Automatic Control of Synchronous Machines

Abstract

The terminal voltage and speed of a synchronous generator are two important quantities on which the operation depends, and special means are necessary to control them if the best operating conditions are to be obtained. The voltage and speed of a generator operating by itself can be wholly controlled by an excitation regulator actuated by a voltage feedback and a turbine governor actuated by a speed feedback. When a generator is connected to a large power system, however, its voltage and speed are determined much more by the voltage and frequency of the system than by the generator conditions. Nevertheless voltage regulators and speed governors play a large part in the operation of the system and are always provided on a large generator. The voltage of the system is to some extent flexible, but the main effect of a voltage regulator is to control the reactive current. On the other hand the system frequency is nearly constant and the speed changes are small, so that the speed governor is in effect a load controller. A speed signal is not necessarily the best method of actuation, although the governor is required to perform the speed control function in an emergency in order to prevent the generator from running away.