[tankenmate]

As you say, I suspect it probably isn't used much in grid scale applications. The device image in the article would imply that it is just a simple damper, i.e. the angular momentum acts to reduce any frequency change, up or down. It doesn't look like it does any storage, but then the article is somewhat content free.

[mrow84]

What you are describing is essentially a centrifugal governor: https://en.wikipedia.org/wiki/Centrifugal_governor, I think?