Introduction #

This guide is for electrical engineers, facility managers, and power quality specialists who specify or maintain power factor correction (PFC) in facilities with variable-speed drives (VFDs), rectifiers, or other nonlinear loads. It solves the problem of harmonic distortion causing capacitor resonance, equipment failure, and ineffective or harmful PFC. Use this knowledge when selecting between standard and detuned capacitor banks, troubleshooting capacitor or reactor failures, assessing THD before adding PFC, or interpreting power factor measurements in harmonic-rich systems.

For a comprehensive overview of power factor concepts, why it matters, and how correction works, see our Power Factor Guide.

What Are Harmonics and Why They Matter in PFC #

Harmonics are integer multiples of the fundamental power frequency (50 Hz or 60 Hz). In industrial systems, they are produced mainly by nonlinear loads: VFDs, rectifiers, switching power supplies, and arc furnaces. These loads draw current in short pulses rather than sinusoidally, which distorts the voltage and current waveforms. The most significant harmonic orders are the 5th (250 Hz or 300 Hz), 7th (350 Hz or 420 Hz), 11th, and 13th.

Harmonics affect power factor correction in two ways. First, they change the relationship between real power (kW) and apparent power (kVA). Displacement power factor (from the fundamental) and true power factor (including harmonics) can differ; some meters report only displacement PF, which can mislead sizing and evaluation. Second, and more critical for PFC, capacitors have decreasing impedance at higher frequencies (Z = 1/(2πfC)). When the inductive reactance of the system (transformers, cables) and the capacitive reactance of the PFC bank satisfy a resonance condition at a harmonic frequency, current and voltage at that harmonic can be greatly amplified. The result is capacitor overload, overheating, fuse blowing, and in severe cases, damage to capacitors and upstream equipment.

How Harmonics Cause Capacitor Resonance #

A power factor correction capacitor in parallel with the system forms an LC circuit with the source inductance (transformer and cable). The parallel resonance frequency is:

f_res = 1 / (2π√(LC))

Where:

  • f_res = resonance frequency (Hz)
  • L = equivalent system inductance (H) at the point of connection
  • C = capacitance of the PFC bank (F)

If f_res is close to a dominant harmonic (e.g. 5th at 250 Hz or 7th at 350 Hz on a 50 Hz system), that harmonic is amplified. Current through the capacitor can be many times the fundamental current, leading to overheating and failure. The same applies at 300 Hz and 420 Hz on 60 Hz systems.

Detuned capacitor banks add a series reactor (inductor) between the network and the capacitor. The reactor and capacitor are chosen so that their series resonance occurs at a frequency below the lowest problematic harmonic—commonly 189 Hz (50 Hz systems) or 210 Hz (60 Hz systems). At 5th and 7th harmonic frequencies, the combination is inductive; it presents high impedance to those harmonics and prevents resonance with the rest of the system. The bank still supplies reactive power at fundamental frequency for power factor correction, but it no longer amplifies harmonics.

For capacitor sizing, step-by-step kVAR calculation, and installation choices, see Capacitor Bank Sizing for Power Factor Correction.

When to Use Detuned vs Standard Capacitor Banks #

Condition Recommendation
THD_V > 5% or significant harmonic current Detuned banks
Many VFDs, rectifiers, or electronic loads Detuned banks
THD_V < 5%, mainly motors, heaters, and linear loads Standard capacitors usually acceptable
Uncertain harmonic levels Measure THD first; if >5% or substantial 5th/7th, use detuned

Typical detuning frequencies: 189 Hz (50 Hz) and 210 Hz (60 Hz) to stay below 5th harmonic. For 60 Hz, 134 Hz is sometimes used to also avoid 3rd harmonic. Specify the tuning when ordering; detuned units have different kVAR ratings than the capacitor alone because of the series reactor. For more on common errors in PFC projects, including harmonic-related mistakes, see Power Factor Correction: Common Mistakes.

THD and Resonance: Formulas and a Worked Example #

Total Harmonic Distortion (Voltage) #

Voltage THD is defined as:

THD_V = (√(V₂² + V₃² + V₄² + … + Vₙ²) / V₁) × 100%

Where V₁ is the fundamental voltage and V₂…Vₙ are the rms voltages of the 2nd through n-th harmonics. Similar expressions apply to current THD. Power quality meters and analyzers report THD_V and THD_I directly; use measurements at the point of common coupling (PCC) or at the planned PFC location.

Worked Example: When Resonance Becomes a Problem #

A 480 V, 60 Hz facility has a 300 kVAR PFC capacitor at the main bus. The equivalent source inductance (transformer plus cable) at that point is about 200 µH. For 300 kVAR at 480 V (three-phase), C ≈ 1.15 mF. Then:

  • f_res = 1 / (2π√(200×10⁻⁶ × 1.15×10⁻³)) ≈ 332 Hz.

332 Hz is close to the 5th harmonic (300 Hz) and in the range of the 7th (420 Hz). The 5th and 7th harmonic currents can be strongly amplified. If the facility has many VFDs and THD_V is 8%, this standard capacitor can see high harmonic currents, overheating, and failures. Switching to a detuned bank (e.g. 210 Hz) moves the resonance well below 300 Hz, so 5th and 7th harmonics see high impedance and are not amplified.

Common Mistakes with Harmonics in PFC #

Mistake 1: Installing Standard Capacitors Without Checking THD #

The error: Adding standard PFC capacitors in a plant with many VFDs or rectifiers without measuring harmonic levels. Resonance amplifies harmonic current through the capacitors, leading to trips, fuse blowing, or capacitor failure within months or a few years.

The correct approach: Before installing PFC, measure THD_V and THD_I (and ideally harmonic spectrum) at the PCC or target bus. If THD_V > 5% or there are substantial 5th/7th harmonics, use detuned capacitor banks. If unsure, measure; do not assume.

Mistake 2: Relying Only on Displacement Power Factor #

The error: Sizing or evaluating PFC using only displacement power factor from a meter that ignores harmonics. In harmonic-rich systems, true power factor can be lower, and apparent power higher, than the displacement-based values. Capacitor kVAR may be adequate for displacement PF but resonance and harmonic losses can still cause damage.

The correct approach: Use a power analyzer that reports fundamental and total (true) power factor, kW, kVA, kVAR, and THD. For both sizing and verification, consider the harmonic environment.

Mistake 3: Ignoring Harmonics When Replacing Failed Capacitors #

The error: Replacing failed PFC capacitors with the same standard units without investigating cause. If the failure was due to harmonic resonance, the replacement will fail again.

The correct approach: After a failure, measure harmonics and check whether resonance could explain the damage. If THD is high or resonance is likely, switch to detuned banks rather than like-for-like replacement.

Frequently Asked Questions #

Q1: What THD level should trigger the use of detuned capacitor banks? #

A: A common threshold is THD_V > 5% at the point of PFC connection. If the load mix includes many VFDs, rectifiers, or other nonlinear loads, use detuned banks even at slightly lower THD, or after checking the 5th and 7th harmonic levels. When in doubt, measure and, if significant 5th/7th are present, prefer detuned.

Q2: How do harmonics affect power factor measurements? #

A: Conventional power factor (displacement PF) uses only the fundamental. True power factor includes harmonics: PF_true = kW / kVA, where kVA contains harmonic components. In systems with high THD, true PF can be lower than displacement PF, and kVA higher. Use a meter with harmonic analysis when assessing or verifying PFC in such systems.

Q3: What does “detuned to 189 Hz” or “210 Hz” mean? #

A: The reactor and capacitor are tuned so that their series resonance is at 189 Hz (50 Hz systems) or 210 Hz (60 Hz systems). At that frequency the branch is resistive (minimum impedance). At 5th and 7th harmonic frequencies (250 Hz, 350 Hz for 50 Hz; 300 Hz, 420 Hz for 60 Hz), the branch is inductive and presents high impedance, avoiding parallel resonance with the rest of the system. The bank still corrects power factor at the fundamental.

Q4: Can I use standard PFC capacitors in a facility with some VFDs? #

A: It depends on the total harmonic distortion and how much of the load is nonlinear. If THD_V is below about 5% and VFDs are a small share of load, standard capacitors are often acceptable. If THD_V exceeds 5% or there are many VFDs, use detuned banks. Measure first; do not guess.

Q5: Do active harmonic filters replace the need for detuned PFC capacitors? #

A: Not necessarily. Active filters inject canceling harmonic currents and can improve THD and sometimes true power factor, but they do not provide the bulk reactive power (kVAR) that PFC capacitors do. In many plants, both are used: detuned capacitors for fundamental reactive power and power factor correction, and active filters where THD must be reduced beyond what detuning alone achieves. For typical PFC-only needs in harmonic-heavy systems, detuned banks are the usual first step.

If you need to convert between kW, kVA, and power factor when assessing systems with harmonics, use our PF & kW/kVA Converter.

Conclusion #

Harmonics in power factor correction require a different approach than in purely linear systems. In plants with VFDs, rectifiers, or other nonlinear loads, harmonic distortion can cause parallel resonance between PFC capacitors and system inductance, leading to overload, overheating, and failure. Measure THD before adding or replacing PFC; where THD_V exceeds about 5% or 5th/7th harmonics are significant, use detuned capacitor banks (e.g. 189 Hz or 210 Hz) instead of standard units. Always verify with a power analyzer that reports true power factor and harmonics, and do not reuse standard capacitors after harmonic-related failures without changing the design. Proper treatment of harmonics in PFC protects equipment and keeps correction effective.

About the Author: Sarah Martinez, P.E. is a licensed electrical engineer with 13+ years of experience in power systems design and energy management. Former utility engineer specializing in power quality, power factor correction, and industrial energy optimization. Has designed power factor correction systems for manufacturing facilities, data centers, and commercial buildings. All content in this guide has been reviewed and validated by licensed engineers.