Power Factor Correction for Industrial Sites

Power factor is the ratio of real power (kW) to apparent power (kVA) in an AC circuit. When motors, transformers, and other inductive loads draw reactive power from the supply, the power factor drops below 1.0, and the supply authority charges a penalty. Power factor correction uses capacitor banks to supply the reactive power locally, reducing the apparent power drawn from the grid. This guide explains the power triangle, the correction formula, fixed vs automatic systems, and harmonic resonance risks. The Power Factor Correction Calculator sizes the capacitor bank automatically.

What is power factor and why it matters

Every AC load draws two types of power. Real power (P, measured in kW) does useful work. Reactive power (Q, measured in kVAR) sustains magnetic fields in motors and transformers but does no useful work. The supply must deliver both.

Apparent power (S, measured in kVA) is the vector sum of real and reactive power. Power factor is P / S. A power factor of 1.0 means purely resistive load. A power factor of 0.75 means 25 percent of the apparent power is wasted on reactive current, increasing cable losses and supply charges.

The power triangle

• Adjacent side (horizontal): real power P (kW)
• Opposite side (vertical): reactive power Q (kVAR)
• Hypotenuse: apparent power S (kVA)
• Angle phi: phase angle between voltage and current. cos(phi) = power factor.

Correction reduces Q (the vertical side), shortening the hypotenuse (S) and bringing power factor closer to 1.0.

Leading vs lagging power factor

A lagging power factor (current lags voltage) is caused by inductive loads: motors, transformers, fluorescent ballasts. Most industrial sites are 0.70 to 0.90 lagging.

A leading power factor (current leads voltage) is caused by overcorrection with capacitor banks. Leading power factor increases terminal voltage and can interfere with generator regulation. Avoid overcorrection.

AS/NZS 61000 standards for harmonics

AS/NZS 61000.3.100 sets limits on harmonic current emissions. When capacitor banks interact with transformer inductance, they can amplify harmonics beyond these limits. Any power factor correction design should include a harmonic survey.

Cost impact: demand charges and penalties

Most Australian supply authorities charge based on kVA demand. A site drawing 200 kW at 0.75 PF has 267 kVA demand. Corrected to 0.95 PF: 211 kVA. The 56 kVA reduction at A$10 to A$20 per kVA per month saves A$6,700 to A$13,400 per year.

Calculating required reactive power

Q_remove = P x (tan(phi_old) - tan(phi_new))

For 200 kW at 0.75 PF targeting 0.95:

phi_old = arccos(0.75) = 41.4 deg, tan = 0.882
phi_new = arccos(0.95) = 18.2 deg, tan = 0.329
Q_remove = 200 x (0.882 - 0.329) = 110.6 kVAR

Fixed capacitor banks

A fixed bank provides constant reactive power regardless of load. Simple and cheap, suitable for constant loads. Risk: overcorrects at light load, causing leading power factor.

Automatic power factor correction (APFC)

An APFC system uses a relay that monitors power factor in real time and switches capacitor steps in and out. Typical banks have 6 to 12 steps. More expensive but avoids overcorrection at light load and undercorrection at heavy load. Standard for variable-load sites.

Resonance and harmonic interactions

A capacitor bank and transformer inductance form a parallel resonant circuit:

f_res = f_supply x sqrt(S_sc / Q_cap)

If this frequency coincides with the 5th harmonic (250 Hz), harmonic currents are amplified dramatically. Solution: detuned reactors in series with the capacitor bank, shifting resonance below the lowest significant harmonic.

Worked example: workshop (PF 0.75 to 0.95)

A metalworking workshop draws 150 kW at 0.75 PF.

Q_remove = 150 x (0.882 - 0.329) = 83.0 kVAR

Select a 100 kVAR APFC bank with 4 steps of 25 kVAR. No VFDs on site, so detuned reactors are not required.

Worked example: commercial site with VFDs

A commercial building draws 300 kW at 0.80 PF. Four VFD-fed chillers. THD at main switchboard: 8 percent.

Q_remove = 300 x (0.750 - 0.329) = 126.3 kVAR

Select a 150 kVAR APFC bank with 12.5 percent detuned reactors on each step. Detuning shifts resonance to approximately 190 Hz, below the 250 Hz 5th harmonic.

Where the calculator fits in

The ElecCalc Power Factor Correction Calculator calculates Q_remove, recommends a capacitor bank size, and estimates annual savings. For motor reactive power contribution, use the Motor Calculator.