Power correction capacitor sizing
Three Phase Systems · Lesson 17 · Intermediate
Purpose
Calculate a target reactive-power correction and translate it into a qualified capacitor-bank specification.
Learning objectives
- Calculate kvar correction
- Avoid overcorrection
- Distinguish star and delta capacitance
- Identify resonance and switching checks
Power-factor correction supplies leading reactive power locally so the source carries less lagging reactive current. The required kvar follows from the before-and-after power triangles, not from active power alone.
Core theory
For constant active power P, required correction is Qc = P(tanφ1 − tanφ2), where cosφ1 and cosφ2 are the initial and target displacement factors.
Capacitance depends on system frequency, voltage, and connection. For an ideal three-phase bank, Qc = 3ωC Vph²; substituting the correct star or delta phase voltage is essential.
A practical bank requires staged control, capacitor duty rating, switching/contactors, protection, enclosure/temperature, discharge devices, residual-voltage control, harmonic/resonance study, and avoidance of leading operation or generator/inverter conflict.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Correction kvar | Leading reactive-power rating needed to reach a target displacement factor | Qc | var |
| Detuning | Reactor-capacitor design intended to avoid problematic harmonic resonance | Not applicable | Not applicable |
| Discharge device | Means to reduce stored capacitor voltage after isolation | Not applicable | Not applicable |
Qc = P(tanφ1 − tanφ2)Subtract target reactive demand from initial reactive demand at unchanged active power.
var when P is WAssumptions: Sinusoidal displacement PF; Active power remains 100 kW.
- Initial: tan(arccos 0.80) = 0.750.
- Target: tan(arccos 0.95) ≈ 0.329.
- Correction: Qc = 100(0.750−0.329) ≈ 42.1 kvar.
Ideal correction is approximately 42.1 kvar capacitive.
Reasonableness check: The target remains lagging, so correction is smaller than the original 75 kvar demand.
- Using PF difference directly
- Ignoring star/delta voltage
- Installing capacitors without harmonic study
Where this appears in practice
Correction banks reduce upstream current and release capacity in industrial/commercial distribution.
Knowledge check
Why is a target of exactly unity PF often avoided in variable-load systems?
Load variation can cause leading overcorrection. Staged automatic control and a suitable target margin reduce this risk.
Answer: Load variation can cause leading overcorrection. Staged automatic control and a suitable target margin reduce this risk.
Practical exercise
Calculate correction kvar for the example at targets 0.90 and 0.95, then list the non-calculation design gates.
Summary
- Correction comes from tangent difference
- Connection determines capacitance
- Harmonics and stored charge govern safety
Sources and review
- IEC 60038: IEC standard voltages: IEC; 2009+A1:2021; International
- IEC 61921: Low-voltage power-factor-correction banks: IEC; 2017; International
- Harmonics and Power Quality Analysis webinar Q&A: IET; Current online guidance; United Kingdom
Editorial review date: 2026-08-22. Professional electrical review is pending.