When does a single phase inverter become a 3 phase inverter?

I have a 42KW installation using 6 SMA7000 inverters hanging on a Sunny Tower and 2 SMA6000's mounted on the wall. We balanced each of the 2 legs of the 8 inverters onto each of the 3 phases i.e. AB, BC, AC etc. (There is a slight imbalance, but that's not the issue according to the utility and NEC 690.63)


The local utility refused to allow me to interconnect, claiming that the inverters were "3 phase" and that when one phase of the 3 phase power is pulled, not all of the inverters shut down or "ceased to energize".
The IEEE 1547 interconnection standards differentiate between single phase and three phase precisely in the cease to energize requirement. If a single phase is pulled, a three phase inverter should shut down. Using single phase string inverters (which, according to SMA, has a lot to recommend i.e. modularity, better power production through better MPP etc.... so far they haven't been much help with THIS issue) it isn't a requirement.

Instead of being helpful, the utility and their 40 or 50 engineers just cross their arms and say "Please submit the design you plan to use to rectify this situation for our review." Of course, they had the design and one line for 4 months prior to the witness test and didn't say anything except take my $800 for the interconnection application and witness test. Oh, and begrudgingly point out the IEEE 1547 standards that aren't included in the utility guidelines but that you must abide by (and pay $138 to buy them).

So what makes these inverters 3 phase inverters? Just b/c they're hooked up in a 3 phase environment? And what about a small pv system, say a 2KW with one inverter hooked up to a single phase? How is that different than what we installed? It won't shut down when the phases it is NOT connected to go offline.

Thanks for any help in this regard.

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    I'll take a stab,,, and belie my own ignorance.

    With a conventional split phase 120/240 service,, it is served by only one phase of the primary and transformer on the pole (or on the ground). If that transformer goes dark due to it's own fuse blowing or the line opening somewhere, the entire phase,,, both sides of the split phase 120/240 go dark,,, in turn killing the inverter.

    Now in your case, it sounds like the utility is arguing that because you have an inverter on each phase, when any one phase of the transformer goes dark, only the inverter on that phase will go dark, leaving the others to produce power.

    While I personally can't see why this would matter, since the utility would know when it's own phases are dark anyway, (or live for that matter!) it shouldn't make a difference that all three inverters go dark.

    Perhaps some smarter mind than mine can elucidate as to why this should be a problem.

    Tony
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    maybe i'm wrong here, but why would they have 2 sides of the 3 phase energized too? why should it be you to have a complete shutdown if they don't? remember the inverters, barring inverter failure, are only active if their lines are active making them in violation of the same ruling and it belies me to think they would blame you for piggybacking their violation. how do they shut them all down when a generator or leg of their transformer fails might be a clue as to what you can do? my experience with 3 phase is none so i just throwing my thoughts out.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    Let's see ...

    Are you actually feeding 3-phase back to the grid?

    From the utility's POV 3-phase is 3-phase: AB-BC-CA It is generated that way to begin with, off one generator. If that goes down, all three lines de-energize. All switches and controls actuate on all poles simultaneously.

    3-phase isn't like single phase: a scope picture would show three separate overlapping sine waves. Single phase is one, and the 'half phase' we use for 120 is only part of that.

    If one leg goes down, that must be detected so that the other two can be automatically killed, regardless of cause of fault. Otherwise you end up with a partially energized 3-phase motor running somewhere, which can be a hazard. Think low voltage on a standard AC induction motor: it's bad.

    If you have 3 (6) inverters arranged to provide 3-phase power, they should also be 'control connected' to act as one inverter for that reason. Only the inverter manufacturer would know if such a set-up would truly function that way. Loss of draw on one leg may cause that particular inverter to shut down, but the others wouldn't so the line could remain partially energized.

    There is a surprisingly large amount of resistance in the established grid power system personal against solar. (Pun intended.) My B-in-L is one such engineer; absolutely insists there's no future in solar.

    I've either just made the issue clearer, or muddier. :p
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    actually you have reiterated basically what i said in that the whole thing goes down at the same time on the utility's end. that doesn't matter that the inverters watch their leg part then because all 3 will go down if nothing is there just like 2 individual inverters set to each portion of the 120/240vac system that would go down too if the whole thing goes down. if their ac isn't there then neither is the inverter ac, period, as they are anti-islanding and that's the automatic shutoff requirement needed. the only possible problem as i see it would be if 1 or 2 inverters were to fail that there's an imbalance of power being sent to the 3 phase grid. again, if the grid is up then why would it matter if you are only feeding 1 or 2 of the phases as they'll be absorbed by the grid? there's still no hazard from the inverters that wasn't presented by the utility too, follow me? does it matter if i were to feed only 120vac to the 1 leg of the grid when both legs would be present of a 240vac system as we have? no, and that would be the same scenario of just feeding 1 or 2 of the phases should there be inverter trouble. i don't foresee any circumstance that would make any difference of them not tied together as their utility system will shut them down regardless if individually or tied together. if no utility ac the anti-islanding will shut it down. if there's utility ac on 1 or 2 of the phases (making them the utility in violation) then the inverters will only follow the same violation presented to them by the utility. they are asking for the inverters to be more foolproof than their own utility system as it is possible for lines to go down on 1 or 2 phases from the utility leaving the rest intact and energized. they violate their own rules then because that can happen right outside your place with no safeguards implemented by the utility to stop such an occurence from happening.
    simply, they are yanking your chain because they don't want you to do it and are setting up road blocks to prevent you from doing it by this absurd requirement to you subject to their approval if it's a suitable solution to them. make them show you how it is a violation of ieee and how they aren't in the same violation to you.:grr
  • FL SUN
    FL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    Just looking at SMA's website, found something pertaining to this subject in their Q&A.
    Can I use Sunny Boy Inverters to design a system for a building with 3-phase power?

    Yes you can. In general, the Sunny Boy Inverters may be connected to any 3-phase system, regardless of configuration or grounding scheme, provided the AC voltage is correct for the model of inverter that you are using.
    For more details on all of the design considerations please contact our technical support department at techinfo@sma-america.com or call us at (916) 625 0870.

    And SMA hasn't been too much help with this? I am not familiar with combining inverters in multi-phase applications as of yet, but it seems each inverter should be tied together via communications and react to complete shutdown of system with the loss of a phase or leg to comply with the anti-islanding requirement. That's just too easy to do with today's technology. I am not familiar with the code on 3 phase grid tied systems as of yet either, but it would seem logical to shut down all power manufacturing (on my end) during a phase fault condition.

    That would also limit my liability on the customer's equipment as 3 phase inductive loads don't take to single phasing. By "my" equipment shutting down 100%, that eliminates me or "my" equipment from being liable, or "conveniently blamed" for future single phasing damage from the Power Company's fault condition.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    why should the inverters be made to shut all 3 down if the utility is still sending 2 of the 3 phases? the utility is in violation of the ruling and not the owner of the inverters when they allow 2 of 3 to be active. the hazard is present, not by the inverters, but by the utility sending 2 phases without shutdown. isn't that what that ieee rule was about and yet the utility is allowed to violate it while making the inverter owner compliant? if the 3 inverters are shut down the hazard will still be present from the utility sending out 2 of the 3 phases. make the utility compliant and the inverters follow suite because of the anti-islanding circuitry in them. the utility is being ridiculous because they don't want solar introduced.
  • halfcrazy
    halfcrazy Solar Expert Posts: 720 ✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    I am with Neil on this if a failure acured that made a phase go dark then so what if the other 2 phase's still have inverters running.
  • FL SUN
    FL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    I see your points of view on the issue.

    Mine is through my contractor's spectacles. It's my responsibility to insure the job goes seamless from start to finish. That way is most efficient of all. Customers are happy with no doubt that everything was handled professionally, job is done with no hitches, and I get payment with no reservations on the customer's part.

    All the bugaboos should get worked out and coordinated with the power company even before the tools are taken out of the truck. I'm not there to fight with anyone and prove this or that in the middle of the job.

    If they have a beef with my renewable energy job, then that can be dealt with before the cart gets before the horse.

    Where's the OP anyway? There may be more than meets the eye here.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    he has not been back since asking the question, but he could've checked our answers without logging in. not too worried about it for it is his question and it would be foolish to ask and not come back for the answers. we gave our opinions and it's here for him in any case.
  • System2
    System2 Posts: 6,290 admin
    Re: When does a single phase inverter become a 3 phase inverter?

    Thanks for all of your input. I've been so frustrated with this issue that I stayed away from the computer over the weekend and went fishing.

    I agree with the comment re: having all the details before tools leave the truck. In this case, I filed the one line and specs about 3 months ago long before the installation, and did an additional schematic for the utility b/c of the slight imbalance (8 legs into 3 phases, NEC 690.63 mentions this). It's frustrating. They had the design in hand, and the first clue I had that they were going to focus on this single phase v. three phase issue was the day before the witness test when I saw the test criteria written specifically for this installation.

    If I plopped a 3 phase inverter like a PVPowered or SatCon on this job, I would not have had a problem b/c the circuitry is built in. But there is a valid argument for using string single phase inverters... each inverter tracks only a few strings in the array. Instead of each string being subject to the weakest MPP, only that inverter would generate less power. Modularity is supposed to be a good thing when the inverters need to be replaced down the road.

    I still don't understand how they can say these inverters are three phase. The Sunny Tower is just a racking system for six single phase inverters. Nothing changes inside; each inverter still is what it is.

    There are two EDC's in my state, and they share the same interconnection guidelines. The other EDC, which serves about 80% of the population, has approved this installation in another location without question.

    I'll let you know how my meeting goes next week. Thanks, you've given me some more talking points.
  • mradtke
    mradtke Solar Expert Posts: 53 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    DSimon,

    I am not an expert on any of this, so please consider the source.

    I have a 5kW residential system. I also have 3 phase power. I feed only one phase and the power company is happy with that.

    You have a much larger PV system which makes me think that you might be a business customer with large power demands. If so, perhaps you can just feed a single phase as well and the power company would be happy with that since they don't seem to mind the imbalance that you mentioned. You should be able to use the inverters that you have already invested in, but there may be issues about wire sizes and breakers.

    For what is was worth ...

    Mike
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    just for the record, a single phase inverter does not become a 3 phase inverter. 3 single phase inverters can feed all 3 phases of a 3 phase system if balance is desired, but as i had said 1 of the phases, or even 2, can be fed and absorbed by the grid with no problem. they (the utility) are yanking your chain as these inverters are set to disengage should no power be present from the utility and if the utility presents 2 of the 3 legs you are merely piggybacking their violation that they made no provision for complying with their own requirement. wouldn't that be stupid for you to shut down all 3 phases and they still present 1 or 2 of the phases of the 3 phase? that's a violation on their part that they can't guarantee all 3 would be disconnected.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: When does a single phase inverter become a 3 phase inverter?

    dsimon,
    please let us know how it all comes out.