Handbook of Electrical Installation Practice - BeKnowledge

636 Handbook of Electrical Installation Practice
supply authorities often offer reduced terms to consumers whose power factor is
high, or impose penalties for those with low power factor.
THEORY OF POWER FACTOR CORRECTION
The kVA in an a.c. circuit can be resolved into two components: the in-phase component
which supplies the useful power (kW), and the wattless component (kvar)
which does no useful work. The phasor sum of the two is the kVA drawn from the
supply. The cosine of the phase angle f1 between the kVA and the kW components
represents the power factor of the load.
The phasor diagram for this is shown in Fig. 23.1. The load current is in phase
with the kVA so that it lags the supply voltage by the same phase angle.
To improve the power factor, equipment drawing kvar of approximately the same
magnitude as the load kvar, but in phase opposition (leading), is connected in parallel
with the load. The resultant kVA is now smaller and the new power factor, cos
f2 is increased. Thus any value of cos f2 can be obtained by controlling the magnitude
of the leading kvar added. This is shown in Fig. 23.2.
POWER FACTOR IMPROVEMENT
In practice two types of equipment are available to produce leading kvar:
(1) Rotary equipment Phase advancers, synchronous motors and synchronous
condensers. Where auto-synchronous motors are employed the power factor
correction may be a secondary function.
(2) Static equipment Capacitors.
Fig. 23.1 Phasor diagram of plant operating at a lagging power factor.