Power Factor Compensation
Electric loads, whether pure admittances, PVpf loads, or PQV loads, may have power
factors that differ from 1. A unity power factor is however desirable for a number of
reasons. One is that at a power factor of one, the device consumes only real power, the
imaginary power being zero. The other is that with pf=1 the total current drawn by the
device is there uniquely for the purpose of doing useful work. A device that consumes (or
generates) reactive power requires a current flow, current that the supplying utility must
send through its transmission system wires. Even a device that consumes zero watts but a
non-zero amount of vars requires some current. This purely reactive current flow,
although resulting in zero watt load consumption, nonetheless produces some real losses
in the transmission lines leading to the load, in addition to causing network voltage
stability problems. The transmission utility therefore has to supply the resulting
transmission losses and must provide voltage stability resources, all of which cost money.
Consequently, large electricity consumers who use up or generate vars are charged a rate
for both reactive and real power, one in c/kVAr-h and the other in c/kWh. It is therefore
economically important for consumers to adjust their loads to reduce their var
requirements as much as possible, or equivalently to bring their power factor close to
unity.
Power Factor Compensation in Admittance or Impedance Loads
If the load behaves as an impedance with
Z
R
jX
=
+
, then the consumer can add a
compensating device in series whose impedance is
c
c
Z
jX
=
. The combined impedance
is then
(
)
c
c
Z
Z
R
j X
X
+
=
+
+
, which if
c
X
X
= −
, becomes purely resistive and
therefore has unity power factor.
The phasor diagram of the above load compensated to obtain a pf=1, could be denoted
by,
Z
R
jX
=
+
c
c
Z
jX
=
V
+
−
I
'
V
+
−
I
V
'
V
c
jX I

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