Power factor improvement

Low power factor means current lags far behind of voltage. Due to inductive load, circuit takes lagging current.
So, if we want to improve p.f of the system, we should counter against inductive reactance. That means we should install a device which will take leading current. Capacitor does the same.
In case of capacitor, it draws the leading current which partly or completely neutralizes the lagging current, therefore power factor improves.

Other way, capacitive reactance partly neutralizes the inductive reactance; therefore power factor improves ø1 to ø2.
Improvement of power factor is achieved by 1) Static capacitor or Capacitor bank, 2) Synchronous condenser, 3) Phase advancers.
1) Static Capacitor: By using capacitor in parallel with the equipment, the p.f can be improved. Capacitor draws the leading current and increases capacitive reactance which partly or fully neutralizes inductive reactance. The capacitor may connect in star or delta form in case of 3 phase load. By using more static capacitor for correction of power factor, capacitor bank is used.
Capacitors are connected in series or parallel known as series capacitor or shunt capacitor.

Series capacitors are connected in series with the line and the load current always flows through the capacitor and hence capacitive reactance improved which improve voltage regulation. But in faulty condition, voltage across capacitor may be raised up to 15 times more than the value. So, protective equipment should install along with series capacitor. So cost becomes high.
Shunt capacitor draws a fixed amount leading current which neutralizes the reactive component of load current, hence p.f of the system is improved.
So, capacitor, which is connected in series used for better voltage regulation of line whereas, connected in parallel used for power factor improvement.
2) Synchronous condenser: An over excited synchronous motor also works as power factor improvement equipment. Actually it works more finely than static capacitor. When a synchronous motor is over excited, it takes the leading current, which partly neutralizes the inductive reactance of the circuit and act as a capacitor. By varying the motor excitation the output can be controlled and also it has high thermal stability of winding for short circuit current. But this device is very costly as compared to static capacitor; its maintenance cost is very high as well as auxiliary equipment is necessary for starting.
3) Phase advancers: In case of induction motor, the stator winding draws exciting current which lags the voltage by 90˚. By an exciter, known as phase advancer, if we provide the exciting ampere turn to the rotor circuit at slip frequency, the induction motor can also be operate on leading power factor. The cost of this type of device is not economical for motor below 200H.P.

Capacitor Rating :

To improve the power factor from cos ø1 to cos ø2, we should know the active power of the circuit that is KW and the rating of the capacitor should be in KVAR.
OX = KW =Active power.
OY= KVA or Reactive power with power factor, cosø1,
OZ= KVA or Reactive power with power factor cosø2 after p.f improvement.
So, leading power factor supplies by YZ = XY-XZ = KVAR1-KVAR2,
=OA (tanø1-tanø2)
=KW (tanø1-tanø2).
So, the KVAR rating should be KW (tanø1-tanø2).

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