The wide application of nonlinear power equipment produces a lot of distorted current harmonics, which cause harmonic voltage when flowing in power grid. This harmonic pollution threatens the safety and stability of power system seriously. Together with electromagnetic interference, power factor reduction, are the three major public hazards for power system.

Active power filter(APF/AHF for short) is a perfect comprehensive solution to power quality problems with power grid such as harmonic wave(can filter 2nd to 50th harmonic interference completely), reactive power, and 3 phase load unbalance.APF is connected in parallel in power grid, to detect the harmonic wave in power grid in real time, generate the reverse-phase compensation current through the converter, and dynamically filter the harmonic wave in power grid. The operation of APF is unaffected by power grid structure and load type, and it will not produce harmonic oscillation with the system, thus perfectly realizing harmonic wave control of various loads. APF can also realize dynamic reactive compensation, and control the capacitor switching, to improve the power factor of power grid. Meanwhile, APF has the function of controlling the 3 phase load current unbalance, thus comprehensively solving various power quality problems with power grid.