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?