Which of the following best describes 'excitation' in a generator?

Excitation in a generator primarily refers to the supply of reactive power. This process is crucial for maintaining the voltage levels across the generator and the power system as a whole. Essentially, excitation involves providing DC current to the rotor's windings, creating a magnetic field necessary for inducing voltage in the stator windings. By adjusting the amount of excitation, operators can control the reactive power output of the generator, thereby helping to stabilize voltage levels in the grid.

While you may hear the term used in connection with other aspects of generator operation, such as mechanical input and voltage adjustments, these are either secondary to or influenced by the process of excitation itself. Frequency control is not specifically tied to excitation; rather, it is managed through coordination of generation and load balance within the system. Thus, the best descriptor for excitation in this context is its role in supplying reactive power.