PV energy feed-in to the grid
The problem you are facing here is the individual PV inverters are having a different idea of what the AC power flow is. They only see voltage and the phase angle between voltage and current at their own location.

If the grid is reasonably soft, as most low voltage grids are, turning on/off a big inductive/capacitive consumer can make the local grid "bounce" and cause "ringing" between the regulation circuits of multiple PV inverters. This is especially bad with washing machines and microwave ovens, which are controlled in an off/on fashion.