Voltage, frequency and waveform are the three key indexes of the power quality. Along with the development for power electronic. There are used to the greatly non-linear load it makes the power-net seriously polluted with harmonic. Due to harmonic Voltage distortion, the electrical equipments should be damaged. They also makes influence on the power factor decreasing, wire loss increasing and neutral current seriously go beyond its index and brings a hidden danger on safety to supply line.

The passive power filter, active power filter installation and the continuous compensation of reactive power installation are the main equipments for raising the power factor and purifying the power network. These devices are used to mine, factory, medical, scientific research, oil, coalfields, port, metal, power, chemical, railway, car, residence, public facilities etc.