In addition to the basic components of a synchronous generator (the rotor, stator, and their windings), auxiliary devices are used to help maintain the machine’s operation within acceptable limits. These devices include the following:
GOVERNOR
The function of the governor is to control the mechanical power input to the generator. The control is via a feedback loop where the speed of the rotor is constantly monitored. For instance, if this speed falls behind the synchronous speed, the input is insufficient and has to be increased. This is accomplished by opening a valve to increase the amount of steam for turbo-generators or the flow of water through the penstock for hydro-generators. Governors are mechanical systems and, therefore, usually have some significant time lags (many seconds) compared to other electromagnetic parameters associated with the machine.DAMPER WINDINGS
(armortisseur windings). These windings are special conducting bars buried in notches on the rotor surface (the rotor resembles a squirrel-cage-rotor induction machine). The damper windings provide an additional stabilizing force for the machine during certain periods of operation. As long as the machine is in a steady state, the stator field rotates at the same speed as the rotor, and no currents are induced in the damper windings. However, when the speeds of the stator field and the rotor become different (because of a load disturbance), currents are induced in the damper windings in such a way as to keep the two speeds from separating.EXCITATION CONTROL SYSTEM
Modern excitation systems are fast and efficient. An excitation control system is a feedback loop designed to maintain the voltage at the machine terminals at a set level. Figure illustrates the mechanisms at work. Assume that a disturbance occurs in the system, and as a result, the machine terminal voltage Vt drops. The excitation system boosts the internal voltage EF. This action can increase the voltage Vt and also tends to increase the reactive power output.From a system viewpoint, the two controlling mechanisms of excitation and the governor rely on local information (the machine terminal voltage and rotor speed). In other words, they are decentralized controls. For large-scale systems, such designs do not always guarantee stable behavior because the effects of the interconnection system and other elements in the network are not taken into account.
An analysis of the operation of centralized control systems is beyond the scope of this book; however, it is instructive to examine some of the principles of decentralized control systems. Many of these principles apply in a modified form to centralized control techniques.
 |
| Figure: The per-phase equivalent circuit of a round-rotor synchronous machine. EF is the internal voltage (phasor form) and Vt is the terminal voltage. |