3. Miscellaneous. In addition to the basic components of a synchronous generator (rotor, stator, and thei windings), there are auxiliary devices which help maintain the machine's operation within acceptable limits. Three such devices are mentioned here: governor, damper windings, and excitation control system Governor. This is to control the mechanical power input Pin. 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 done by opening up the valve to increase the steam for turbogenerators or the flow of water through the penstock for hydrogenerators. Governors are mechanical systems and therefore have some significant time lags(many seconds)compared to other lectromagnetic phenomena associated with the machine. If the time duration of interest is short, the effect of governor can be ignored in the study; that is, Pin is treated as a constant. Damper windings(amortisseur windings). These are special conducting bars buried in notches on the rotor surface, and the rotor resembles that of a squirrel-cage-rotor induction machine(see Section 66.2) The damper windings provide an additional stabilizing force for the machine when it is perturbed from an equilibrium. 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. That is, these windings exhibit no effect on a steady-state machine. However, when the speeds of the stator field and the rotor become different (because of a disturbance, currents are induced in the damper windings in such a way as to keep according to Lenz's law, the two speeds from separating Excitation control system. Modern excitation systems are very fast and quite efficient. An excitation ontrol system is a feedback loop that aims at keeping the voltage at machine terminals at a set level. To explain the main feature of the excitation system, it is sufficient to consider Fig. 66.4. Assume that a disturbance occurs in the system, and as a result, the machine's terminal voltage V, drops. The excitation stem boosts the internal voltage EF; this action can increase the voltage V, and also tends to increase the reactive power output. From a system viewpoint, the two controllers of excitation and governor rely on local information(machine terminal voltage and rotor speed). In other words, they are decentralized controls. For large-scale systems, such designs do not always guarantee a desired stable behavior since the effect of interconnection is not taken into account in detail Synchronous Machine Parameters. When a disturbance, such as a short circuit at the machine terminals, takes place, the dynamics of a synchronous machine will be observed before a new steady state is reached. Such a process typically takes a few seconds and can be divided into subprocesses. The damper windings(armortis seur)exhibit their effect only during the first few cycles when the difference in speed between the rotor and ne perturbed stator field is significant. This period is referred to as subtransient. The next and longer period which is between the subtransient and the new steady state, is called transient. Various parameters associated with the subprocesses can be visualized from an equivalent circuit. The d-axis and q-axis(dynamic) equivalent circuits of a synchronous generator consist of resistors, inductors, and voltage sources. In the subtransient period, the equivalent of the damper windings needs to be considered. In the transient period, this equivalent can be ignored. When the new steady state is reached, the current in the rotor nding becomes a constant(dc); thus, one can further ignore the equivalent inductance of this winding. This approximate method results in three equivalent circuits, listed in order of complexity: subtransient, transient, and steady state. For each circuit, one can define parameters such as(effective)reactance and time constant For example, the d-axis circuit for the transient period has an effective reactance X' and a time constant Tdo omputed from the R-L circuit) when open circuited. The parameters of a synchronous machine can be mputed from experimental data and are used in numerical studies. Typical values for these parameters are given in Table 66.1 References on synchronous generators are numerous because of the historical importance of these machines in large-scale electric energy production. [Sarma, 1979] includes a derivation of the steady-state and dynamic models, dynamic performance, excitation, and trends in development of large generators. [Chapman, 1991] e 2000 by CRC Press LLC© 2000 by CRC Press LLC 3. Miscellaneous. In addition to the basic components of a synchronous generator (rotor, stator, and their windings), there are auxiliary devices which help maintain the machine’s operation within acceptable limits. Three such devices are mentioned here: governor, damper windings, and excitation control system. • Governor. This is to control the mechanical power input Pin. 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 done by opening up the valve to increase the steam for turbogenerators or the flow of water through the penstock for hydrogenerators. Governors are mechanical systems and therefore have some significant time lags (many seconds) compared to other electromagnetic phenomena associated with the machine. If the time duration of interest is short, the effect of governor can be ignored in the study; that is, Pin is treated as a constant. • Damper windings (armortisseur windings). These are special conducting bars buried in notches on the rotor surface, and the rotor resembles that of a squirrel-cage-rotor induction machine (see Section 66.2). The damper windings provide an additional stabilizing force for the machine when it is perturbed from an equilibrium. 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. That is, these windings exhibit no effect on a steady-state machine. However, when the speeds of the stator field and the rotor become different (because of a disturbance), currents are induced in the damper windings in such a way as to keep, according to Lenz’s law, the two speeds from separating. • Excitation control system. Modern excitation systems are very fast and quite efficient. An excitation control system is a feedback loop that aims at keeping the voltage at machine terminals at a set level. To explain the main feature of the excitation system, it is sufficient to consider Fig. 66.4. Assume that a disturbance occurs in the system, and as a result, the machine’s 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 controllers of excitation and governor rely on local information (machine’s terminal voltage and rotor speed). In other words, they are decentralized controls. For large-scale systems, such designs do not always guarantee a desired stable behavior since the effect of interconnection is not taken into account in detail. Synchronous Machine Parameters. When a disturbance, such as a short circuit at the machine terminals, takes place, the dynamics of a synchronous machine will be observed before a new steady state is reached. Such a process typically takes a few seconds and can be divided into subprocesses. The damper windings (armortis￾seur) exhibit their effect only during the first few cycles when the difference in speed between the rotor and the perturbed stator field is significant. This period is referred to as subtransient. The next and longer period, which is between the subtransient and the new steady state, is called transient. Various parameters associated with the subprocesses can be visualized from an equivalent circuit. The d-axis and q-axis (dynamic) equivalent circuits of a synchronous generator consist of resistors, inductors, and voltage sources. In the subtransient period, the equivalent of the damper windings needs to be considered. In the transient period, this equivalent can be ignored. When the new steady state is reached, the current in the rotor winding becomes a constant (dc); thus, one can further ignore the equivalent inductance of this winding. This approximate method results in three equivalent circuits, listed in order of complexity: subtransient, transient, and steady state. For each circuit, one can define parameters such as (effective) reactance and time constant. For example, the d-axis circuit for the transient period has an effective reactance X ¢ d and a time constant T ¢ do (computed from the R-L circuit) when open circuited. The parameters of a synchronous machine can be computed from experimental data and are used in numerical studies. Typical values for these parameters are given in Table 66.1. References on synchronous generators are numerous because of the historical importance of these machines in large-scale electric energy production. [Sarma, 1979] includes a derivation of the steady-state and dynamic models, dynamic performance, excitation, and trends in development of large generators. [Chapman, 1991]