Active filters are called "active" because, as the name implies, the device requires a power supply (to compensate for harmonics in the main circuit). Their application overcomes the drawbacks of traditional harmonic suppression and reactive power compensation methods, such as LC filters (which can only provide fixed compensation), achieving dynamic tracking compensation. Moreover, they can compensate for both harmonics and reactive power.
An Active Power Filter (APF) is a new type of power electronic device used for dynamically suppressing harmonics and compensating reactive power. It can quickly track and compensate harmonics of various sizes and frequencies. The term "active" is used in contrast to passive LC filters, which can only passively absorb harmonics of fixed frequency and size. The APF can sample load current, separate each harmonic and reactive power, control, and actively output the magnitude, frequency, and phase of the current. It responds quickly, counteracting the corresponding current in the load, achieving dynamic tracking compensation. Moreover, it can compensate for harmonics, reactive power, and unbalance simultaneously.

An active power filter, in essence, is a power source that can supply the harmonic current required by the load, thus reducing its impact on the power grid and other equipment. With an increasing awareness of the hazards of harmonic currents, most new projects now incorporate active power filters. Moreover, as the structural design of the APF becomes more compact and power density increases, modular products like the iNAS-MA series are now widely promoted in the construction industry, offering individual compensation capacities of 30/50/75/100/150, etc.

Active Power Filter, a new type of power harmonic control equipment made with modern power electronics technology and digital signal processing based on high-speed DSP devices. It consists of two main parts: the command current computing circuit and the compensation current generating circuit. The command current computing circuit monitors the current in the line in real-time, converts the analog current signal into a digital signal, and sends it to the high-speed digital signal processor (DSP) for processing. It separates harmonics from the fundamental frequency and sends out driving pulses in the form of pulse width modulation (PWM) signals to the compensation current generating circuit. These pulses drive the IGBT or IPM power module, generating a compensation current with an amplitude equal to but opposite in phase to the grid harmonic current, injecting it into the grid to compensate or cancel out harmonic currents and actively eliminate power harmonics.

Based on the differences in the DC-side energy storage elements of the main circuit of the active power filter (APF), the types of APF can be categorized as: voltage-type APF and current-type APF.
The main circuit of the Active Power Filter (APF) employs an IGBT power converter, utilizing detection technology based on the instantaneous reactive power theory to automatically track changes in grid harmonics, offering high controllability and rapid responsiveness. It overcomes the drawbacks of traditional passive filters, such as poor filtering effects, prone to resonance, and limited to compensating fixed-order harmonics, providing excellent filtering and compensation for various rapid and transient shock loads.