Adding panels via grid-tie micro-inverters to an off grid system

[miahallen]

The brother of a friend operates an off-grid community well & water treatment system. The system runs fairly well at 240V 60Hz 3-ph using 3 Outback inverters. They setup the system several years ago, and are looking for more power. While discussing micro-inverters for separate projects they were intrigued that the micro-inverters were compatible with a wide range of grid types, including a 3-phase setup like I described above. They proceeded to ask if they could increase the system they had with grid-tie micro-inverters.

My thought is that it would not work, but I thought I'd ask if anyone had experimented with such a possibility...and if so, what were the results? Thanks

BTW - The brother doesn't speak English and my friend doesn't know many details about the system. If we need more specific about the system, I would have to dig a bit. TIA

[Amy@altE]

That's called AC Coupling. It's a little complicated, but the output of the Outback and the Enphase would be connected, and the Outback would trick the Enphase into thinking the grid is still up. I haven't done it with 3-phase, but it might work. I think the simpler way to go is to just add another charge controller for the new panels and go the old fashioned DC Coupled route.

Here's an article Enphase wrote about it. http://enphase.com/global/files/Enph...ed-Systems.pdf

[miahallen]

Thanks for the reply Amy...as I read that document, it occurred to me that all of the safety & protection mechanisms referenced are already present in the Outback inverters on the "AC-input" connection, where you would normally connect either grid-tie or a genset (see diagram below).

FX.JPG

It seems this could potentially offer the protections necessary. So the question in this case would be; could it handle the fluctuation power coming in from a PV array?

[Amy@altE]

However, the AC input is just that, an input, so the Enphase would not see the grid as up and would turn off. It needs to be connected to the output so it sees the power and thinks the grid is on. But the output doesn't have the 3-stage AC charger, so tricks have to be deployed to turn off the Enphase when the battery is full.

[miahallen]

Oh that makes sense, anti-islanding would kick in huh? Thanks again Amy.

[Mike90250]

possible, if you keep the added PV, below the limit of what would cook the batteries if the pump was off while the water tanks were full.

The 3 (or 6, or 9) added panels to the AC grid, will, as demand allows, backfeed the main inverters and recharge the batteries. Not a problem with a large battery bank and a small amount of added power.

[Amy@altE]

It just seems like a round about way to add more solar to an off-grid system. You primarily see AC Coupling used as a way to add batteries to a grid-tied system, not the other way around. Don't complicate the system, just add another charge controller and panels.

[miahallen]

Yeah, the more I'm seeing/reading, the more I'm agreeing with your last comment Amy....thanks all