In power systems, automatic generation control (AGC) is the control mechanism responsible for:
- maintaining the system frequency to the desired value
- maintaining real power interchanges between balancing authority areas at the scheduled values
Generating units with AGC capabilities are units whose outputs can be remotely controlled by the system operator’s energy management system (EMS) at time intervals as short as every four seconds. Units with AGC are used to manage the second-by-second fluctuations in system balance between supply and demand caused by load fluctuations, variable renewable supply, and other supply fluctuations. AGC-capable units are ramped up and down remotely by the EMS and are used within successive five-minute periods to keep supply and demand in balance.
Typical sources of AGC include hydropower, gas combustion turbines, gas combined-cycle gas turbines, and gas or coal steam turbine units that are providing energy from a portion of their capacity but have additional unused capacity. In some control areas, batteries, fly wheels, and flexible loads also provide AGC capabilities. In regions with Independent System Operators (ISOs), capable resources provide AGC through the frequency regulation market, which is one of the ancillary services markets.