Capacitor Banks (Power factor correction)

Capacitor Banks for Power Factor Correction What is it A capacitor bank is a group of capacitors connected to an electrical system to improve its power factor. Inductive loads—such as motors, transformers, pumps, and some lighting—cause a lagging power factor, meaning the current lags the voltage. Utilities charge for low power factor because it increases current, loads transformers, and wastes energy. By adding leading reactive power with a capacitor bank, the overall power factor moves closer to unity (1.0), reducing current and improving efficiency. How it works - Real power (P) is the actual useful work (kilowatts), while reactive power (Q) is stored and released by inductive elements (kVAR). - A capacitor supplies leading reactive power, which cancels part of the lagging reactive power from inductive loads. - The result is a lower apparent power (S) drawn from the grid, since S = sqrt(P^2 + Q^2) and Q is reduced. - With a better power factor, electrical losses in cables and transformers drop, and voltage regulation improves. Sizing and calculation (a simple approach) - You know the plant’s real power P (in kW) and the current power factor pf1 (e.g., 0.70, 0.80, etc.). You may have a target power factor pf2 (e.g., 0.95). - Compute the current reactive power Q1 = P × tan(acos(pf1)). - Compute the desired reactive power Q2 = P × tan(acos(pf2)). - Required capacitor reactive power Qc = Q1 − Q2. - Choose a capacitor bank with a rating near Qc, and add a margin (often 10–20%) to cover load growth and measurement uncertainty. Example: If P = 100 kW and pf1 = 0.75, pf2 = 0.95: - phi1 = acos(0.75) ≈ 41.4°, Q1 ≈ 100 × tan(41.4°) ≈ 87 kVAR - phi2 = acos(0.95) ≈ 18.2°, Q2 ≈ 100 × tan(18.2°) ≈ 32.5 kVAR - Qc ≈ 87 − 32.5 ≈ 54.5 kVAR A ~60 kVAR capacitor bank would be a practical match, with some headroom. Types and control - Fixed banks: Always connected, simple and inexpensive, best for loads with a constant inductive profile. - Switched/Automatic (APFC) banks: Banks are switched in and out to maintain the desired PF as load changes. Controllers monitor PF and adjust capacitor steps in real time. - Detuned banks: In systems with harmonics, capacitors are paired with reactors to avoid resonance at harmonic frequencies. Placement and integration - Place capacitor banks close to the inductive loads or on the same feeder to maximize voltage support and minimize distribution losses. - Avoid placing banks across highly sensitive equipment where voltage spikes or transients could cause problems. - Coordinate with the electrical system: transformer rating, cable sizes, and existing harmonic content. Harmonics and protection - Capacitors can interact with harmonic currents and cause resonance, potentially damaging equipment. - In systems with significant harmonics, use detuning reactors or harmonics filters and ensure appropriate protection (fuse/RC network, proper switching devices, etc.). - Regular maintenance is important: capacitors can degrade, bulge, or fail, and switched banks require proper control logic to prevent simultaneous switching that could cause transients. Benefits and considerations - Benefits: reduced reactive power demand, lower kVA from the utility, lower energy losses, improved voltage profiles, and potentially lower demand charges on the utility bill. - Considerations: initial cost, ongoing maintenance, need for proper protection and harmonic mitigation, and the importance of correct sizing to avoid over- or under-correction. Implementation steps (short) - Survey loads to identify inductive-heavy equipment and measure current PF. - Decide target PF based on utility requirements and economic return. - Size the bank with a margin for future growth and tolerate load variability. - Select appropriate bank type (fixed vs automatic) and add harmonic mitigation if needed. - Install with proper protection, control wiring, and commissioning tests. - Monitor performance and adjust as loads evolve. A well-implemented capacitor bank can be a cost-effective way to improve power quality and efficiency for many industrial and commercial applications. For best results, work with a qualified electrical engineer to tailor the solution to your specific system and local standards.