An excitation system is a system that provides regulated DC current to the field windings of a generator to produce an output voltage to the field. The generator turns mechanical energy from a prime mover, such as a hydro, gas, steam, or wind turbine, into electrical energy for transmission to customers. The excitation system excites the armature by creating a magnetic field on the rotor via a DC current. The output voltage of the armature varies with the strength of the magnetic field. Thus, the excitation system controls the output voltage of the generator by adjusting DC current to the generator field winding.

Excitation systems have two general classifications: rotating exciters or static exciters. Rotating exciters include brushless and brushed types, while static exciters include compound sources and potential sources. Modern excitation systems are static, where DC current is created by rectifying AC power using saturable current transformers (SCTs) and power potential transformers (PPTs) .

Subsystems for the excitation system include the processors and I/O devices that monitor generator terminal voltage and current, field voltage and current, rotating exciter field voltage and current, control switches, breaker status, and safety permissives. Outputs include annunciation, alarms, meters, and a full range of data for the distributed control system.

The field flashing equipment is used to generate the initial electromagnetic field until the generator creates enough voltage to self-excite and sustain the mechanical to electrical power conversion. Self-excited generators must be started without any external load attached, as external load will sink the electrical power from the generator before the capacity to generate electrical power can increase).