In short. Reactive power does no useful work but loads cables, transformers and alternators. A study finds where kVAr is needed, how much correction is justified, and what happens on generator supply, where too much capacitance can cause leading power factor and unstable voltage.
Relationships
kVA² = kW² + kVAr²
kVAr needed = kW × (tan φ₁ − tan φ₂)
Improving power factor from φ₁ to φ₂ reduces current for the same kW, easing loading and losses.
Why it matters
| Equipment | Effect of low power factor |
|---|---|
| Utility supply | Penalties and higher demand charges |
| Cables and transformers | Higher current, losses and voltage drop |
| Generator | kVA limits the alternator even when engine kW is spare |
Study steps
- Measure or estimate load kW and power factor by operating scenario, including light load.
- Set targets for the utility and for generator operation.
- Size correction in steps so power factor does not overshoot at low load.
- Check harmonics and resonance before adding capacitors. See resonance analysis.
- Check generator operation with capacitors connected, since leading power factor can reduce excitation margin and risk loss of control. See harmonics for related topics.
Generator cautions
- Alternators are rated at 0.8 lagging; leading operation is limited by the capability curve and stability margin.
- Automatic capacitor banks should be disconnected or controlled on generator supply.
- Switching a bank causes transients; check with the AVR.
Correction methods
- Fixed or automatic switched capacitor banks.
- Detuned reactors where harmonics are present.
- Active compensation or static var devices for fast loads.
- Motor-level correction for large constant loads.
Illustrative example
A 600 kW load at 0.78 power factor needs to reach 0.95. The kVAr required is 600 × (0.802 − 0.329), about 284 kVAr. The study chooses a 300 kVAr detuned bank in steps and adds an interlock that disconnects it on generator supply, because light-load operation could push the generator toward leading power factor.
Power factor in one triangle
kVA² = kW² + kVAr²; power factor = kW ÷ kVA
Reactive power (kVAr) is the magnetising current for motors and transformers. It loads cables and transformers but does no useful work.
Capacitor sizing example (illustrative)
A 500 kW load at 0.80 power factor has kVAr = 500 × tan(cos⁻¹ 0.80) = 500 × 0.75 = 375 kVAr. To reach 0.95: target kVAr = 500 × 0.329 = 164 kVAr. Capacitor required = 375 − 164 = 211 kVAr.
Generator interaction
| Issue | Consequence |
|---|---|
| Low lagging power factor on generator | Alternator limited by field heating; kW capacity falls below nameplate |
| Capacitor banks left connected on generator supply | Leading power factor, risk of overvoltage, AVR instability, loss of excitation trip |
| Switching capacitors against light load | Resonance with harmonic sources |
Design points
- Disconnect or automatically control capacitor banks when on generator supply.
- Use detuned reactors where harmonics are present.
- Check the alternator’s capability curve at the study’s operating points.
- Consider utility penalties and incentives for power factor on the grid side.
Study results should show the power factor at the incomer under mains and generator supply, and capacitor stage switching logic.
See harmonic filter application and kW and kVAr sharing.
Common mistakes
- Fixed capacitors with no light-load check.
- Skipping harmonic checks.
- Ignoring generator mode.
What to send us
Send load profile and bill data. Send your application details.
Questions engineers ask
Why can’t the generator use capacitors?
Over-correction can cause leading current and voltage instability.
What is detuning?
Series reactors move the resonance below troublesome harmonics.
What target power factor is right?
Choose based on tariff, equipment and safe margin, not the highest number.
Does low power factor reduce generator kW?
The engine kW is unchanged, but alternator kVA may limit useful kW.
Values are illustrative. Confirm with measured data.