How Low-Voltage Active Power Filters Compensate for Harmonics in the Power Grid

16 Jun 2025

With the increasing use of nonlinear loads such as variable frequency drives, rectifiers, and other power electronic devices, power grid harmonics have become a common problem in low-voltage electrical systems. Low-voltage active power filters (APFs) play a key role in detecting and mitigating harmonic distortion to improve power quality. Here’s how APFs work to actively compensate for harmonics in real time.

1. Harmonic Detection and Source Identification

The first step in harmonic compensation is to detect the presence and characteristics of harmonic currents in the grid. These harmonics are typically generated by nonlinear loads, such as:

Inverters

Rectifiers

Switch-mode power supplies

Other power electronic devices

APFs continuously monitor voltage and current waveforms. By using advanced signal processing algorithms—such as instantaneous power theory (p-q theory), Fast Fourier Transform (FFT), or adaptive filtering—the filter can accurately extract harmonic components from the power signals.

2. Calculating the Required Compensation Current

Once the harmonic components are identified, the APF determines how much compensating current is needed to restore waveform purity. The objective is to reshape the distorted current waveform into an ideal pure sine wave, thus reducing total harmonic distortion (THD) and enhancing power quality.

The compensation current typically includes two components:

Harmonic current compensation: Targets specific harmonic frequencies present in the grid

Reactive power compensation: Offsets reactive currents caused by inductive or capacitive loads, improving the power factor close to unity

3. Generating the Compensating Current

The APF uses its internal inverter—typically based on IGBTs or MOSFETs—to generate a compensating current that is equal in magnitude but opposite in phase to the detected harmonic current.

By injecting this inverse current into the power grid, the APF effectively cancels out the harmonic currents caused by the original loads. As a result, the total current drawn from the power source becomes cleaner and closer to a sine wave.

4. Control Strategy and Feedback Mechanism

To ensure high accuracy and real-time responsiveness, APFs are equipped with closed-loop feedback control systems. These systems constantly compare actual grid current with the ideal waveform and dynamically adjust the inverter output.

Common control methods include:

Instantaneous power theory (p-q theory): Decomposes power flow into active, reactive, and harmonic parts to calculate compensation signals

Fast Fourier Transform (FFT): Provides frequency-domain analysis to isolate specific harmonic orders

Current feedback control: Continuously adjusts compensation in response to load or grid fluctuations