One, two or three inverters for a three phase system.

[Ozymandias]

Hello,
I am learning all I can about PV systems and want to design a system for the shop I work for. I am working towards the NABCEP installer certification but I need to get some systems under my belt. I have been searching the forum to find some answers but not having any luck so moderators please move or remove my thread if it is redundant.

The service from the pole is 208/120Y, this goes to an 400 amp service disconnect and 400 MDP. I am designing a 25 kw flat roof system and considering a supply connection. My original design was to have a single undersized inverter then entertained two SMA 10.3 kw inverters to manage the array but as I research the subject I wonder if I should have three SMA 7 kw inverters so that I can connect each inverter to two of the busbars of the three phase legs thus balancing the load. Or keep it at two inverters, each connected to two of the busbars of the three phases but not truly balanced across the phases. These inverter(s) will installed on the roof to satisfy rapid shutdown requirements and run into a PV service disconnect near the existing main disconnect. I hope I am making sense.

 

[ggunn]

Hello,
I am learning all I can about PV systems and want to design a system for the shop I work for. I am working towards the NABCEP installer certification but I need to get some systems under my belt. I have been searching the forum to find some answers but not having any luck so moderators please move or remove my thread if it is redundant.

The service from the pole is 208/120Y, this goes to an 400 amp service disconnect and 400 MDP. I am designing a 25 kw flat roof system and considering a supply connection. My original design was to have a single undersized inverter then entertained two SMA 10.3 kw inverters to manage the array but as I research the subject I wonder if I should have three SMA 7 kw inverters so that I can connect each inverter to two of the busbars of the three phase legs thus balancing the load. Or keep it at two inverters, each connected to two of the busbars of the three phases but not truly balanced across the phases. These inverter(s) will installed on the roof to satisfy rapid shutdown requirements and run into a PV service disconnect near the existing main disconnect. I hope I am making sense.

10kW of imbalance may be more than your POCO is willing to accept. Of the two scenarios involving single phase inverters, three 7kW's is better than two 10kW's.

A three phase inverter or a couple of them would be a more straightforward installation, and with a 400A main disco and a 400A bus in the MDP you can land up to 80A on a backfed breaker. You could, for example, use a pair of Fronius 11.4kW three phase inverters with their output combined in a solar load center. The total maximum output would be 63.2A @208V. Multiply times 1.25 to get 79A (minimum OCPD and the number to use in the 120% calculation in the MDP). Backfeed a 80A breaker in the MDP. 25kW overloads the inverters by about 10%, which is reasonable.

 

[jaggedben]

I sure as heck would think exactly as ggunn. Those Fronius inverters would seem to be ideal.

 

[Ozymandias]

Thank you both. I do like the three phase inverter(s) option and landing into a breaker since there's room in the panel. There's plenty of room in the roof for expanding but 25kw is the max for utility rebates here in Missouri. Combining the output of the inverters has me stumped, a solar load center looks like a meter can with a bus for breakers. At the moment the meter can is on the outside being fed by 10 AWG jumpers from the service point splice while the rest of it goes indoors to the 400 amp disconnect. Is it just combining the inverter outputs into a junction box in the roof and piping into the building to a (un)fused disconnect next to the existing main disconnect and then to a backfed 80 amp breaker in MPD?

 

[jaggedben]

You said a bunch there that raises questions...

- Is a separate solar production meter required? (by the utility, by the rebate program, or whoever?). This depends on your local regulations. If one is required and you're talking about using a meter main combo that would probably only work for a supply side connection, because those come with bonded neutrals. A separate meter socket is not expensive.
- Typically one would use a small subpanel to combine inverter outputs. I believe splicing in a j-box is technically allowed but you'd be required to use 80A wire everywhere and that might lead to problems terminating in the inverters.
- If you've got 10awg going to the meter on a 400A service that is almost certainly a CT meter.

Replace the j-box in your proposal with a subpanel and it sounds ok. Whether you need the unfused disco depends on utility requirements, as one can usually get a locking device for the breaker to satisfy the NEC.

 

[Ozymandias]

I don't necessarily need a separate meter, I am not familiar with a solar load center term and after a quick search a meter/main combo came up. And you're correct about the CT.
So far it seems the layout will be
25kw array feeding into 2 fronius 11.4 kW each (combiners if needed which depends on mppt options and string calculations) which will be placed on roof to satisfy rapid shutdown. The output of these inverters will combine into a subpanel that will feed into the MPD hopeful satisfying the AHJ. It's good to know about the locking device on the breaker. This is all in the planning stages. I want to involve my employer in the solar business so I'm trying in earnest to learn as much as I can on the subject.

 

[ggunn]

Combining the output of the inverters has me stumped, a solar load center looks like a meter can with a bus for breakers. ... Is it just combining the inverter outputs into a junction box in the roof and piping into the building to a (un)fused disconnect next to the existing main disconnect and then to a backfed 80 amp breaker in MPD?

A solar load center is nothing more than a panel like an MLO MDP. The inverter breakers are where load breakers would normally go, and the conductors feeding the backfed breaker in the MDP are landed on the lugs of the solar load center. As long as all the breaker ratings in the solar load center add up to less than the rating of the busbars, the bus qualifies under 705.12(D)(2)(3)(c) if you are operating under the 2014 NEC. Many (most?) AHJ's will approve such a configuration even under 2011, although it's not explicitly allowed.

 

[Ozymandias]

Great! Thank you.

 

[BillK-AZ]

Newer Fronius inverters

Newer Fronius inverters

The Fronius IG series with the 11 kw inverter is obsolete. The newer Fronius 3-phase Symo 12KW works well for this. Lighter, lower cost, and easier to install. There are two MPPT inputs that can be paralleled if desired.

There is a handy online tool for sizing the array:
http://www.fronius.com/froniusdownload/tool.html

You can learn a lot with this tool.