Also its not uncommon for me to have to go look up a utilities own specs myself rather than rely just on what the tech tells me.

 

[kwired]

So that's a clearly documented policy choice which is causing the OP's problem.

But is there any technical justification for such a policy choice? I.e. if on an open delta service we just ignore the stinger transformer, and then apply the single phase DER interconnection rules, what possible problems could occur for the utility that wouldn't occur if the stinger transformer were absent?

Thanks, Wayne

My guess is they assume most open delta systems will have a minimal sized stinger transformer in comparison to the 120/240 transformer, which probably is true most the time. We do have some occasionally around here where that is not the case, often in remote locations where the way they ended up even having three phase capability is by bring one primary phase and neutral from one direction and another primary phase and neutral from another direction.

 

[wwhitney]

My guess is they assume most open delta systems will have a minimal sized stinger transformer in comparison to the 120/240 transformer, which probably is true most the time.

OK, but why is that a problem? The sample calculation in post #26 suggests it wouldn't be.

Cheers, Wayne

 

[jaggedben]

Wayne, I don't really understand the notation your using in post #26, but I think what we're interested in here is not a +/-% from nominal voltage but the imbalance between the different three phase voltages. Your calculation indeed shows that backfeeding the split-phase imbalances the voltage. I'm curious what the effect is of a three phase load given a stinger transformer rating of, say 1/3rd the kVa of the split-phase. And I'm curious what happens when the two effects are added together. I admittedly don't understand how post #26 shows there isn't a problem.

 

[wwhitney]

Wayne, I don't really understand the notation your using in post #26

Just cartesian coordinate geometry in the plane. If the neutral point is the zero volt reference, that means N is the point (0,0). Then if we choose to draw the point C to the right, that puts it at the point (120,0), and A at (-120, 0). Then if B is straight up instead of down, B = (0,208). Etc.

but I think what we're interested in here is not a +/-% from nominal voltage but the imbalance between the different three phase voltages. Your calculation indeed shows that backfeeding the split-phase imbalances the voltage.

Yes, if there's voltage rise from an inverter it will imbalance the voltage, just like voltage drop from loads will. But the point of the computation in post #26 is that if the single phase inverter moves the A-C voltage to 242V from 240V, and the A-B stinger is unloaded, so its voltage stays at 240V, the B-C voltage becomes 241V.

I.e. unless the allowable voltage imbalance criterion for a 3-phase delta service is stricter than for a single phase service, I'm not seeing how the presence of the stinger transformer makes anything worse. But maybe I'm overlooking something.

Cheers, Wayne

 

[kwired]

OK, but why is that a problem? The sample calculation in post #26 suggests it wouldn't be.

Cheers, Wayne

I guess I was going down the road of the concerns of possibly overloading the stinger transformer since it often designed for minimal load in comparison to the other transformer. Three phase PV system is going to try to push equal load through all three lines regardless what is there for transformers.

Reality is they need to look at each individual installation to verify what will work but maybe the average install request is pushing the limits and they just decided they aren't doing this on open delta systems even though there may be some that will be ok.

 

[jaggedben]

But the point of the computation in post #26 is that if the single phase inverter moves the A-C voltage to 242V from 240V, and the A-B stinger is unloaded, so its voltage stays at 240V, the B-C voltage becomes 241V.

I.e. unless the allowable voltage imbalance criterion for a 3-phase delta service is stricter than for a single phase service, I'm not seeing how the presence of the stinger transformer makes anything worse. But maybe I'm overlooking something.

Backfeeding an unloaded single phase service doesn't result in any voltage imbalance at all (on the service that is). So it seems to me like your example makes the opposite of the point you're saying it does.

 

[wwhitney]

I guess I was going down the road of the concerns of possibly overloading the stinger transformer since it often designed for minimal load in comparison to the other transformer. Three phase PV system is going to try to push equal load through all three lines regardless what is there for transformers.

Right, so don't use a 3 phase PV system on an open delta. Just use a single phase PV system for the "lighting" transformer.

Cheers, Wayne

 

[wwhitney]

Backfeeding an unloaded single phase service doesn't result in any voltage imbalance at all (on the service that is). So it seems to me like your example makes the opposite of the point you're saying it does.

OK, maybe we are using terms differently here. I have been considering imbalance to be any deviation from the nominal voltages. But sounds like you mean imbalance is any deviation from the L-L voltages being equal, regardless of whether the average L-L voltage matches nominal. In which case it's impossible to imbalance a single phase service, as there is only one L-L voltage.

Then with this meaning of imbalance I certainly agree that a single phase PV backfeed on a 3 phase open delta can cause imbalance. But so do single phase loads. To first order, the computation I did will give comparable answers whether you have a net single phase load or backfeed. Namely if the A-C voltage changes by X (positive or negative), and the A-B voltage remains fixed, the B-C voltage will change by 0.5X. That 0.5 factor comes from the cosine of 60 degrees.

So as long as the worst case PV single phase backfeed voltage rise is no worse than the worst case single load voltage drop, I'm not seeing how the PV single phase backfeed is worse for voltage imbalance than normal loading is.

However, I haven't worked out the voltage drop effect from 3 phase loading on the open delta--since it's not symmetric, I expect that will cause imbalance. And it could be the imbalance caused by 3 phase loading and by single phase loading are subtractive, while the imbalance caused by 3 phase loading and single phase backfeeding are additive. Which is maybe what you were trying to say in post #25.

So I'll try to work that out. But I'm still unclear on (a) whether the utility regulates or is regulated as far as imbalance directly, as opposed to just deviation from nominal L-L voltages and (b) whether 3 phase loads suffer from imbalance more than they do from a balanced voltage drop/rise. Would appreciate comments on those points.

Cheers, Wayne

 

[jaggedben]

OK, maybe we are using terms differently here. I have been considering imbalance to be any deviation from the nominal voltages. But sounds like you mean imbalance is any deviation from the L-L voltages being equal, regardless of whether the average L-L voltage matches nominal. ...

Correct. And it's my understanding that at a certain severity the difference in voltage between different phases can be damaging to three-phase motors. For example as discussed here. https://forums.mikeholt.com/threads/3-phase-high-leg-problem.144501/

...

However, I haven't worked out the voltage drop effect from 3 phase loading on the open delta--since it's not symmetric, I expect that will cause imbalance. And it could be the imbalance caused by 3 phase loading and by single phase loading are subtractive, while the imbalance caused by 3 phase loading and single phase backfeeding are additive. Which is maybe what you were trying to say in post #25.

That is exactly what I was trying to say in that post. But admittedly I'm not the one able to show why mathematically, I'm just coming at it intuitively.

So I'll try to work that out. But I'm still unclear on (a) whether the utility regulates or is regulated as far as imbalance directly, as opposed to just deviation from nominal L-L voltages and (b) whether 3 phase loads suffer from imbalance more than they do from a balanced voltage drop/rise. Would appreciate comments on those points.

My understanding, which might be wrong or simplistic (it's mostly from indirect discussion on this forum), is that the utility sizes the stinger pot to have a low enough impedance to handle the anticipated three phase load while keeping the difference between phase voltages under the desired/required percentage. So if the effect of that load in addition to backfeeding the split-phase is indeed additive, then their calculation would be no longer valid with PV backfeeding. (Nevermind that the calculation was likely done decades ago on paper, or that the three-phase load it was done for is arguably likely to no longer exist. But you can see why this situation might result in a confused policy choice.)

 

[tortuga]

Reality is they need to look at each individual installation to verify what will work but maybe the average install request is pushing the limits and they just decided they aren't doing this on open delta systems even though there may be some that will be ok.

I am going with this.
The solution they train their personnel on is have the customer convert to single phase.
My favored solution is to close the delta, which they are not trained on but can do.
In town it should be a no-briner, but you need to shift some 240 load to B-C legs to have an even sized delta bank.
But as kwired mentioned in a remote area its not always feasible due to the lack of the 3rd primary.

 

[jaggedben]

Yeah, but it is also weird that they demanded the OP backfeed all three phases on an open delta. (See posts 8 and 13.)

@Babu62 Did you propose just backfeeding the split phase? Like ggunn suggested in post #2. But I didn't see you say you proposed that.

 

[wwhitney]

That is exactly what I was trying to say in that post. But admittedly I'm not the one able to show why mathematically, I'm just coming at it intuitively.

OK, I've thought about the geometry a while, and I came up with the diagram below, where the voltage point A is taken as the fixed reference. Here for simplicity I'm assuming the coil impedances are predominantly resistive, and the the load currents (for three phase and single phase loads separately) are balanced and in phase with the applied voltages. Which means I omitted the single phase neural point N and any unbalanced single phase load; the single phase current L can just be taken to be the average current through the A-C coil.

So the point of the diagram is to show the phase relationships, i.e. the direction of the voltage drops. The direction I've drawn each current is its phase (up to an unspecified choice of +/- 1). The stinger pot resistance brings the high leg voltage point B downward, which is towards the center of the ABC triangle (what would be the neutral point of a wye secondary). Likewise, the 3 phase current brings the voltage point C towards that same center point. And if the single phase current is a net load, it further moves the voltage point C to the left. If it's a net backfeed, the voltage rise would move it to the right instead.

Implicit in this picture is that this is for small effects where we can consider each effect separately and add them up. E.g. if we think of the three phase current causing a large move of the voltage point C, and then add a large resistive single phase A-C load, the single phase voltage drop would move the voltage point C directly towards A, not to the left.

Now what I'm missing is an appropriate measure of "imbalance" for the 3 phase system. The 3 phase loading on an open delta transformer inherently unbalances the system; the voltage angle BAC becomes less than 60 degrees, and all the voltages get smaller but in an unbalanced manner. Adding a single phase load makes the voltage angle BAC even smaller, but it reduces the voltage A-C which reduces the spread of the L-L voltages when X < Y. Whereas with a net single phase backfeed, we get the opposite.

So without such a measure of imbalance, it's not clear to me if single phase backfeed will cause the the imbalance to get worse. But now I agree that it's a possibility.

Cheers, Wayne