I don’t see many (any?) plants using them, is the technology there yet? Genuinely curious.
And on an unrelated note, it seems that hybrid power plants are becoming a talking point now and they seem to make a lot of sense. Like covering dams with solar panels saves on water, make PVs cooler/work more efficiently and you don’t need to worry about permits/building more grid connections because you can simply slow down hydro during daytime (and get along with grid following inverters just fine).
California requires all new solar installations to contribute to stabilizing the grid by helping out with regulating the voltage and frequency https://www.openintl.com/california-rule-21-interconnection/
The technology to act as inertia isn't actually hard, is it? You deliberately offset your voltage by a fraction of a millisecond and you have to handle some percent extra current.
I believe that frequency is also variable so you'd need to sample that too.
ERCOT frequencies have sagged when the grid was stressed.