Well within a fraction of a cycle/second. One of the big reasons is that a generator's mechanical speed is directly tied to the electrical frequency. This incl

Well within a fraction of a cycle/second. One of the big reasons is that a generator's mechanical speed is directly tied to the electrical frequency. This includes the prime-movers that actually drive the generators. Many of these large machines don't react well to operating far from their design speed, which can cause huge issues.