Reactive Power Compensation: Capacitor Bank Sizing Guide

Why Field Measurement Beats Rule-of-Thumb Sizing

Sizing a capacitor bank from nameplate transformer kVA and an assumed displacement power factor often produces banks that are too large, too small, or that excite harmonic resonance with the supply transformer. A short field study with a power quality analyzer captures actual demand profile, harmonic content, and existing capacitor health, leading to a bank that delivers promised savings without surprises.

Step 1: Measure Load Profile

Log voltage, current, real power, reactive power, and apparent power on the main switchboard for at least one full production week. Identify the peak kW demand and the corresponding kVAR. Capture short-cycle loads (welders, presses) that may need fast-switching banks instead of fixed compensation.

Step 2: Calculate Required kVAR

For target power factor PF₂ from existing PF₁:

kVAR = kW × (tan(arccos PF₁) − tan(arccos PF₂))

Example: 500 kW at PF 0.78 lagging, target 0.95 → kVAR = 500 × (0.802 − 0.329) = 237 kVAR. Round to standard step sizes (typically 25 or 50 kVAR steps).

Step 3: Assess Harmonic Risk

Capacitors form a parallel resonance with the source impedance. Resonance frequency: f_r ≈ 50/60 × sqrt(MVAsc / kVARcap). If f_r falls near a dominant load harmonic (5th, 7th, 11th), the bank will overheat and may explode. Either de-tune with series reactors (typically 7%) or move the bank elsewhere on the system.

Step 4: Verify Existing Capacitor Health

Measure each capacitor can with a clamp meter and capacitance function. Capacitance loss greater than 5% from nameplate indicates impending failure. Replace before adding new banks; otherwise, the new bank will compensate for failed capacitors that should simply be replaced.

Step 5: Document and Re-Survey

After installation, re-run the same week-long study. Verify achieved power factor, capacitor current draw with harmonics.