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2

Power systems

engineering ±

fundamental concepts

2.1 Reactive power control

In an ideal AC power system the voltage and frequency at every supply point would
be constant and free from harmonics, and the power factor would be unity. In
particular these parameters would be independent of the size and characteristics of
consumers' loads. In an ideal system, each load could be designed for optimum
performance at the given supply voltage, rather than for merely adequate perform-
ance over an unpredictable range of voltage. Moreover, there could be no interfer-
ence between different loads as a result of variations in the current taken by each one
(Miller, 1982).
In three-phase systems, the phase currents and voltages must also be balanced. 1
The stability of the system against oscillations and faults must also be assured. All
these criteria add up to a notion of power quality. A single numerical definition of
power quality does not exist, but it is helpful to use quantities such as the maximum
fluctuation in rms supply voltage averaged over a stated period of time, or the total
harmonic distortion (THD), or the `availability' (i.e. the percentage of time, averaged
over a period of, say, a year, for which the supply is uninterrupted).
The maintenance of constant frequency requires an exact balance between the
overall power supplied by generators and the overall power absorbed by loads,
irrespective of the voltage. However, the voltage plays an important role in main-
taining the stability of power transmission, as we shall see. Voltage levels are very
sensitive to the flow of reactive power and therefore the control of reactive power is

1
Unbalance causes negative-sequence current which produces a backward-rotating field in rotating AC
machines, causing torque fluctuations and power loss with potential overheating.