I do work with power plants in the US, control systems for major hydro
facilities.

A _lot_ of effort goes into maintaining a highly accurate long-term
frequency.

Basically, the power plants deliberately overgenerate to push the frequency
a little higher and undergenerate to pull it back down. Obivously all the
plants on the same power net must remain in sync,

Some plants are given the job of driving things in the right direction by an
"Energy Control Center" which tracks the long term error in the frequency.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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