# Active harmonic filter

Canonical URL: https://eloragrid.ai/symbols/active-harmonic-filter
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: AHF
Drawing use: SLD, Schematic
Reference families: IEC, IEEE / ANSI

A shunt converter that measures load harmonics and injects opposing currents to cancel them at the connection point.

## In plain English

### What does Active harmonic filter mean?

An active harmonic filter is a shunt-connected voltage-source converter that measures load current through current transformers, calculates the harmonic content and injects equal and opposite currents. It can also correct power factor and balance load between phases. Response adapts to changing loads, so it does not need tuning to particular orders.

### A simple example

Single-line diagrams show a converter block with a wave or harmonic mark, connected to the switchboard bus through a breaker, with current transformers on the load side or on the supply side depending on the control method. Several modules are often paralleled.

### What the drawings help you do

The filter rating is in amperes of compensation current, not in kvar, and it can only cancel harmonics within that current and frequency range. The symbol does not tell you where the current transformers are, which decides whether the filter compensates the whole board or one load.

### This particular reference

A shunt converter that measures load harmonics and injects opposing currents to cancel them at the connection point. An active filter cancels harmonics with a converter; a passive filter sinks them with tuned capacitors and reactors.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- Phase: One of the related AC waveforms in a power system.
- Rating: A value stating the conditions for which equipment is specified.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [STATCOM: converter-based reactive power compensation](https://www.hitachienergy.com/products-and-solutions/facts/statcom)
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/)


## What it represents

An active harmonic filter is a shunt-connected voltage-source converter that measures load current through current transformers, calculates the harmonic content and injects equal and opposite currents. It can also correct power factor and balance load between phases. Response adapts to changing loads, so it does not need tuning to particular orders.
Single-line diagrams show a converter block with a wave or harmonic mark, connected to the switchboard bus through a breaker, with current transformers on the load side or on the supply side depending on the control method. Several modules are often paralleled.
The filter rating is in amperes of compensation current, not in kvar, and it can only cancel harmonics within that current and frequency range. The symbol does not tell you where the current transformers are, which decides whether the filter compensates the whole board or one load.

## How to read it

- Find the current transformers; their position defines what is being compensated.
- Ratings are given in compensating amperes.

## Information to show or reference

- State compensating current and harmonic range.
- Show current transformer location.
- Reference parallel module arrangement.

## Important distinction

An active filter cancels harmonics with a converter; a passive filter sinks them with tuned capacitors and reactors.

## Facts

- Core element: Shunt voltage-source converter
- Rating: Compensating current in amperes
- Extra functions: Power factor and load balancing
- Preview: Independent educational illustration; consult the applicable standard and project legend for the exact graphic

## Public references

- [STATCOM: converter-based reactive power compensation](https://www.hitachienergy.com/products-and-solutions/facts/statcom) — Hitachi Energy. Public converter-based compensator page noting active filtering capability.
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/) — IEC. IEC graphical symbol context for converters.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.