Three Phase Grid Interconnection (PV System)

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SEGuser

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I am currently designing a 20kw photovoltaic system for a large commercial application (480/277WYE) and the customer has requested we engineer the system to hit exactly 20kw. In attempting to do so we have determined one possibility would be the following:

(2) 7000W Inverters (277V output)
(1) 6000W Inverter (277V output)

These three inverters would each utilize a Line to N connection. But I know that the (2) 7kW inverters will be producing at ~25.24A while the (1) 6kW inverter will be at ~21.66A. To my understanding this could be an issue.

Now, please note I am a mechanical engineer by education I have 2.5 years experience with single phase interconnections in the solar industry. But I am still a more or less in-experienced with three-phase.

So I am wondering how I calculate the phase imbalance (if any). The local utility, like most, has a restriction on the imbalance of the power we back-feed. And I can not remember or find instructions on how to calculate this, so if there is a specific formula or process I should go about to determine this I would greatly appreciate any help. OR if there is some other problem regarding this set up please let me know.

Thanks for any/all of your help in advance.

Taylor
Solar Engineering Group
 
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SEGuser

Member
Ahh, now we get into the fun part of solar... the politics and rules of the Utilities.

The utility in question splits their incentive program at 20kw, below 20kw the incentive is based off installed watts, while above 20kw is production based. And as I'm sure you would agree, the up front pay out of the incentive sounds better than the incentive that requires 10 years to pay out in full.

So, if there is a well known/good 20kw 3 phase inverter someone knows of please let me know, I am familiar with all the major players in the U.S. and the pair of 7kw and single 6kw is the best I can think of, barring the phase imbalance.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Ah you kind of lost me here, you could use 26 KW of inverters with only 20 KW of panels installed.

In my opinion the total wattage of the panels is the system size not the inverter size.

But that said I see no real issue with your first plan, the amount of imbalance is pretty small and there is not requirement to be in perfect balance.
 
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SEGuser

Member
Ya, it is strange that the utility has their incentives based on the AC output of the inverter(s).

But that is the way they are doing it apparently.

So as far as the (2) 7kW and (1) 6kW they list there phase imbalance in terms of kW difference in some region. And from what I can tell it's typically ~6kW variance between inverters. I am going to find out for sure. But I should be good with this set up if that is the case.

Thanks for the input.
 
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BillK-AZ

Senior Member
Location
Mesa Arizona
Three Phase Grid Interconnection

Three Phase Grid Interconnection

I just discussed having 2 11.4 KW Fronius inverters connected to a 400-amp 3-phase service with Arizona Public Service and they were not concerned about imbalance because the inverters were much smaller than the loads. The need is to install these this year on a 1-phase 240/120V system and be able to connect in about a year to a planned 208/120V 3-phase with a multi-pack six meter service entrance (3? in/ 1? out) after a building expansion.

The Fronius IG Plus line has both 1 and 3-phase inverters.

At 25 KW you need to consider all be back feed issues at the service entrance.

In Arizona the rebates are based on PV DC watts, but drawings required by the utility must have titles based on nominal inverter ratings.
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
SEGuser said:
I am currently designing a 20kw photovoltaic system for a large commercial application (480/277WYE) and the customer has requested we engineer the system to hit exactly 20kw. In attempting to do so we have determined one possibility would be the following:

(2) 7000W Inverters (277V output)
(1) 6000W Inverter (277V output)

These three inverters would each utilize a Line to N connection. But I know that the (2) 7kW inverters will be producing at ~25.24A while the (1) 6kW inverter will be at ~21.66A. To my understanding this could be an issue.

Now, please note I am a mechanical engineer by education I have 2.5 years experience with single phase interconnections in the solar industry. But I am still a more or less in-experienced with three-phase.

So I am wondering how I calculate the phase imbalance (if any). The local utility, like most, has a restriction on the imbalance of the power we back-feed. And I can not remember or find instructions on how to calculate this, so if there is a specific formula or process I should go about to determine this I would greatly appreciate any help. OR if there is some other problem regarding this set up please let me know.

Thanks for any/all of your help in advance.

Taylor
Solar Engineering Group
Click to expand...

No problem at all. It will cause a slight imbalance in the Utility power draw or backfeed, but they are used to it. Guess what happens when you loose one of the inverters? You still be OK, but most likely have a slight voltage imbalance unless the Utility is fairly stiff.
 
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JayP

Member
SEGuser said:
The utility in question splits their incentive program at 20kw, below 20kw the incentive is based off installed watts, while above 20kw is production based. And as I'm sure you would agree, the up front pay out of the incentive sounds better than the incentive that requires 10 years to pay out in full.
Click to expand...

I confused if 20 kw is split point - wouldn't you want to be a little above or below?

also I don't see why anyone would use three single phase units instead of a three phase unit.
I would think your efficiencies would suffer.

note- I'm a power EE and actually restored a 100kw solar field from 1980 - a few years ago.
 
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broadgage

Senior Member
Location
London, England
As others post, an imbalance of only about 1KW is most unlikely to be a concern, I bet that the load is more unbalanced than that !

Under very favourable conditions, a PV array can produce more than its rated wattage.
If the PV array and the inverter are of eqaul rating, then under favourable conditions the PV output will exceed the inverter rating, the inverter will then limit its output power to avoid overheating, and thus waste some of the available PV output.

I would suggest the use of three inverters each rated at 7,000 watts, and not quite fully loaded, with the 20KW of PV modules divided equally among the 3 inverters.
Use of only one inverter type may be slightly cheaper, and might make stocking a spare worth while.
It should be possible to claim any rebate etc. for a 20 KW system even is the system is slightly larger.
Grid tie inverters tend to introduce third harmonic currents in the neutral if used line to neutral in a three phase system.
This is unlikely to be of practical concern if the inverters are small in relation to the service, as is often the case.
 
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