Phase Imbalance

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Braden
Braden Registered Users Posts: 9 ✭
in Advanced Solar Electric Technical Forum #1
In creating commercial scale grid direct system designs I have received somewhat conflicting opinions on the number of 3-phase inverters to use to avoid phase imbalance.

I have had a master electrician tell me that an even number of 3-phase inverters is needed to avoid phase imbalance entirely (unless only 1 inverter is used). And that while minimal phase imbalance is okay, its far better to avoid it altogether.

I have had another master electrician tell me that while an odd number of inverters can create a phase imbalance, in the case of the system design he was examining, it was small enough to pass utility regulations for interconnection.

I have had an electrical engineer tell me that in order to avoid phase imbalance in a 3-phase system, the number of inverters used, if not 1, must be divisible by 3.

I am neither a master electrician nor an electrical engineer. In fact, I have a decidedly non technical background and am just now learning PV system design for my company. So I guess my question is twofold:

1) can anyone point me to a good resource where someone with a technical background limited to college Physics 101 can understand what it is, how it relates to inverters, and how the inverters cause it?

2) if I fully get 1 answered, I should be able to figure this out on my own but just in case: In your expert opinion, who is correct regarding the number of inverters used in a system.

Thanks in advance.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
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    Re: Phase Imbalance

    i'm not sure if i'm fully understanding your concerns on this, but if 1 inverter can generate all 3 phases then one would assume those that designed it did so with it balanced as far as power distributed to each phase. the phasing itself, however, is dictated by the grid. if the power company has a leg out of phase then the inverter will follow it. the reverse can't happen.
    i would also imagine that for large scale systems that individual inverters would be used to provide more power. each inverter should provide the same power to the grid for power balance and by definition there would need to be an inverter for each phase or because it's 3 phase there are to be 3 inverters. to expand upon any 1 leg with another inverter would create a power imbalance so the other legs would also need an inverter of equal power added. this means multiples of 3 inverters that are of equal power to be added at each time. as an example now, say you have 3 inverters on the grid connection each at 2kw. you could add 3 more inverters be they all 1kw, 2kw, 3kw, or whatever kw as long as they are the same with each other. they don't necessarily have to match the 3 that are already there. each inverter will lock onto its own phase and feed power to it.
    hope this covers what you were asking.
    someone can correct me if i'm wrong, but i also believe the nec is requiring some way of keeping the balance should any inverter fail. that means taking the other 2 out of line automatically.
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  • newenergy
    newenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    #3
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    Re: Phase Imbalance

    Say you have 3-phase 208/120 "Y" where there is 208V between any two lines and 120 volts between any line and the neutral.

    In this system you could have 3 120V inverters all line to neutral and it would be balanced. Or, you could have one 208V inverter between two lines and one 120V inverter between the other line and neutral and if sizing were right, they could balance.

    If you have a 3 phase system with no neutral and all legs at the same voltage I don't see how you can precisely balance the loads without 3 inverters or a number divisible by 3 unless you have an inverter that has 3-phase output.

    Although, if you really wanted to balance 2 inverters, I reckon you could combine their output and then use transformers create 3-phase output.

    Of course you could have any number of inverters that have 3-phase output.

    ----

    I know there was a discussion in some other thread about the NEC and balancing like this, but I don't remember where. I've never seen anything in the NEC about this specifically about inverters, but maybe it's interpreting something more general about load balancing.
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  • newenergy
    newenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    #4
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    Re: Phase Imbalance

    OP - this is a good article that has a lot about 3 phase power.

    http://science.howstuffworks.com/power3.htm
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