An APFC panel that was perfect in 2015 can be a fire risk in a 2026 plant full of drives and inverters. Nothing in the panel changed. The load did.
Automatic power factor correction (APFC) panels are among the most common pieces of equipment in Indian industry, because distribution licensees penalise low power factor and reward correction. They are also among the most frequently misapplied. Capacitor banks sized on a tariff bill, switched without regard to harmonics, placed on a bus full of variable-frequency drives and solar inverters, fail early and sometimes dramatically. This guide explains how to calculate the kVAr you need, why harmonics change the design, when a detuned or thyristor-switched bank is required, and what to specify.
Formulas and limits reflect standard engineering practice and published standards as of October 2026; the plant’s measured load data must drive the final design.
Key takeaways
- Size on measured load: Q (kVAr) = P × (tan φ1 − tan φ2); aim for 0.98 to 0.99 lagging, never unity.
- Choose steps for smooth control: 6 to 14 steps of 5 / 10 / 25 / 50 kVAr so the controller can track the load without hunting.
- Harmonics decide the design: drive- and inverter-heavy buses need detuned (7 % or 14 %) reactors, never plain capacitors.
- Switching follows the load: thyristor switching for fast-changing loads such as welders and chargers; contactor switching for steady loads.
- Mind the voltage: capacitors on detuned banks must be rated 440 to 525 V; 800 V solar and BESS buses need purpose-rated banks.
In this guide
What power factor correction does
Induction motors, transformers and many electronic loads draw reactive power (kVAr) alongside real power (kW). Reactive power does no work but loads cables, transformers and the utility’s network, and it shows up on the bill as a low power factor. Capacitors supply reactive power locally so that less is drawn from the grid. An APFC panel switches capacitor steps in and out automatically to hold a target power factor, typically 0.98 to 0.99 lagging, as the load changes.
Step 1: Calculate the kVAr
For a load P (kW) at an existing power factor cos φ1 to be corrected to cos φ2:
Q (kVAr) = P × (tan φ1 − tan φ2)
| Existing PF | Target 0.95 | Target 0.98 | Target 0.99 |
|---|---|---|---|
| 0.70 | 0.69 × P | 0.82 × P | 0.88 × P |
| 0.75 | 0.55 × P | 0.68 × P | 0.74 × P |
| 0.80 | 0.42 × P | 0.55 × P | 0.61 × P |
| 0.85 | 0.29 × P | 0.42 × P | 0.48 × P |
| 0.90 | 0.16 × P | 0.28 × P | 0.34 × P |
Example: a plant with 1,000 kW average load at 0.80 power factor, to be corrected to 0.98, needs about 0.55 × 1,000 = 550 kVAr. Use measured average and peak kW and power factor from the energy meter or a one-week power-quality log, not the connected load; connected load over-sizes the bank and the smallest step then overshoots at light load.
Step 2: Choose the steps
The smallest step should be small enough that the controller can hold the target without hunting at minimum load; a common rule is that the smallest step is no more than about 10 to 15 per cent of the total. Banks are built in 6 to 14 steps with step sizes such as 5, 10, 25, 50 or 100 kVAr, arranged in a 1:1:2:2 or 1:2:4 pattern so that many combinations are available. A microprocessor controller measures current and voltage, computes the reactive demand and switches steps with a delay that respects capacitor discharge time.
Step 3: Account for harmonics
This is where most APFC failures start. Variable-frequency drives, UPS systems, LED lighting, welding machines and solar inverters draw non-sinusoidal current. Capacitors have low impedance at harmonic frequencies, and a capacitor bank with the plant’s inductance forms a resonant circuit. If the resonant frequency lands near the 5th or 7th harmonic, harmonic current is amplified, capacitors overheat, fuses blow and contactors weld.
The usual thresholds in practice:
- Non-linear load below roughly 15 per cent of the transformer rating and current THD low: standard capacitors to IEC 60831 may be adequate.
- Non-linear load above roughly 15 to 20 per cent, or current THD rising: use a detuned bank. A series reactor with each capacitor shifts the resonance below the lowest significant harmonic. Common detuning factors are 7 per cent (tuned near 189 Hz) and 14 per cent (near 134 Hz). Detuned capacitors are rated at higher voltage, typically 440, 480 or 525 V, because the reactor raises the voltage across them.
- Very high distortion or a need to actually remove harmonics: an active harmonic filter or a tuned passive filter, designed from a harmonic study.
Reactors should be specified to IEC 60076-6 with a linearity rating, and the panel needs ventilation designed for the reactor losses, which are far larger than capacitor losses.
Step 4: Contactor or thyristor switching
| Switching | Best for | Notes |
|---|---|---|
| Capacitor-duty contactors | Slowly varying loads: pumps, compressors, general industry | Pre-charge resistors limit inrush; switching delay of seconds to minutes |
| Thyristor switching modules | Fast-changing loads: welding, cranes, presses, lifts, rolling mills | Transient-free switching within cycles; no contactor wear; higher cost; often called RTPFC (real-time PFC) |
Step 5: Solar and 800 V systems
Solar inverters usually operate at or near unity power factor, so a plant with rooftop solar may see its displayed power factor fall during the day as the grid supplies only reactive power. Controllers for such sites need leading-and-lagging control and current transformers placed on the grid incomer, not on the plant bus, so that correction follows what the utility meter sees. On utility-scale plants with 800 V AC collection, reactive-power support may be required by the connection agreement; capacitor banks for these systems must be rated for the higher voltage and coordinated with the inverter’s own reactive capability.
What to specify
- Total kVAr, number and size of steps, target power factor, and whether leading control is required.
- Capacitor type and voltage rating (IEC 60831-1/2), reactor detuning factor if applicable (IEC 60076-6), switching type.
- Controller with Modbus communication for energy management.
- Incomer and per-step protection (MCCB or HRC fuses), discharge resistors, and the assembly to IEC 61439-1/-2 with a design-verification basis.
- Enclosure IP42 or IP54 indoors with thermostat-controlled forced ventilation, IP55 outdoors.
- Ambient temperature and derating, and the ventilation calculation for reactor losses.
A modern APFC panel for Indian conditions is typically offered from about 20 kVAr to 3000 kVAr in 6 to 14 steps, with contactor or thyristor switching, 7 or 14 per cent detuned reactor options, and controllers with a 0.99 target and optional Modbus communication.
Frequently asked questions
How is the kVAr rating of an APFC panel calculated?
The kVAr rating is calculated as kW × (tan φ1 − tan φ2), where φ1 is the existing power factor angle and φ2 is the target power factor angle. The actual working load in kW should be used, not the connected load.
What APFC panel size is required for a 100 kW load?
If the power factor is raised from 0.75 to 0.98, about 68 kVAr is required. Therefore, a 70 kVAr or 75 kVAr APFC panel is usually selected.
Why should power factor not be corrected to unity?
At unity, a small error in step switching can push the system into a leading power factor. For this reason, a target of 0.97 to 0.99 lagging is preferred.
How many steps should an APFC panel have?
In most industrial plants, four to eight steps are used. The smallest step should be smaller than the reactive demand at minimum load, so hunting is avoided.
What is a detuned APFC panel?
A detuned APFC panel is one in which a reactor is connected in series with each capacitor step. As a result, harmonic resonance is prevented and the capacitors are protected from harmonic overload.
When is a 7% reactor used instead of a 14% reactor?
A 7% reactor is used where 5th and higher harmonics from VFDs are dominant. A 14% reactor, on the other hand, is chosen where a significant 3rd harmonic is present.
Why do capacitors fail in plants with VFD loads?
Harmonic current produced by VFDs is drawn into the capacitors, and resonance may also be created with the transformer. Consequently, the capacitors overheat, swell, and fail early.
Why are 480 V or 525 V capacitors used in a 415 V detuned panel?
When a series reactor is added, the voltage across the capacitor rises above the system voltage. Therefore, higher-rated capacitors are used to keep them within safe limits.
Can an active harmonic filter be used with an APFC panel?
Yes. When non-linear load makes up more than about half of the transformer rating, a detuned APFC panel is often combined with an active harmonic filter. In this way, power factor is corrected and harmonic distortion is reduced together.
Which standards apply to APFC panels?
The panel assembly is covered by IEC 61439-1 and IEC 61439-2, and the LV shunt capacitors are covered by IEC 60831.
What data is needed before an APFC panel is designed?
The existing PF, target PF, maximum and minimum kW, transformer rating and impedance, harmonic levels, and details of VFD or UPS loads should be provided. In addition, future load expansion should be considered.
Related reading
Disclaimer. This article is for general technical education. Formulas, thresholds and standards references reflect common engineering practice and public sources current as of October 2026. Capacitor bank design must be based on the plant’s measured load and harmonic data and on the current editions of the applicable standards; nothing here is a design specification or a guarantee of performance.
