On many posts about the doom-and-gloom of Net Metering run amok, there is talk of "grid stability" being an issue. I frequently see references to the grid frequency changing during times of over/under production. Why is that the case? How much does the frequency vary from true 60 Hz? I'm assuming I just don't understand how the grid uses the frequency of the AC power. My thinking is that the grid frequency comes from the speed that turbines spin at, which I don't necessarily see how those would be impacted. Does the grid use the frequency of the AC power as a sort of power line communication channel to communicate between various power plants on the power requirements at that time? Why does over/under production change the grid frequency?