Electrical Power System Essentials

5.3 ACTIVE POWER AND FREQUENCY CONTROL

5.3.1 Primary Control

A change in the balance between the generation of active power and the consumption of active power changes the kinetic energy of the rotating mass of the generators, and alters the system frequency. Without control actions – such as increasing (decreasing) the input of mechanical power to the generator in order to restore the active power balance and thus the frequency – the deceleration (acceleration) continues as is already explained in the introduction of this chapter.

The active power balance is restored by a so-called speed governor, as is shown in Figure 5.4. The speed governor can be set with a characteristic (frequency-power relation)as is depicted in Figure 5.5 (a): the rotational speed (and thus the frequency) is independent of the generator loading. Such a control becomes problematic when generators are connected to run in parallel: when a frequency drop occurs, caused by an increased active power consumption in the system, each speed governor will try to restore the original frequency by increasing the power to the prime mover (in Figure 5.4 the steam valve will be opened further to supply more steam to the turbine). The generators equipped with fast-operating controllers will produce more active power than the generators with slow-operating controllers and after a number of control actions, the active power to be produced is distributed more or less randomly over the generators. This is an unwanted situation ...