The basic idea behind power factor improvement is to look for the devices in which current
leads the voltage and connect it to load
in such a way that the current drawn by it neutralises the lagging reactive
component of load current. There are three methods to accomplish this.
(1)Power
factor correction using capacitor :- Power factor
correction by using capacitor bank is economical and is widely used. Since a capacitor takes leading current ,therefore, if we
connect it in parallel with the load pf can be improved. Lets see how.
Consider a load taking lagging current I at a
pf of cos Φ1 as shown along with its phasor diagram.
Lets connect a capacitor C in parallel as shown. The
phasor diagram gets slightly modified.
We find that current drawn from supply is the vector
sum of load current I and capacitor
current Ic and ∴
by changing these two we can change I
' and ∴
Pf can be improved.
But since load ZL is fixed ∴ I
(I=V/ZL) cannot be
changed and we can change Ic
only.
Therefore, for a given voltage and frequency, we can change Ic
only by varying capacitance. ∴
appropriate capacitor value must be choosen.
Lets assume we want to improve pf to cos Φ2
. In this scenario, the phasor diagram is in second diagram. From the diagram we find that prior to pf
correction, reactive power supplied by supply
V was VIsin Φ1 . But after pf
correction only VI 'sin
Φ2 is supplied. This
means ( VIsin Φ1 - VI
'sin
Φ2 ) must have been supplied
by the capacitor.
∴ VIc = (
VIsin Φ1 - VI 'sin Φ2 )
we get, Ic = ( Isin Φ1 - I 'sin Φ2 )
(2) Synchronous
Condenser :- when
a three phase synchronous motor with no
load is over-excited ,it takes leading current which makes it behave like a
capacitor and hence the name synchronous condenser. If we connect it in parallel
with the load ,it will improve the power factor just the way capacitor does in the
above method.
(3) Phase
advancers :- are used to
improve the power factor of induction motors. The low power factor of an
induction motor is due to the fact that its stator winding draws more current
for excitation which lags the supply
voltage. So if the excitation ampere turns can be provided from some other ac
source, then stator winding can be relived and the pf can be improved.
The phase advancer
circuit is mounted on the rotor shaft and connected to rotor circuit for
providing extra ampere turns at slip frequency for the motor during starting or
less load and thus improves the power factor.
Important Points :-
(1) Reactive
power taken by load = voltage across load (V)× Current through the load (I) ×
Sine of angle between V and I
Above formula shows that reactive power requirement by
the load does not change. It is only supplied by some device locally, thus
reducing the reactive power burden on source.
(2) Power factor is corrected only to the value upto which the savings on - line losses and power plant investment - are greater than the cost of power factor improvement device , measured over the device lifetime.
(2) Power factor is corrected only to the value upto which the savings on - line losses and power plant investment - are greater than the cost of power factor improvement device , measured over the device lifetime.
Closely related concepts :-
