Automatic generation control (AGC)

AGC is a generator control system that adjusts the real power output of generators in response to control signals from the system operator’s energy management system (EMS) within a time frame that is typically two to five seconds. The EMS monitors system frequency and sends signals to generators to adjust supply as needed to maintain the system frequency (50 or 60 Hz depending on the region). Control signals are transmitted via telemetry to remote terminal units (RTU) at the generator. The RTUs convert the raise/lower megawatts (MW) into instructions to the generator governor, which results in a change in the generator output power. 

AGC is used to maintain acceptable frequencies during normal operation due to fluctuations in load and variable resources, and as an early response to system contingencies such as the unexpected loss of a generator or a transmission line. AGC units are used to provide frequency response reserves.

Typical sources of AGC include hydropowergas combustion turbinesgas combined-cycle gas turbines, and gas or coal steam turbine units that are providing energy from a portion of their capacity but that have additional unused capacity that can be ramped in the two- to five-second time frame. The equivalent of AGC can be provided by battery storage.