It's not solid state,just non mechanical, lithium batteries do undergo a chemical and toacertain degree physical state change as ions move and covert compounds from one to another. True solid state would use so called "supper caps", which provide physical spaces for electrons to rest, until discharged, with at present no swelling or physical movement,though swelling might become apparent with high enough capacities, which on consideration seems like it could come with a whole new way for things to go suddenly and drasticaly wrong, where something goes from a solid to a completly disasociated plasma instantly. nothing that will surprise an experienced linesman as initialy it would probable sound alot like the insulation starting to break down just before something carrying a lot of current at very high voltages makes just befor it finds ground

Seems incredibly expensive to build spinning rust generators with the sole intention to solve ancillary services?

We can of course also simply add a clutch between the turbine and the generator allowing the grid to drive the generator and providing that physical response if we somehow realize we can’t do without it.

A response in milliseconds is effectively instantaneous. Also there is no coal in Scotland since 2016.

And if you want physical spinning machines, there is hydro which is obviously big in Scotland because there's plenty rain.

Sure. But, how much?

> It programmed Blackhillock’s inverter to hit 250 percent above nominal current to deliver the 140-millisecond pulse that NESO requires, says Aaron Gerdemann, a business-development manager for SMA. After that, the device will back down, allowing the circuits to cool.

Kinetic UPSes can fit in that niche. They're not exactly expensive to build, nor is there any new technology that needs to be developed. Like many power grid improvements the constraining factors are will and money.