Comment by namibj

Note that it's typically far easier to just teach the grid behavior aspects to anyways-existing battery interfaced AC/DC converters than to buy separate high power AC/AC converters, along with a flywheel and high power motor for said flywheel.

It's rare that you actually have good reason not just through easily avoidable architectural choices to have dedicated 1~100 second range energy storage. Below that you can just use aluminium wet electrolytic capacitors, and above that you can just use [very] high power lithium ion batteries.

Inherently mechanical systems are different, of course.