Active harmonic filter (AHF) — dynamic compensation versus the tuned passive filter

The guide on tuned reactors and capacitor banks covers the passive detuned filter: a fixed reactor-capacitor combination, tuned just below one specific harmonic order, which avoids resonance with the grid but otherwise has a fixed, non-adaptive compensation behaviour. This article covers the alternative: the active harmonic filter (AHF), also called an active power filter.

Operating principle

An AHF is a power-electronic device (typically IGBT-based, with PWM control) connected in parallel (shunt) with the load. The filter continuously measures the distorted load current, calculates in real time which harmonic components are present (see the [guide on harmonics and THD](/guides/nen-1010/harmonischen-thd) for the background of that phenomenon), and injects a current that is in anti-phase with those harmonic components — so that the current the grid "sees" at the connection point remains nearly pure sinusoidal.

The key difference from a tuned passive filter

Tuning: a passive detuned filter is physically tuned to one (or a limited number of) harmonic orders and does not change that behaviour when the load changes. An AHF continuously measures which harmonic orders are actually present and compensates for them — usually several orders at once, within the bandwidth of its controller and switching frequency.
Resonance risk: a passive filter consists of an LC network and can, if mistuned relative to the grid impedance, actually amplify resonance (see the detuned-filter guide for those risks). An AHF is a current-source-controlled device, not an LC network, and carries no such resonance risk with the grid.
Adaptability: if the load's harmonic spectrum changes (for example due to added variable-frequency drives or LED lighting), an AHF keeps compensating effectively within its power rating without redesign — a passive filter would need to be retuned or supplemented in that case.
Additional functions: many AHFs can, besides harmonic compensation, also correct reactive power (see the [guide on power factor correction](/guides/nen-1010/vermogensfactorcorrectie)) and, in three-phase versions, compensate part of the phase unbalance — functions a standard passive filter does not offer.

Sizing: on harmonic current, not full load current

An AHF is sized to the RMS level of the harmonic currents to be compensated, not the full (fundamental plus harmonic) load current. An AHF with too low a rating relative to the actual harmonic level of the installation only partially compensates — the remaining, uncompensated harmonic current can, for example, still lead to increased loading of the neutral conductor (see the guide on the neutral conductor and third harmonic).

Note: there is no specific IEC design-testing standard for the AHF itself, unlike, for example, IEC 62040-3 for a UPS. IEC 61000-3-2 describes emission limits for harmonic currents from smaller equipment, and IEC 61000-4-7 the corresponding measurement method for harmonics and interharmonics; these standards form the reference framework within which an AHF is sized, together with system-level guidelines such as IEEE 519.

Practical relevance

For an installation with a strongly varying or growing harmonic profile — for example due to a growing number of variable-frequency drives, charge points, or LED lighting circuits — an AHF is often a more suitable choice than a passive filter, precisely because the compensation does not need to be tuned in advance to one fixed situation. For a stable, well-characterised dominant harmonic (such as a large, constantly operated 6-pulse converter), a passive tuned filter often remains the simpler and cheaper solution per compensated kVAr.

Common mistakes

Sizing an AHF to the full load current instead of the actual harmonic current — this leads to a substantially oversized and unnecessarily expensive filter.
Applying an AHF without first measuring the actual harmonic spectrum (see the guide on measuring harmonics with a power analyser) — without that measurement, it is not possible to determine the required rating and bandwidth.
Treating an AHF and a tuned passive filter as fully interchangeable — the choice depends on the stability of the harmonic profile, the resonance risk, and the desired additional functions (reactive power compensation, phase unbalance).