IEEE 421.5-1992 pdf free.IEEE Recommended Practice for Excitation System Models for Power System Stability Studies.
IEEE 421.5 In some particular system configurations,continuous excitation control with terminal voltage and power systemstabilizing regulator input signals does not ensure that the potential of the excitation system for improving systemstability is fully exploited. For these situations, discontinuous excitation control signals may be employed to enhancestability following large transient disturbances, see [5],[10], and [13].
The type DEC1A discontinuous excitation control model, shown in Fig 17, is used to represent a scheme that boostsgenerator excitation to a level higher than that demanded by the voltage regulator and stabilizer immediately followinga system fault. The scheme, which has been applied to a number of large synchronous generators with bus-fed staticexciters (ST1A), adds a signal proportional to rotor angle change to the terminal voltage and power system stabilizingsignals. This angle signal is used only during the transient period of about 2 s because it results in steady-stateinstability if used continuously.The objective of such a control is to maintain the field voltage and, hence, the terminalvoltage high until the maximum of the rotor angle swing is reached.This control is used specifically for instances inwhich both local and interarea oscilations are present in the transient, and in which the back swing of the local modewould otherwise bring the excitation off ceiling before the true peak of the angular swing is reached.Excessiveterminal voltage is prevented by the use of a terminal voltage limiter circuit.
The effect of this discontinuous control, in addition to increasing generator terminal voltage and air-gap power, is toraise the system voltage level and load power, thereby contributing to unit deceleration.
As shown in Fig 17, the speed (or equivalent) PSS signal provides continuous control to maintain steady-state stabilityunder normal operating conditions. For the discontinuous control, a signal proportional to change in the angle of thesynchronous machine is obtained by integrating the speed signal. It is not a perfect integrator, i.e., the signal is resetwith the time constant, TAN.IEEE 421.5 pdf download.