The flywheel is directly connected to the synchronous condenser's rotor, and that rotor is locked to the grid's frequency. The grid's frequency must be maintain

The flywheel is directly connected to the synchronous condenser's rotor, and that rotor is locked to the grid's frequency. The grid's frequency must be maintained within a very narrow range (for a variety of reasons), so the actual speed difference of the flywheel between "full" and "the grid is shutting down" is very small and so most of the energy cannot be put into the grid. This is why flywheel storage uses variable-frequency drive systems; to be able to pull energy out of the flywheel over the entire range of full-speed to ~standstill.