One of the parent posts correctly identified there are thermal issues with operating at significantly off nominal frequencies for extended time periods. System operators will specify frequency bands and minimum time a generator has to stay online for each band. For example we have to stay online indefinitely for frequencies 58.5 to 61 Hz, and can trip after 60 seconds for 57.5 to 58.5 Hz, and can trip instantly below 57.5 Hz.
Some plants the system operator has the ability to send a signal to instantly open the generator breakers to shed generation if necessary.
>> It can be applying its maximum power into a grid that is still short of power, which is probably a better situation than if it was not doing that.
> One of the parent posts correctly identified there are thermal issues with operating at significantly off nominal frequencies for extended time periods
Yeah, that's very much the problem. Power sources that burn fuel to generate electricity generally don't like being run at 100% throttle for long periods of time. In a low frequency situation you've got potentially multiple countries of generators all running flat out trying to get the frequency back to where it's supposed to be. If they reduce throttle without having something else come online to take the place of the energy they were producing, the line frequency drops further.
Nuclear has its own issue with running full throttle: once a plant has been running at 100%, it takes time for it to throttle back down and might not be possible to immediately throttle back up. There's decay heat from the fission products and there's short-ish lived (up to 40-50 hours though) neutron poisons.
Some plants the system operator has the ability to send a signal to instantly open the generator breakers to shed generation if necessary.