Wye Inverter on a Delta Service

[wwhitney]

So the next question would be whether UL 1741 permits such an inverter.

A quick scan of UL 1741 suggests that the answer would lie in IEEE 1547, which UL 1741 references. The IEEE standard, unfortunately, does not appear to be freely available.

Cheers, Wayne

 

[ggunn]

Clearly this is an example of a ungrounded 480V service, one of many many thousands of ungrounded systems in use. Hopefully the inverter manufacturers take notice that its not just 120/240, 208/120 and 480/277 out there. An inverter is supposed to be a 'smart' thing it should figure out that there is no neutral or L-N voltage reference.

It still could be a grounded wye with the neutral not run from the transformer to the CT can.

I remember seeing in an installation manual for a specific three phase inverter that it could run on a delta service, but I do not remember which manufacturer it was. Solectria, maybe?

 

[tortuga]

OP posted pics of ground detectors, that pretty much means its ungrounded.
Well glad is possible, eliminating the need for transformers in PV systems has to be low hanging fruit for improving system efficiency

I am thinking I may need to install two Delta to Wye transformers now, ~ 50 kVA and 125 kVA (backfeeding to two identical services, four total inverters)

I mean what are the idle hours per year of those transformers? If you figure no-load losses at 4380 hours per year that's gotta be over 3000 kwh per year of vampire load.

 

[ggunn]

OP posted pics of ground detectors, that pretty much means its ungrounded.

I'm not sure how those things work; does a lit light signify continuity with ground or potential to ground?

 

[tortuga]

I'm not sure how those things work; does a lit light signify continuity with ground or potential to ground?

I am no expert but the basic idea is the lights measure phase to ground, (actually its a lower voltage lamp via a little control transformer).
When a ground fault occurs, one light goes out, but the other 2 are reading 480V (phase to phase) because of the ground fault.
Probably just the connections of those wye inverters would trip a ground fault lamp.

 

[jaggedben]

Supposing it is a nominally ungrounded service that is actually wye grounded at the utility transformer(s)...

Is that not rather dangerous, to have a grounded system with no effective ground-fault current return path? Can the utility actually get away with that?

 

[jaggedben]

I am no expert but the basic idea is the lights measure phase to ground, (actually its a lower voltage lamp via a little control transformer).
When a ground fault occurs, one light goes out, but the other 2 are reading 480V (phase to phase) because of the ground fault.

Right, three dim lights is good but two bright ones (or anything else, really) is bad.

Probably just the connections of those wye inverters would trip a ground fault lamp.

Why?

 

[ggunn]

Supposing it is a nominally ungrounded service that is actually wye grounded at the utility transformer(s)...

Is that not rather dangerous, to have a grounded system with no effective ground-fault current return path? Can the utility actually get away with that?

I do not know; I was just looking at the observable facts and hoping for a way that the OP might not have made a potentially expensive mistake. Of course, he did indeed make a mistake by designing with and installing an inverter without knowing for sure what the service configuration is.

 

[ggunn]

Right, three dim lights is good but two bright ones (or anything else, really) is bad.

See post #32. Warning lights you cannot trust are no better than none at all.

 

[tortuga]

@jaggedben Looking up the data sheet for

Sunny Tripower CORE1 (STP 50-US-40) inverters require connection to a grounded Wye transformer winding. A neutral conductor connection to the transformer's grounded neutral point is optional.
..../....
Another post on the forum mentioned that VFDs, MOVs, innate capacitance etc can provide the illusion of a center grounded system. I can’t find that again, so not sure what that’s about.

https://www.stellavolta.com/content/SMA%20Sunny%20Tripower%20Core1%20-%20Stellavolta.pdf

My guess and its just a guess they have surge protection devices (MOVs) that are in a Wye configuration referenced to ground.

 

[ggunn]

@jaggedben Looking up the data sheet for

https://www.stellavolta.com/content/SMA%20Sunny%20Tripower%20Core1%20-%20Stellavolta.pdf

My guess and its just a guess they have surge protection devices (MOVs) that are in a Wye configuration referenced to ground.

We are all guessing, hence my advice to the OP that he turn off the inverters and leave them off until and unless it can be confirmed that the inverters and the service are compatible.

 

[pv_n00b]

It still could be a grounded wye with the neutral not run from the transformer to the CT can.

I remember seeing in an installation manual for a specific three phase inverter that it could run on a delta service, but I do not remember which manufacturer it was. Solectria, maybe?

CPS has them and a couple of others, but not many.
If you see this in the manual then you can connect to an ungrounded service:




If you only see this though:

You cannot assume you can connect to an ungrounded service. It may be using the EGC to connect back to the neutral at the service entrance.

 

[wwhitney]

If you see this in the manual then you can connect to an ungrounded service:

What brand/model is your example from, and are there others you are aware of that are similar?

I guess the existence of that manual settles the questions in my earlier posts today.

Cheers, Wayne

 

[ggunn]

Is there any difference between a floating 3W wye service and a 3W delta service other than the way the transformer secondary is wound?

 

[Elect117]

Is there any difference between a floating 3W wye service and a 3W delta service other than the way the transformer secondary is wound?

That is like asking what the difference is between wye and delta lol. Ya the behavior is different for both systems but primarily faults is what makes the wye more dangerous than a delta.

We have seen meters blow out of sockets during a fault that would not trip the main (because they don't trip on the first ground fault). The meter can has a reference to ground through the panel which is bonded to ground. That meter has a MOV that will fail in a not so friendly way. One customer of the utility built a cage around the meter because it happened twice in one year and someone almost got knocked out by it.

https://www.eaton.com/content/dam/eaton/markets/healthcare/knowledge-center/white-paper/transient-overvoltages-on-ungrounded-systems-from-intermittent-ground-faults.pdf

It is ill advised to interconnect to ungrounded systems since they are notably unstable and unsafe. They are made safe by certain NEC practices but the utility side on the interconnection is a gamble that can result in expensive and dangerous issues.

 

[electrofelon]

That is like asking what the difference is between wye and delta lol. Ya the behavior is different for both systems but primarily faults is what makes the wye more dangerous than a delta.

Can you be specific about what the difference would be between a fault on an ungrounded Delta and a ungrounded wye? I'm not seeing why in the case of the eye it would be more dangerous.

We have seen meters blow out of sockets during a fault that would not trip the main (because they don't trip on the first ground fault). The meter can has a reference to ground through the panel which is bonded to ground. That meter has a MOV that will fail in a not so friendly way. One customer of the utility built a cage around the meter because it happened twice in one year and someone almost got knocked out by it.

I don't understand this. Is the wrong meter being used? Who is designing a meter for an ungrounded system that explodes if there is a first ground fault?? Seems like that's a problem with the meter, not with the ungrounded service.

It is ill advised to interconnect to ungrounded systems since they are notably unstable and unsafe. They are made safe by certain NEC practices but the utility side on the interconnection is a gamble that can result in expensive and dangerous issues.

Not sure I agree with you here. I would not call them unstable or unsafe. Grounded systems are not without disadvantages. Can you expand on what you mean by "the utility side of the interconnection is a gamble?"

 

[ggunn]

We have gotten pretty far afield of the original question. Can you connect a three phase inverter (specifically an SMA Tripower) that is designed for only wye connectivity to a delta service?

In a word: No.

 

[electrofelon]

We have gotten pretty far afield of the original question. Can you connect a three phase inverter (specifically an SMA Tripower) that is designed for only wye connectivity to a delta service?

In a word: No.

Let's not keep things TOO simple here....

 

[Rock-Noll]

I appreciate all of your discussion and especially the Eaton paper link. Definitely learning a lot here! One of my Dad's favorite sayings was:

"Experience is what you get, when you don't get what you want"

I'll be in touch with the utility engineer and be figuring things out.

 

[ggunn]

"Experience is what you get, when you don't get what you want"

Cue the Rolling Stones...