In short. An automatic power factor correction panel switches capacitor steps to supply the reactive power that inductive loads need. Sized well it lowers current and penalties; sized or placed badly, especially with harmonics or on a generator, it can resonate or trip.
Sizing
Required kVAr = kW × (tan φ1 − tan φ2)
Here φ1 is the existing and φ2 the target phase angle. Use measured kW and power factor at typical load.
Worked example (illustrative)
A 200 kW load at 0.80 power factor, corrected to 0.98: tan φ1 = 0.75, tan φ2 = 0.20, so about 200 × 0.55 = 110 kVAr, split into several steps.
| Item | Consideration |
|---|---|
| Steps | Fine steps follow load variation |
| Controller | Correct CT location and settings |
| Detuning reactors | Needed where harmonics exist |
| Contactors | Capacitor-duty rated |
| Protection | Fuses, discharge, temperature |
Harmonics and resonance
Capacitors with the system inductance can resonate at a harmonic frequency. Studies and detuned reactors avoid it. See harmonic studies and reactive power studies.
On generators
Do not leave capacitors connected when running on a generator without checking: over-correction can lead to leading power factor and voltage rise. Interlock or control the panel on generator supply. See kW and kVAr sharing.
Common failure causes
- Resonance with harmonics.
- Overheating from poor ventilation.
- Wrong controller CT arrangement.
- Worn contactors from frequent switching.
- Operation at over-voltage.
Staging and controller settings
An APFC panel has several capacitor steps, switched by a controller based on measured reactive power. Typical practice is to use steps of unequal size, so that fine adjustment is possible with fewer contactors. Controller settings include the target power factor, the time delay between switching operations, and the smallest step size, which relates to the CT ratio and the minimum current at which the controller is active.
CT placement
The current transformer must measure the total current of the loads being corrected, in the correct phase and direction. A wrongly placed CT makes the controller think the power factor is poor when it is fine, or the reverse. Test with a clamp meter and a power analyser during commissioning.
Detuned reactor selection
| Site condition | Approach |
|---|---|
| Little nonlinear load | Standard capacitors may suffice |
| Moderate drives or UPS | Detuned reactors, chosen to avoid resonance |
| Heavy harmonics | Harmonic study, active filter or tuned filter |
Detuning shifts the resonant frequency below the dominant harmonics, so the capacitor bank does not amplify them. Select reactor percentages on the basis of a harmonic measurement or study.
Operation with generators
An alternator is not designed to absorb large leading reactive power. If capacitors remain connected while the load is light, the generator sees a leading power factor, which can cause over-voltage and loss of stability. Use controls that disconnect capacitors on generator supply or limit steps by generator capability.
Maintenance
- Check capacitor current against rating.
- Thermography of connections and capacitors.
- Contactor wear.
- Discharge resistors.
- Controller log and settings.
The master engineering question
At the lowest load and on generator supply, does the panel still avoid over-correction and resonance?
What to send us
Send bills or logs and single-line diagram. Send your application details.
Questions engineers ask
Why do capacitors fail?
Often harmonics, heat or over-voltage.
Do I need reactors?
If harmonics are present, usually yes.
Can it run on a generator?
Only with checks and control logic.
What target pf?
Set by tariff rules and site, commonly near unity without over-correction.
Values are illustrative. Design needs measured data.