How it works
Reactive power Q (measured in VAr) is the component of apparent power S that oscillates between source and load without doing real work. The power triangle relates S (kVA), P (kW), and Q (kVAr) by S² = P² + Q². Power factor PF = P/S = cos φ, where φ is the angle between voltage and current phasors. To improve PF from cos φ1 to cos φ2 at a load of P kW, the required capacitor bank is Qc = P(tan φ1 − tan φ2) kVAr. Shunt capacitors at 11 kV or 33 kV buses supply lagging VAr locally, reducing reactive current flow on the upstream network and improving voltage regulation.
Key points to remember
Unity power factor is the target, but most industrial installations operate between 0.85 and 0.95 lagging. A power factor of 0.7 means only 70% of the kVA drawn is doing useful work, with 30% circulating as reactive current that heats conductors without contribution to output. The Bureau of Energy Efficiency in India mandates a minimum PF of 0.85 for HT consumers, with surcharge below that. Synchronous condensers and STATCOMs provide variable reactive power, while fixed capacitor banks provide constant VAr. Leading power factor (capacitive load) causes voltage rise and must also be controlled.
Exam tip
Every Anna University and GATE paper asks you to calculate the capacitor bank rating in kVAr needed to improve power factor from a given lagging value to 0.9 or unity — the formula Qc = P(tan φ1 − tan φ2) must be written with the triangle diagram alongside.