Back in "my day" the grid was mostly (exclusively?) served by rotating generators where the rotation was directly linked to the frequency and phase of the grid. For a reason that escapes me at the moment, the grid frequency was the primary indication of supply vs demand - and the machines were regulated by speed so that if the frequency were to drop then they would increase their output, or if the frequency rose then their output would reduce.
As I understand it now, even big wind turbines use electronic invertors to generate the AC and sync with the grid. How do these interact with the grid?
For example in a situation where a lot of wind power was poured into the grid resulting in an oversupply, would the wind turbine cut their output or would they be relying on conventional power stations making the correction?