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Electrical Power Systems
demand to maintain the frequency deviation and tie-power deviation–within the specified limits. As mentioned in section 12.1, small changes in real power are mainly dependent on changes in rotor angle d and thus, the frequency. The reactive power is mainly dependent on the voltage magnitude (i.e., on the generator excitation). The excitation system time constant is much smaller than the prime mover time constant and its transient decay much faster and does not affect the LFC dynamic. Thus the coupling between the LFC loop and the AVR loop is negligible and the load frequency and excitation voltage control are analyzed independently.
Fig. 12.1: Schematic diagram of LFC and AVR of a synchronous generator.
12.3 FUNDAMENTALS OF SPEED GOVERNING SYSTEM The basic concepts of speed governing can be illustrated by considering an isolated generating unit supplying a local load as shown in Fig. 12.2
Tm = mechanical torque Pm = mechanical power
Te = electrical torque Pe = electrical power
Fig. 12.2: Generator supplying a local load.
PL = load power.