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Grid tie solar with backup

fatjay

Solar Enthusiast
Joined
Oct 31, 2022
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166
I like to think that I don't sniff glue, but I had a thought in the shower last night.

I have a small array I'm building, 4.2kw. I'm picking up a growatt 5k grid tie inverter. The inverter needs a sine wave supply from the grid to start taking power from the panels and supplying it to the house.

When power goes out, I currently switch off my main, switch on my gen breakers, and fire up my gas generator that backfeeds the whole house. My thought is, that will also activate my grid tie inverter, and utilize my solar panels.

But if my generator runs out of gas, what if I utilize my oukitel aio to provide that sine wave with a suicide cord of sorts. Obviously I'd only get one leg out of it, and it would have to be on the same leg of the growatt. obviously I would need to limit my consumption, and watch my battery level, but if i charged it from the panels and grid tie inverter at the same time i'm supplying the sine wave, i should be fine as long as panels are producing and i don't drop to low overnight.

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Very good chance that you'll create havoc and blue smoke.

Grid tied inverters do not self-regulate. They are designed to send the maximum available PV power to the "infinite" grid, so whever is forming grid, must be able to accept the maximum array output at all times.

The concept you are describing is "AC coupling," but the "grid device" needs to be able to frequency shift the Hz to force the GT inverter to disconnect rather than try to dump all its power into something that can't accept it. Inverters capable of AC coupling can also use the surplus AC on their output to charge the batteries.

 
The solar inverter is going to inject power into the generator which is not designed to absorb it.
Best case this will result in a small overvoltage then the solar inverter will shut down. Worst case the generator will smoke.

It would be better to get an all-in-one solar inverter like DEYE which has ports for grid, generator, solar, and house... and it'll do whatever is needed. The nice thing is, if there's enough sun you don't need to burn gas, and if the house's power use is low you can also run the generator part-time. Since these generators use a lot of gas just to idle it should use significantly less gas.
 
I think i understand what you're saying. Conventional grid makes the power available, my generator makes the power available, even if I don't use it. But the grid tie inverter, it must send the power it produces somewhere?
 
I think i understand what you're saying. Conventional grid makes the power available, my generator makes the power available, even if I don't use it. But the grid tie inverter, it must send the power it produces somewhere?

Yes. Again, grid-tie inverters are designed to output maximum possible power into an "infinite" grid. You either need an "infinite" grid or an off-grid inverter that is designed for AC coupling with the frequency shifting feature.

Frequency shifting is a method by which the off-grid "source" inverter alters it's frequency to something around 58Hz instead of 60 as the batteries hit absorption voltage. The GT inverter doesn't like this, so it disconnects and waits for the "grid" to get back to "normal." As loads are used, and as battery voltage drops, the off-grid inverter shifts the frequency back to 60Hz. The GT inverter re-syncs with the "grid" and then begins outputting max PV power. The "grid" inverter uses that power to power loads and/or charge the battery.

It's also important that the "grid" inverter is at least as powerful as the GT inverter.
 

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