Power Factor Correction Calculator

Step 1 — reactive power now. φ₁ = arccos 0.75 = 41.4°, so Q₁ = P tan φ₁ = 100 × 0.882 = 88.2 kvar.

Step 2 — reactive power wanted. φ₂ = arccos 0.95 = 18.2°, so Q₂ = 100 × 0.329 = 32.9 kvar.

Step 3 — bank size. Q_c = Q₁ − Q₂ = 55.3 kvar. In practice you would specify the next standard step up, typically a 60 kvar bank in 5 or 10 kvar stages.

Step 4 — what it buys. Apparent power falls from 100/0.75 = 133.3 kVA to 100/0.95 = 105.3 kVA, and line current from 192 A to 152 A — a 21% reduction in both. That current reduction is the part that matters for cable and transformer loading.

Step 5 — capacitance. For a delta-connected bank at 400 V, C = Q_c/(3ωV_LL²) = 367 µF per phase. The same duty in star needs 1101 µF per phase, because each capacitor sees only 231 V. Delta is the usual choice at low voltage for exactly this reason: three times the voltage means a third of the capacitance and a smaller, cheaper unit.

1. Sinusoidal, balanced supply. Q_c = P(tan φ₁ − tan φ₂) assumes a clean fundamental waveform. With significant harmonic distortion the displacement power factor this calculates is no longer the true power factor, and correcting to the displayed target will not give the expected result.

2. Harmonic resonance is not checked. This is the one that damages equipment. A capacitor bank forms a parallel resonant circuit with the supply transformer inductance. The approximate resonant harmonic order is h_r ≈ √(S_sc/Q_c), where S_sc is the fault level at the busbar. For a 1000 kVA transformer at 6% impedance, S_sc ≈ 16.7 MVA, and a 55 kvar bank gives h_r ≈ 17 — comfortably clear. The same bank on a 315 kVA transformer gives h_r ≈ 9.5, close to the 9th and 11th harmonics of a six-pulse drive. That is a resonance waiting to happen, and it calls for detuned reactors.

3. Fixed correction assumes a fixed load. Sizing at full load and leaving the bank permanently connected produces leading power factor at light load, which some utility tariffs penalise as heavily as lagging. Variable loads want automatic stepped banks with a power-factor controller.

4. Motor terminal correction has a limit. Capacitors connected directly at motor terminals must stay below the motor's no-load magnetising kvar, or the motor can self-excite when the supply is interrupted while it is still spinning, generating damaging overvoltage.

5. Inrush is not sized. Switching a capacitor step draws a high-frequency inrush current, and back-to-back switching of parallel steps is worse. Contactors need capacitor-duty rating or damping reactors.

Capacitor units: IEC 60831-1 and -2 for low-voltage self-healing units up to 1 kV; IEC 60871-1 for shunt capacitors above 1 kV. Both set tolerance, loss, discharge and endurance requirements. Capacitors are rated for 1.1 × rated RMS voltage continuously and 1.3 × rated RMS current, the latter allowing for harmonic content.

Harmonic assessment: IEEE 519 for voltage and current distortion limits at the point of common coupling. IEC 61000-2-4 for compatibility levels inside an industrial installation.

Before ordering a bank: measure over a full production cycle rather than at one instant — a power quality logger over a week will show the load variation that decides fixed versus automatic. Record the harmonic spectrum at the same time. Check the transformer fault level and run the resonance estimate above. If h_r lands within two orders of a harmonic your drives actually produce, specify detuned reactors (commonly tuned to 189 Hz or 134 Hz on a 50 Hz system) rather than plain capacitors.

Qc = P[tan(arccos PF1) − tan(arccos PF2)]. It is the reactive power that must be supplied locally to move from the existing to the target power factor.

Loads vary. Correcting exactly to unity at one operating point can produce leading power factor at lighter load. Automatic stepped banks are commonly used for variable loads.

Capacitor banks can resonate with system inductance and amplify harmonic currents. Harmonic surveys, detuned reactors or filtered banks may be needed.