What Is an Active Harmonic Filter? Types and Selection Criteria
An active harmonic filter (AHF) measures the harmonic currents drawn by the load with current transformers and generates their opposite to stop them reaching the grid.
An active harmonic filter (AHF) is a device that continuously measures the harmonic currents drawn by the load with current transformers and, with a power electronics stage, generates the opposite of these harmonics to stop them reaching the grid. Non-linear loads such as drives, UPS units, rectifiers, LED lighting and welding machines draw non-sinusoidal current from the grid; these harmonics heat cables and transformers, stress capacitors and mislead protection devices. Because an active filter adapts to the harmonic spectrum as the load changes, it is used in plants with variable loads.
Active and passive filters
| Feature | Active harmonic filter | Passive harmonic filter |
|---|---|---|
| Operation | Measures harmonics and injects opposite current | Filters with a reactor and capacitor circuit tuned to a specific harmonic |
| Variable load | Adapts to the load | Works best at the harmonic and load it is tuned for |
| Resonance risk | Does not resonate with the grid impedance | Resonance calculation needed in design |
| Sizing | By harmonic current (A) | By harmonic order and correction power |
Modules in the catalog
The Tense TNS-AHF range consists of 50 A, 100 A and 150 A modules. According to the product data, all three work at 400 V AC, are wall or cabinet (rack) mounted, have an instantaneous response time below 200 µs and communicate over Modbus-RTU.
Selection criteria
- Harmonic current measurement: The filter is chosen by the plant's total harmonic current, not by the load current. First measure current harmonics and THD with an energy analyser during the busiest operation.
- Rating and margin: The module rating (50, 100, 150 A) must cover the measured harmonic current and leave room for load growth. Check in the datasheet whether several modules can be used in parallel.
- Current transformers: The filter takes its measurement from current transformers. The CT location (grid or load side), ratio and direction must be connected according to the installation manual; wrong connection makes the filter increase harmonics.
- Additional functions: Some active filters also compensate reactive power and phase imbalance. Check the presence and priority of these functions in the datasheet.
- Mounting and cooling: For wall or cabinet mounting, leave the ventilation clearance given in the datasheet around the module; power electronics produce heat.
- Communication: Over Modbus-RTU the filter's status and measurements can be passed to SCADA or an energy monitoring system.
Common mistakes
- Sizing the filter by load current: An oversized, expensive filter is bought, or it falls short in a plant with high harmonic current.
- Connecting a CT in reverse: The filter increases harmonics instead of filtering them.
- Ignoring the correction panel: On a grid with harmonics, capacitors without reactors keep the resonance risk; check whether correction is done with a harmonic filter reactor.
Active harmonic filters at Welibaba
The Active Harmonic Filters category has 3 active products from Tense, all sold on quotation: TNS-AHF50-W-R, TNS-AHF100-W-R and TNS-AHF150-W-R. Passive solutions are in the Harmonic Filters category and explained in the harmonic filter guide.
Frequently asked questions
What does an active harmonic filter do?
It measures the harmonic currents produced by non-linear loads and generates their opposite to keep them off the grid; it reduces cable and transformer heating, capacitor failures and nuisance protection trips.
What rating of active filter is needed?
The total harmonic current measured in the plant, plus a margin for load growth. Base the choice on a measurement with an energy analyser, not on an estimate.
Active or passive filter?
An active filter in plants where the load and harmonic spectrum change often; a passive filter can be economical in plants with a steady load and a few dominant harmonics.