Power factor and disadvantages of low pf

Power factor is the cosine of the phase angle φ between the voltage and current in an AC circuit. It is also the ratio of active power to apparent power:

cos φ = P / S = R / Z = kW / kVA

It lies between 0 and 1, and is called lagging for inductive loads and leading for capacitive loads.

Disadvantages of low power factor

• Large current for the same power. I = P/(V cos φ) for single phase, or P/(√3 V_L cos φ) for three phase. A low cos φ means a larger current.
• Higher copper loss. Loss = I²R rises as 1/cos²φ in the cables, transformers and generator windings.
• Poor voltage regulation. The large current causes a bigger voltage drop (IR and IX) in lines and transformers, so the load voltage falls.
• Larger equipment size and cost. Generators, transformers, switchgear and cables must be rated in kVA, not kW, so they are bigger and costlier for the same useful power.
• Reduced capacity. The same kVA rating delivers less kW (kW = kVA × cos φ), so a plant cannot supply as many loads.
• Lower efficiency and heavier tariff. Utilities usually charge a penalty on low power factor (below about 0.8 to 0.9).