PV Inverter output to 2 out of 3 phases...

[BretHeilig]

I'm trying to propose a commercial PV array that will be intertied to a 120/208Y service. There is only enough room for about 7kw worth of modules, however, which means I have to go with an inverter that only outputs to 2 of the 3 phases. The inverter does have a neutral.

I understand how load-balancing works with a split-phase system (worst-case scenario, the neutral is carrying all the return current), but I don't really get how the concept applies to a three-phase system. Is this something I just shouldn't do? Or is there some rule I can follow ("keep the neutral the same size as the hots") or some existing condition I need to verify, which will make this okay?

Thanks for any advice, or links to explanations


Bret

 

[augie47]

No neutral sounds like a European inverter.
My opinion would be that you could only supply 240 volt loads with that inverter. You could always add a transformer to obtain 240/120, but I think it would be more practical to seek a different inverter.

 

[BretHeilig]

The inverter does have a neutral the SB6000US is definitely rated for use in the United States.

 

[augie47]

sorry, I misread.
You definitely would not want to supply and pure 3 phase loads with the inverter so connecting it directly to a 3 phase panel would cause problems.
From a pure electrical standpoint you could feed any single phase load
from the inverters and were you to supply 2 of the three phases of a MWBC there should be no problem with the shared neutral.
If it's a utility-interactive system I see no way to connect it to a 3 phase panelboard. If it's a standby system thru a transfer switch, I could see picking up selected MWBC loads, or single phase loads, on a subpanel fed by both thru a transfer switch.

 

[LarryFine]

. . . I have to go with an inverter that only outputs to 2 of the 3 phases. The inverter does have a neutral.

Are you saying that the inverter's output is 120/240v 1ph, or that it's a 4-wire 3ph output that can be rewired for 120/208 1ph?

If it's the former, you will have intertie issues, as Gus mentioned.

. . . is there some rule I can follow ("keep the neutral the same size as the hots") . . .

Keep the neutral the same size as the hots.

 

[dbuckley]

The Sunny Boy 6000 is a single phase inverter, and on a 208/120 system its connected between any two hots as a 208V single phase source.

 

[LarryFine]

The Sunny Boy 6000 is a single phase inverter, and on a 208/120 system its connected between any two hots as a 208V single phase source.

Using the neutral also, of course. Without it, you'd have typical missing-neutral imbalances on line-to-neutral loads.

Will the output be 120/240 or 120/208? If the former, you need to be sure any line-to-line 208v loads can take 240v.

 

[LarryFine]

Double post due to a glitch in the matrix.

 

[dbuckley]

Using the neutral also, of course. Without it, you'd have typical missing-neutral imbalances on line-to-neutral loads.

I think thats the point: its single phase inverter, with two output terminals, which for 208 is wired across two hots, no neutral, connected to a 208Y/120V system. The inverter can't supply 120V loads on its own, only the utility can do that, and as soon as the utility fails the inverter shuts off too, otherwise (quite apart from the risks associated with islanding) the voltages would, as you note, be all over the shop due to the lack of a neutral reference.

On a 208Y/120 system this inverter cant have a "real" neutral as then it would need to generate two phases 120 degrees apart, and as a single phase inverter, thats beyond its abilities.

 

[iwire]

First off keep in mind we are talking grid tied inverters not stand alone.

The single phase inverter will do it's job perfectly well connected to a three phase system. I have connected a 50 KW straight three phase inverter to a 208Y120 service, it does not need a neutral to lower the buildings power consumption.

 

[augie47]

Lesson learned Bob.
Always follow your first instincts I should have deferred to someone like yourself with knowledge of PV systems.
Another lesson learned (about PV systems and about answering) :grin:
as always, a tip of my hat, Sir.

 

[iwire]

Lesson learned Bob.

Only because I asked the question earlier.


No neutral? How does it work?