For a stable synchronous power system, frequency all over the system is equal in steady state.

a) Load Generation Imbalance causes decline or rise in the speeds of synchronous machines connected in the system.

b) If prime movers deliver a constant power, the equilibrium value of frequency is determined by the frequency dependence of loads. Since this dependence is weak, the frequency decline or rise due to load-generation imbalance can be quite high.

c) Prime mover control is done by making prime mover power a function of the speed. This mechanism is called a speed governor. Speed Governors prevent sudden large rise or fall in speed.

d) The sharing of extra load by generators is determined by the droop of speed governors. The droop is related to the steady state proportional gain given to the speed error. Due to proportional gain feature, a steady state error in frequency will remain when a governor causes a steady state change in prime mover power. Therefore frequency regulation by a governor is not perfect, but permits a droop so that sharing can be achieved conveniently.

e) Frequency can be brought back to the nominal value and inter-area power flow can be regulated by another slow acting control loop which controls prime mover output in a few generators in each area. This is called Automatic Generation Control. It is important to note that an AGC is much slower acting than a speed governor.

f) The steady state scheduling of generators based on economic factors is done by changing the load reference of the prime mover manually.

Therefore there is a hierarchy of prime mover controls, operating on three different time scales which decide the final generator output.

In emergency situations wherein large load-generation imbalance exists generator or load tripping may be required to prevent large frequency changes.