Turbine speed is controlled by a device called a governor. When the power in exceeds the power out and the speed (frequency) increases the governor lays off the

Turbine speed is controlled by a device called a governor. When the power in exceeds the power out and the speed (frequency) increases the governor lays off the throttle to stop then speed from increasing further. Throttle could be water, steam, gas, etc. Then the grid operator will have some units that operate in “AGC” automatic generation control which would reduce their output until the frequency is back at 60Hz. Units start to trip below 58 and above 62 Hz ish, and turbines should respond to 0.01 Hz change in frequency within 200 ms according to the IEEE 125 guideline, so even a 0.5 Hz deviation is huge. A 1 Hz deviation is a major event that would result from a mismatch on the order of gigawatts