https://spectrum.ieee.org/baltic-power-grid

This is where synchronous condensers step in. Synchronous condensers (also called synchronous compensators) are essentially generators that, in normal operation, are spun by an AC grid’s power and synced to its frequency (rather than driven by their own fuel). When power plants and/or transmission lines shut down unexpectedly, the momentum in their spinning mass offers an instantaneous supply of energy that cushions the blow, thus protecting equipment and preventing outages.

 

So it is. The remaining connected to one-eyed russian gangsters rope is hellish risky, but cut off the wires is risky as well. Therefore new peak power recuuperator stations are essential, however the Riga HES cascade have peak power 400 MW what can be activated in two or three seconds, however at this power the 100 km long lake will be dried out in 12 minutes. Thus, the fact it is peak energy station. But for compensation of faster than 2...3 seconds deviations we have nowhere nothing, until now.

 

A synchronous compensator is a short timescale device but it is very similar to a long timescale device such as pumped storage. It is roughly equivalent to the capacitors in a big linear supply (hence condenser) versus the battery in a UPS.

Really, it is not any different than flywheel storage except that it is explicitly deigned to float when things are happy and dump when the sadness starts.