Harmonic mitigation

Active vs Passive Harmonic Filters: Selection Guide

Power-quality guide · Reviewed 22 Sep 2026 · ElectroDesigner

Active and passive harmonic filters solve distortion in different ways. An active harmonic filter measures distortion and injects compensating current dynamically. Passive filtering uses inductors and capacitors arranged for the target application; depending on topology, it may be a tuned shunt network or a line-side broadband/series filter supplied for a nonlinear branch.

In this article1. Technology difference 2. Selection logic 3. Worked example 4. THDi, TDD and PCC 5. Assumptions and limits

1. What changes between active and passive filtering?

Active harmonic filter (AHF)

An AHF is normally connected in parallel with the system and injects a compensating waveform to reduce harmonic current seen upstream. Because the compensation is controlled electronically, it is well suited to mixed or changing nonlinear loads and can compensate several branches from a common point when correctly selected and installed.

Passive filter

Passive solutions use L/C components and are application-specific. Tuned shunt filters provide a low-impedance path for selected harmonic orders. Broadband drive filters may instead be supplied as line-side branch equipment matched to a stable nonlinear load. Their performance depends on the actual spectrum, source impedance, loading and filter topology.

Do not select an LC network from THDi alone. The harmonic spectrum and network impedance determine resonance and component stress.

2. Practical selection logic

Changing load mix / multiple nonlinear loads: AHF is usually the more adaptable architecture.
Stable repetitive drive branch: a manufacturer-engineered passive filter can be efficient and simple.
High pre-existing THDv, capacitor banks or weak source: perform an impedance/resonance study before final selection.
Triplen harmonics in 3P4W systems: include neutral-current behavior and topology in the study.
Compliance target: define the actual PCC and measurement basis before sizing equipment.

3. Worked example: preliminary AHF compensation current

Use the same simplified screening model as the ElectroDesigner calculator: two 80 A six-pulse VFD branches at 35% THDi plus one 40 A UPS branch at 10% THDi. For each load, estimate fundamental current from RMS current and THDi:

I₁ = I_RMS / √(1 + THDi²)

I_h = I₁ × THDi

Conservative arithmetic summation gives:

Total fundamental current I₁ ≈ 190.82 A

Estimated harmonic current I_h ≈ 56.84 A

Estimated source THDi ≈ 29.79%

If the screening target is 5% THDi:

Target harmonic current = 190.82 × 0.05 = 9.54 A

Compensation before margin = 56.84 − 9.54 = 47.30 A

With 10% design margin = 47.30 × 1.10 = 52.02 A

A preliminary module screen would therefore move to the next suitable manufacturer rating, for example about 55 A where such a module exists. This is a screening result, not a final filter design.

Run the Harmonic Filter Sizing Calculator

4. THDi, TDD and the point of common coupling

IEEE 519-2022 applies harmonic voltage and current distortion objectives at the user point of common coupling (PCC) for the overall installation. Its current-distortion framework uses TDD and the source-strength ratio Isc/IL; it is not an equipment-level THDi limit. A low THDi value at one drive therefore does not, by itself, prove facility compliance at the PCC.

5. Assumptions and limits

The worked example uses conservative arithmetic harmonic-current summation and does not model phase-angle cancellation between sources.
Individual harmonic orders, network frequency-dependent impedance and resonance are not solved.
AHF thermal derating, CT arrangement, voltage, topology and manufacturer overload behavior must be checked.
Passive filter selection requires the real spectrum and the manufacturer's application rules.
Generators, weak grids, capacitor banks, high THDv and significant triplen content justify a detailed harmonic study.

Adjacent checks

Read Power Factor Correction Worked Example before adding capacitor banks to a nonlinear network. Use the Power Factor Correction Calculator for reactive-power sizing and keep the harmonic and PFC assumptions in the same design review.

Engineering references

References: Schneider Electric Electrical Installation Guide — Harmonic filtering; Schneider Electric — Active harmonic mitigation at low voltage; and IEEE 519-2022 scope at the PCC. No IEEE harmonic-limit tables are reproduced here.