3 phase 240v delta high leg: single phase or 3 phase interconnection?

[MOsolar]

I am looking at a job with a 3 phase, 240v delta high-leg service. From the main disconnect they have a 3phase, 100A, 20 space panel with only two 3phase circuit breakers in it. Also form the main disconnect they have a 1phase, 200A, 40 space panel that is full (minus every third spot of course). My question is, would it be better to design a 3phase interconnection or a 1phase? We are looking to install 25kW of solar.

 

[SolarPro]

It sounds like the system is too large to design for a single-phase interconnection, since doing so will result in an imbalance on the 3-phase service. Typically, you can interconnect in a situation like this using an inverter that doesn't need or use a neutral connection. As long as you interconnect line-to-line, the voltages are all in range for the inverter. You might find a single 3-phase inverter that fits the bill; if not you can probably stack three single-phase inverters on the 3-phase service. If you do the latter, you probably only need to operate one inverter w/out the neutral connection. Note that in some inverters you can enable operation without a neutral connection by moving a jumper inside the device.

While I've made connections like this in the past, we always engaged an inverter manufacturer's applications engineer, just to make sure they signed off on the application and we understood what was required to make everything work. You'll also want to make sure the AHJ understands what you are proposing.

Based on your description of the service, the challenge sounds like finding a Code-compliant place to make an interconnection. If you can't make a supply-side connection on the utility side of the main disco, you might need to replace the main disconnect with a custom (because of the high leg) MLO panel that provides room for up to six service disconnects. (Or something along these lines.)

 

[ggunn]

I am looking at a job with a 3 phase, 240v delta high-leg service. From the main disconnect they have a 3phase, 100A, 20 space panel with only two 3phase circuit breakers in it. Also form the main disconnect they have a 1phase, 200A, 40 space panel that is full (minus every third spot of course). My question is, would it be better to design a 3phase interconnection or a 1phase? We are looking to install 25kW of solar.

Well, an SMA 5kW inverter has 20.8A max current output at 240V. Five of them in parallel split phase would take a 104A X 1.25 = 130A, defaulting to 150A two pole breaker. I think that means you are going to have to go with a supply side tap rather than go into either panel. If that is true you might have no choice but to go three phase.

 

[jaggedben]

While I've made connections like this in the past, we always engaged an inverter manufacturer's applications engineer, just to make sure they signed off on the application and we understood what was required to make everything work.

I agree with that completely.

I would talk to Fronius. Their inverters seem to be the most flexible on the AC side. I contacted them about precisely this question regarding their 3-phase delta inverter (I believe it is 11 or 12 kW) a few months ago. I got what I thought was a yes, although it was a little vague. (That sale ended up not going through, so I can't tell you how it turned out.) In any case, they did state definitively that their 240 single phase inverters will operate without a neutral reference, if set up for it. So you could do, say, 3 8kW inverters, with only one set up to monitor the neutral on the split phase.

I do not believe SMAs UL-listed 240V single phase string inverters will operate without a neutral reference, although you could confirm this with them. I believe SMA is shipping new 3-phase models now though, and I don't about those.

Power-One does not have a solution for this, at least as of a couple months ago when I talked in person to one of their engineers.

 

[BillK-AZ]

You really need to talk with the utility on this. I have come across these that were really of the open-delta configuration, intended for a single 3-phase load such as an air conditioner. The utility (Arizona Public Service) advised that we not connect an inverter across the open part of the delta because the voltage regulation line-to-line was not controlled and would trip out the inverter from time to time. APS advised that we use only the heavier single-phase A-C connection in the attached diagram. Phase balance was not an issue, most of the load is on the basic A-C 120/240V lines.

If you do a line side connection, you have adequate capacity for a 25 kW PV system.

 

[ggunn]

I do not believe SMAs UL-listed 240V single phase string inverters will operate without a neutral reference, although you could confirm this with them. I believe SMA is shipping new 3-phase models now though, and I don't about those.

The standard SMA Sunny Boys can operate without a neutral reference; page 24 of their SB5000-6000-7000-8000 manual shows a three inverter interconnection to 240 delta with a stinger. One inverter has a neutral and the other two do not.

I believe the transformerless models all require a neutral but I am not sure. The new three phase SB's are 480VAC output only.

 

[kwired]

It sounds like the system is too large to design for a single-phase interconnection, since doing so will result in an imbalance on the 3-phase service. Typically, you can interconnect in a situation like this using an inverter that doesn't need or use a neutral connection. As long as you interconnect line-to-line, the voltages are all in range for the inverter. You might find a single 3-phase inverter that fits the bill; if not you can probably stack three single-phase inverters on the 3-phase service. If you do the latter, you probably only need to operate one inverter w/out the neutral connection. Note that in some inverters you can enable operation without a neutral connection by moving a jumper inside the device.

While I've made connections like this in the past, we always engaged an inverter manufacturer's applications engineer, just to make sure they signed off on the application and we understood what was required to make everything work. You'll also want to make sure the AHJ understands what you are proposing.

Based on your description of the service, the challenge sounds like finding a Code-compliant place to make an interconnection. If you can't make a supply-side connection on the utility side of the main disco, you might need to replace the main disconnect with a custom (because of the high leg) MLO panel that provides room for up to six service disconnects. (Or something along these lines.)

The three phase service is already imbalanced, and often is on a high leg delta system. Often there is a larger "lighting pot" and smaller "stinger pot" that derives the third phase for what is usually limited three phase load. If you had a significant three phase load it may be a full delta system, and all three pots may be the same size.

There is also cases in remote areas where they only bring two conductors and a neutral on the primary to cut down on materials cost and they still provide open delta service even if there is significant three phase load. The transformers need sized larger than they would with a full delta in those cases. The load is balanced on the secondary conductors in that case, but is not balanced within the transformer bank.

 

[MOsolar]

Wow! Thanks for all of the input.

I currently have an email in with the utility, since this is my first interconnection on this utility I do not know anyone in the engineering department...yet.

We were planning on using 2 Fronius 11.4-3 if we go with the 3phase interconnection. We spoke to Fronius about this and they said to make sure to "disable the neutral monitoring" I believe that was the wording that they used. We were worried about how that imbalanced phase would affect the inverter, but I guess if we do a 3 phase interconnect it would be 240 phase-to-phase with no neutral. Like I said, I have requested to talk to someone with the utility but I just wanted some other opinions before I speak with them. Thanks again!

 

[MOsolar]

You really need to talk with the utility on this. I have come across these that were really of the open-delta configuration, intended for a single 3-phase load such as an air conditioner. The utility (Arizona Public Service) advised that we not connect an inverter across the open part of the delta because the voltage regulation line-to-line was not controlled and would trip out the inverter from time to time. APS advised that we use only the heavier single-phase A-C connection in the attached diagram. Phase balance was not an issue, most of the load is on the basic A-C 120/240V lines.

If you do a line side connection, you have adequate capacity for a 25 kW PV system.

I didn't get through to the engineering department at the utility but I got a reply from their Program Supervisor that simply stated, "A three phase service needs to have a three phase inverter. Otherwise you risk having a voltage imbalance that will burn up motors." So I guess I will move forward planning a 3 phase interconnection.

 

[ggunn]

I didn't get through to the engineering department at the utility but I got a reply from their Program Supervisor that simply stated, "A three phase service needs to have a three phase inverter. Otherwise you risk having a voltage imbalance that will burn up motors." So I guess I will move forward planning a 3 phase interconnection.

Um, no. Talk to an engineer.

 

[GoldDigger]

I didn't get through to the engineering department at the utility but I got a reply from their Program Supervisor that simply stated, "A three phase service needs to have a three phase inverter. Otherwise you risk having a voltage imbalance that will burn up motors." So I guess I will move forward planning a 3 phase interconnection.

That answer either means that the PS does not know what he is talking about, or that all of their 3 phase services, even high-leg, are closed delta with equal sized pots!

The answer might be perfectly appropriate for an originally balanced three phase service with mostly three phase load.

 

[jaggedben]

That answer either means that the PS does not know what he is talking about, or that all of their 3 phase services, even high-leg, are closed delta with equal sized pots!

The answer might be perfectly appropriate for an originally balanced three phase service with mostly three phase load.

But regardless, one should be able to use single phase inverters in multiples of 3 (or even not multiples of three, if the imbalance is small enough).

 

[GoldDigger]

But regardless, one should be able to use single phase inverters in multiples of 3 (or even not multiples of three, if the imbalance is small enough).

Absolutely!

No need for a single-unit three phase inverter, and I hope the POCO PS was not intending to imply that. That would be one more point against him.

 

[ggunn]

The answer might be perfectly appropriate for an originally balanced three phase service with mostly three phase load.

I even doubt that. A current imbalance does not necessarily mean a voltage imbalance. At any rate, I think that a voltage imbalance large enough endanger three phase motors would knock the inverters off-line.

We have interconnected banks of inverters in non-multiples of three on three phase services numerous times without having a problem.