How to do a 3 phase balanced load?

Martha, if you had three inverters, the following drawing - that we drew for a customer of ours - would be a solution. 

I am also looking forward to seeing some responses to this question.

Martha,

As the drawing above shows, you would ideally connect three single phase inverters to a three phase service. The reason for this is to avoid unbalanced loads and the resulting unbalanced voltages. 

When voltage on one phase drops enough in relation to the other phases, it can cause electric motors to work too hard and overheat.

There are a couple of ways to do what you want to do:

1. Check the voltage of each phase. Most 3 phase services have a number of 1 phase loads scattered around - there may be one that is higher - say, phase A. If that's the case, and depending on the size of the service, you may be able to connect the two inverters to phases B and C, thus actually helping to balance the voltages in the service.
2. Look at the overall size of the service. If it's an 800A 3 phase service, and you're going to connect two 32A Inverters, then it's unlikely you will cause any significant imbalance.

The industry standard for "unbalanced voltage" is an overall difference of 3% between the three phases. Also, remember to check the service phases for voltage with the service breaker off - this will show if there is any significant imbalance on the utility side of the service breaker that you should alert you power company to.

NEC 705.100 says that 1 phase inverters may not be connected to a 3 phase system "unless the interconnected system is designed to avoid significant unbalanced voltages".

It's a bit complex to explain in a forum. I would recommend looking at the technical documents on the SMA website since you have SMA inverters.

There are a number of good articles in SolarPro magazine on the subject. You need to check with your utility to see if there is a maximum phase imbalance for PV interconnections. Many limit it to 6kW and that would prevent you from using the 2 8kW inverters.

Some AHJs can put weird requirements on using single phase inverters on 3 phase systems, like requiring some type of control system to trip them all off if one of them trips off. This is due to a misinterpretation of the NEC but the SMA inverters seem to have added some software to help do this. It's discussed in the second link above.

Good luck,

Marvin Hamon, P.E.

Wow. Thanks, that did turn out to be a little complex - but the net result is that we are going to add a 3rd SMA 8000 since there is plenty of demand at the site and enough roof space.

Final connection: 277VAC Phase to Neutral x 3

We will use the SMA rs485 Com cards and the Sunny Webbox, so we may investigate the power balancer option as well.

Thanks to all.