Causes, disadvantages and improvement of low pf

Power factor is the cosine of the angle between voltage and current, $\cos\phi = P/S$. A low (lagging) power factor means the load draws large reactive current for the same useful power.

Causes of low power factor

1. Induction motors, especially when lightly loaded (pf about 0.2–0.3 at no load, 0.8–0.9 at full load); they are the largest industrial load.
2. Transformers working at light load, drawing magnetising current.
3. Arc and induction furnaces and welding sets, which have high reactance.
4. Discharge lamps (fluorescent, mercury, sodium) with chokes.
5. Supply voltage above rated, which increases magnetising current.
6. Power electronic loads (rectifiers, drives) with phase-controlled firing.

Disadvantages of low power factor

For the same power, current $I = P/(\sqrt3 V \cos\phi)$ increases, so:

1. Larger kVA rating of generators, transformers and switchgear is needed (higher capital cost).
2. Larger conductor size is needed for the same loss.
3. Higher copper losses ($I^2R$), so lower efficiency.
4. Higher voltage drop, poor voltage regulation.
5. Reduced handling capacity of the system.
6. Consumers pay more under kVA-demand tariffs or pf penalties.

Methods of power factor improvement

1. Static capacitors: connected in parallel (star or delta) with the load, supply leading kVAR. Low loss, little maintenance, no moving parts; used individually on motors or as automatic capacitor banks.
2. Synchronous condenser: an over-excited synchronous motor on no load acts as a variable capacitor; gives stepless control, used in large substations.
3. Phase advancer: an ac exciter on the rotor of an induction motor supplies magnetising ampere-turns at slip frequency, so the motor can run at unity or leading pf.
4. Static VAR compensator / STATCOM: thyristor or IGBT controlled devices for fast, continuous control.
5. Good practice: avoid running motors and transformers on light load, use correct motor size, use synchronous motors where possible.

  Supply ---+---------+-----> to load
            |         |
           === C    [Load]
            |       (lagging)
  --------- +---------+
  capacitor supplies leading kVAR