A motor, a transformer or a fluorescent ballast needs a magnetic field to work. Building that field takes current that does no useful work. The supply still has to carry it. Power factor measures how much of the current does the work.
Real, reactive and apparent power
Three quantities describe an a.c. load:
- Real power, kW. It turns a shaft, heats an element or lights a lamp.
- Reactive power, kvar. It builds and collapses the magnetic field each half cycle. On average it does no work.
- Apparent power, kVA. It sets the current. The cable, the switch and the transformer are sized on it.
The three form a right triangle: kVA² = kW² + kvar². Power factor is kW divided by kVA. A load at 0.8 power factor draws 1.25 kVA for every kilowatt of work.
Why a low power factor costs
The current follows the kVA. At 0.78, a 250 kW load draws 463 A at 400 V. At 0.95 the same work needs 380 A. The extra current heats the cables and loads the transformer. It also uses up supply capacity that new machines could have used.
Many networks bill reactive power, or bill on kVA, when the power factor falls below a set value. 0.95 is a common figure. Check the tariff of the local network.
How a capacitor corrects it
Inductive loads draw lagging reactive power. A capacitor draws leading reactive power. Fitted on site, a capacitor bank supplies the field current locally, so the supply no longer carries it.
The kvar the bank must supply comes from the triangle. With real power P:
- reactive power now = P × tan(arccos PF now)
- reactive power at the target = P × tan(arccos PF target)
- correction = the difference
The power factor correction tool works these out, then rounds up to whole bank steps.
Why banks switch in steps
A factory load changes through the day. A fixed bank sized for the full load overcorrects at night, and the site turns leading. A leading power factor can raise the voltage and upset generators. So an automatic bank splits into steps, and a controller switches them to follow the load. Pick a step no larger than the reactive power of the lightest regular load.
Harmonics and detuned banks
Variable speed drives, rectifiers, UPS units and LED drivers draw harmonic currents. A capacitor has a low impedance at high frequency, so it attracts those currents. Worse, a capacitor bank and the supply transformer form a tuned circuit. Its resonance order is about the square root of the fault level over the bank size. If it lands near the 5th, 7th, 11th or 13th harmonic, the harmonic currents are amplified. Capacitors then overheat and fail early.
A detuned bank puts a reactor in series with each capacitor step. The pair tunes below the 5th harmonic, so the bank cannot resonate with the main harmonics. As a rule of thumb, use a detuned bank above about 10 % THDi. Above about 20 %, a harmonic study should choose between a detuned bank, a filter and an active compensator.
The neutral side of harmonics is covered in Harmonics and neutral sizing.
The bank circuit
A capacitor bank draws its full rated current whenever its steps are in, and harmonics add to it. AS/NZS 3000:2018 Cl 4.15.2.3 sets the supply conductors at 135 % of the bank current at least. The switching devices must be rated for capacitors (Cl 4.15.2.2). Each capacitor needs a discharge path and a warning notice (Cl 4.15.3.1). The tool works the 135 % current for the bank it suggests.
What the tool leaves out
The tool works one operating point. It gives the conductor current but does not pick the cable or the protection, and it does not model a load profile through the day. The harmonic thresholds are rules of thumb, not limits from a standard. A supplier or a power quality study confirms the final bank.