Grid tie inverter synchronicity

[Tired sparky]

Hello, I have had great success cobbling together a DIY solar system that includes panels, grid tie inverter and battery backup.
If there is a mains power cut which fortunately are rare in the UK but maybe will become more frequent soon, my grid tie will automatically island disconnect. Then if I want to use my battery’s/inverter I have to isolate the incoming mains and then connect the battery/inverter system to the house.

So my question is this, if during the power cut/outage I also connect my solar panels, will my Solar grid tie inverter now Sync with the Battery/inverter pure sign waveform that is powering the house?

If it does Sync will back feeding the battery fed inverter harm it? Basically I will be having the battery fed inverter feeding my house with the solar grid tie also trying to feed it.

Does back feeding inverters harm them? I’m thinking not as long as both A.C waveforms are in Sync?

[peakbagger]

Rarely will a standard inverter put out a clean enough voltage and frequency to fool a grid tie inverter. The SMA Sunny Island is one of them that will and actually can control the gird tie inverters to turn on an off if there is not enough demand. I have that setup and it have tested it a few times and my three other grid tied inverters seem to sync in quite well.

[Tired sparky]

Yes fooling the Grid Tie inverter is exactly what I’m after. Great way of describing it lol.

I wonder if the Grid tie inverter simply won’t work if you say any incoming power isn’t clean enough to make it Sync or if something will go ‘pop’ ?

[Tired sparky]

It would be great if they did Sync as I could get 2000 watts from the batteries plus another 1500 watts from the panels both enabling higher power appliances to work and lessening demand on battery capacity.

[Ampster]

..........

So my question is this, if during the power cut/outage I also connect my solar panels, will my Solar grid tie inverter now Sync with the Battery/inverter pure sign waveform that is powering the house?

If it does Sync will back feeding the battery fed inverter harm it? Basically I will be having the battery fed inverter feeding my house with the solar grid tie also trying to feed it.

Does back feeding inverters harm them? I’m thinking not as long as both A.C waveforms are in Sync?

The concept in the US is called AC coupling. As mentioned above Sunny Island inverter is one that can do that. However it is not as simple as "fooling the GT inverter". There needs to be a means to control the output of the GT inverter because it is designed to output to a grid which presents an infinite load to the GT inverter. In the US the UL1741SA standard requires that GT inverters respond to changes in frequency by modulating their output. There are also tests that a GT inverter goes through that test for the presence of a grid so finding a grid forming inverter that can pass the test from a GT inverter may be the hard part. In addition the grid forming inverter must have current measuring devices and an algorithm that can adjust its output and the output of the GT inverter as the loads change. Generally there is a relationship of how much GT inverter capacity can be AC coupled to a battery inverter that is doing the grid forming and some of that depends on the battery capacity of the grid forming inverter. The battery is the buffer which absorbs the energy of the GT inverter when the loads drop quickly and the GT inverter slowly ramps down. If a load suddenly drops the grid forming inverter could go into over volt protection or worse case let the smoke out. I hope that helps and you find products and terminology that work in your market.

[solardreamer]

It would be great if they did Sync as I could get 2000 watts from the batteries plus another 1500 watts from the panels both enabling higher power appliances to work and lessening demand on battery capacity.

That's great until some clouds pass by where solar power can drop to nearly 0 in a few seconds and the higher power appliances can easily overload trip your inverter. In general, you can't safely run continuous unmanaged loads above the capacity of your inverter. AC coupling is great in general but there are some important caveats. If you are seriously considering AC coupling there are many commercial inverters available for you to consider.

[Tired sparky]

Ah things are never as simple as I would like them to be lol. I understand what you are saying and can now see where the problems would arise.
I really don’t feel like ‘letting the smoke out’ of my inverters so will just run them as standard.

Thanks for the detailed responses.