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My question is somewhat related to this one.

Most of the 3 phase inverters used for photovoktaic (PV) on grid installations can work only if there is AC voltage present. After the AC voltage disappears, the inverter is turned off due to safety reasons.

I understand how the inverter can see that there is no AC before it starts working - it can easily get in sync with the AC waves.

The part I don't understand is how can it figure out that the AC voltage dropped after synchronization was in place already? The inverter is "pushing" the current out now, so how does it know that there is no grid voltage, since even if it disappears, its own voltage is there- how can it tell the difference?

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    \$\begingroup\$ There are multiple ways to do this. A good search term is anti-islanding. Ignoring the edge case of the inverter just satisfy the demand in case of island, the PV inverter has no say in maintaining the grid voltage. If you loose your grid connection, voltage will drop significantly if demand>solar and vice versa. You can think of it not as an voltage source inverter with its own sine reference but rather a current source locked to the grids voltage. \$\endgroup\$ – winny Sep 1 at 7:02
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    \$\begingroup\$ Check also the rate of change of frequency ROCOF relay used to detect the loss of grid or islanding. \$\endgroup\$ – Paul Ghobril Sep 1 at 7:35
  • \$\begingroup\$ Ok, I think I get the general idea. In that case, a follow up question- is it technically feasible to in event of AC mains shutdown disconnect from the grid and connect to a battery powered 3 phase inverter that allows for bidirectional current flow (charging and discharging the batteries)? \$\endgroup\$ – Łukasz Przeniosło Sep 1 at 10:09
  • \$\begingroup\$ It's pushing maybe 10A. That isn't going to move the grid very much. So the grid's inherent phase is still visible ... only minutely modified by your contribution. \$\endgroup\$ – Brian Drummond Sep 1 at 10:39
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    \$\begingroup\$ That's why I couldn't find it ... it was moved to Home Improvement diy.stackexchange.com/questions/196197/… In essence, it was a grid tied system normally, but with a separate isolator so it could safely be islanded without killing linemen. Then, the Powerwall acted both as battery powered inverter, and charger. With no load, it charged itself from the PV grid tied inverter. When full, it let the "grid" frequency rise, and the PV inverter shut off. Neat. \$\endgroup\$ – Brian Drummond Sep 1 at 10:54
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A grid tied inverter is a current source, precise synchronisation isn't needed to avoid catastrophe because the current is limited.

The inverter watches the voltage and pumps the current in the appropriate direction.

If it sees the voltage or frequency going out of range it deduces that it has been disconnected from the grid and stops feeding it.

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