Capacitor Bank Sizing Calculator

The Capacitor Bank Sizing Calculator determines the capacitance value per phase, per-step kVAR, rated current, and detuning reactor size for a three-phase automatic power factor correction (APFC) capacitor bank. Properly sized capacitor banks are the primary tool for reactive power compensation in industrial and commercial electrical systems.

A capacitor bank consists of three-phase capacitor units connected to the electrical bus, typically in delta (Δ) or star (Y) configuration. Delta-connected capacitors are exposed to full line-to-line voltage but are easier to switch and provide better harmonic performance. Star-connected banks are common in high-voltage applications. This calculator uses the phase voltage approach valid for balanced three-phase systems.

The required capacitance per phase is derived from: Q = V²/X_C = V² × ωC, so C = Q/(ω × V² × 3) per phase for a three-phase star-connected bank. The capacitive reactance X_C = V_phase² × 3 / Q determines the resonant frequency with any system inductance.

Detuning reactors (series inductors) are critically important in systems with harmonic loads. Without detuning, capacitors can resonate with system inductance at a harmonic frequency, causing catastrophic overcurrent. Detuning reactors are sized to place the LC resonant frequency below the lowest problematic harmonic. A 7% detuned reactor (p = 7%) creates resonance at 50/√0.07 = 189 Hz (between 3rd and 5th harmonics for 50 Hz systems). For 60 Hz systems, the detuning is designed similarly to avoid the 300 Hz (5th harmonic).

APFC panels typically include: a power factor relay (controller) that measures PF and switches capacitor steps as needed, vacuum contactors rated for capacitor switching (which involves high inrush currents), detuned capacitor modules (capacitor + reactor unit), and protection devices (fuses, thermal protection). Standard automatic banks range from 25 kVAR to over 1000 kVAR in modular designs.