Active power and reactive power must remain balanced between generation and usage. This is often controlled from centralized control centers that use automatic generator control (AGC) capabilities to control the multiple generator units.
The AGC has an effect on both frequency and voltage. The frequency is highly dependent on the active power while the voltage is highly dependent on the reactive power. Therefore the control can be decoupled into two independent functions; active power and frequency control while the other is reactive power and voltage control.
The active power and frequency control is referred to as load frequency control (LFC).
