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Active Harmonic Filters
2 Series · 10 Models
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Active Harmonic Filters Overview
Active Harmonic Filters (APF), also called active power filters, measure the load current and inject an equal-and-opposite current to cancel harmonics at the point of common coupling. QDTB APF systems are rated 50–300 A at 400 V or 690 V and reduce total harmonic current distortion (THDi) to below 5%, protecting transformers, cables and capacitor banks while improving power quality and power factor.
Available Series
Modular APF Series
The modular active power filter (APF) uses rack-mounted power modules that can be paralleled to build the required compensation current. Rated 100–300 A per module at 400 V or 690 V, it measures the load current and injects a phase-opposite current to cancel harmonics, reducing total harmonic current distortion (THDi) to below 5% and improving power factor at the point of common coupling.
Cabinet APF Series
The cabinet active power filter (APF) packages the full compensation rating in a single floor-standing enclosure for central installation at a main switchboard or PCC. Rated 100–300 A at 400 V or 690 V, it cancels harmonics, reduces THDi to below 5%, corrects power factor and relieves transformers and cables of harmonic loading.
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Application Scenarios
Data Centers
Mitigates harmonics from UPS and switch-mode power supplies to protect transformers and switchgear.
Industrial VFD Loads
Cancels harmonics from variable frequency drives, welders and rectifiers.
Commercial Buildings
Improves power quality for LED lighting, HVAC and lift drives.
Water & Pumping Stations
Contains harmonics from soft-starters and pump VFDs.
Technical Advantages
THDi < 5%
Closed-loop cancellation reduces total harmonic current distortion to below 5%.
Modular & Cabinet Options
Rack-mounted modules for scalability or single floor-standing cabinets.
50–300 A Ratings
Sized for individual circuits up to central main-switchboard compensation.
400 V / 690 V Systems
Low-voltage design for industrial and commercial distribution.
FAQ - Active Harmonic Filters
How does an active harmonic filter work?
What harmonics can an APF compensate?
Is an APF the same as a capacitor bank?
How do I size an active harmonic filter?
What capacity and voltage range is available for Active Harmonic Filters?
What is the minimum order quantity (MOQ) for Active Harmonic Filters?
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All specifications and dimensions can be customized upon request
Active Filter vs Detuned Bank vs Passive Filter: How to Choose
| Active Harmonic Filter (APF) | Detuned Capacitor Bank | Passive Filter | |
|---|---|---|---|
| Principle | Measures the load current and injects an equal and opposite harmonic current | Reacts to the dominant harmonic through a reactor and capacitor | Tunes an LC branch to absorb one harmonic order |
| Harmonic orders | Compensates the orders present, with selectable priority | Addresses the tuning order only | One branch per order, so several branches are needed |
| Response | Follows a changing load within milliseconds | Fixed reactive output, so it cannot follow a varying harmonic | Fixed, tuned to the design condition |
| Resonance risk | None, since it does not add capacitance at the harmonic | Managed by detuning, but the risk is only reduced | Designed to avoid resonance at the tuned order |
| Footprint | Compact cabinet or modular rack | Moderate, capacitor and reactor per step | Large for multiple orders |
| Cost | Highest per ampere of harmonic current | Lowest | Middle, and rises with each order added |
| Typical application | Data centres and VFD-heavy plants | Steady plants with a dominant harmonic | Known, stable harmonic spectra |
Where QDTB® Active Harmonic Filters Are Specified
An active filter is specified where the harmonic spectrum moves with the load: data centres full of switched-mode supplies, industrial plants with banks of VFDs, commercial buildings with lifts and chillers, and water or pumping stations with soft starters. A passive branch tuned to one order cannot follow that load, which is why the active device is chosen.
The filter samples the load current, extracts the harmonic components and injects the opposite current at the point of common coupling, so the upstream transformer, cable and capacitor bank see a clean current. That protects the transformer from harmonic heating and keeps the capacitor bank from resonating with the very harmonics it was meant to ignore.
Sizing is based on the measured or estimated harmonic current of the load rather than on the load current itself, and individual orders up to the 25th are normally compensated. Where power factor also has to be corrected, the same device can supply reactive current alongside the harmonic component.
Units are rated 50-300 A at 400 V or 690 V and reduce THDi to below 5 %. That limit is what IEEE 519 and IEC 61000-3-6 ask of the point of common coupling, in modular or cabinet construction.
Frequently Asked Questions
How does an active harmonic filter work?
The APF samples the load current, extracts the harmonic components, and injects a current of equal magnitude and opposite phase so harmonics cancel at the measurement point.
What harmonics can an APF compensate?
Active filters typically compensate individual harmonic orders up to the 25th, with selective control over which orders are targeted.
Is an APF the same as a capacitor bank?
No. Capacitor banks provide fixed reactive power and can resonate with harmonics, while an APF actively cancels harmonic current and can also correct power factor.
How do I size an active harmonic filter?
The APF rating is based on the measured or estimated harmonic current of the load, typically 50–300 A per unit with units paralleled for larger installations.
What capacity and voltage range is available for Active Harmonic Filters?
QDTB® supplies Active Harmonic Filters from 50 to 300 A, with a 0.4 kV voltage rating.
What is the minimum order quantity (MOQ) for Active Harmonic Filters?
One unit for standard products. Custom-designed equipment is quoted on specifications — send your parameters or drawings and our sales team will confirm the MOQ for your configuration.