Backfeeding Three Phase 208 With Single Phase Inverters - Omitting C Phase

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Anthonys

Member
Location
WA, USA
I've designed a system with a balanced three phase backfeed, three single phase 208v inverters loaded identically, combined, and then made to connect w/3 pole brk to a 208v three phase main panel.

Is there any reason why 2 larger inverters could not be used instead, and just land with a 2P breaker in the main? My understanding was that unbalanced loads are not favorable. Is this with regards to voltage only?

I know in some cases an unbalanced load can be favorable to rebalance, but assuming there was already a balanced panel, would it be unfavorable to backfeed with only A and B phase, and omit phase C?
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
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Retired PV System Designer
If the service is 208Y/120, then each 208V inverter will connect to two phase lines.
So one inverter might connect from A to B, the other from B to C.
There is no way that can be balanced, but for a small output POCO may not mind and they can balance your generation to some extent with other's loads.
For a high output system, POCO's problem is that they will not have other generation to balance it with and loads will have a different time pattern. :(
If you use only one 2P breaker, you will be putting both inverters on one line-to-line phase, which POCO is not likely to be happy with for a large system.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I've designed a system with a balanced three phase backfeed, three single phase 208v inverters loaded identically, combined, and then made to connect w/3 pole brk to a 208v three phase main panel.

Is there any reason why 2 larger inverters could not be used instead, and just land with a 2P breaker in the main? My understanding was that unbalanced loads are not favorable. Is this with regards to voltage only?

I know in some cases an unbalanced load can be favorable to rebalance, but assuming there was already a balanced panel, would it be unfavorable to backfeed with only A and B phase, and omit phase C?

It is not voltage per se that the power company is concerned with, it's the power imbalance. Most utilities will allow a "small" imbalance, but you'll have to check with them as to what that limit is. I'd be willing to bet that whatever size your two inverters are, the power company would prefer you to not put them both across the same two phases, which means you'll at least have to have a three phase AC combiner panel. Then you'll have to figure your output current from the panel and size your conductors and OCPD accordingly, which is a non trivial exercise for an unbalanced system, and then if you interconnect through a three pole backfed breaker you may have to wire it up as if you had three inverters instead of two, anyway. You might as well just go with the three smaller inverters, IMO.
 
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Anthonys

Member
Location
WA, USA
Great feedback. I would just presume doing the three smaller inverters, and I don't know why I didn't think about using the four outputs for two inverters across three phases, duh. . . But it is good reassurance to my original train of thought.

I'll keep to balanced systems as much as is possible, but I'm eager to learn how to go through the 'non trivial' exercise of calculating unbalanced loads as well. Perhaps I will seek out some research on that. Plenty out there.
 
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