Along with the increasing seriousness of power system harmonic pollution, improving the harmonic well is the necessary condition for assuring normal operation of the electrical equipment as well as keeping production and life in normal operation. For the traditional passive power filter, its compensation characteristic depends upon the parameters of power grid and load, but these parameters are frequently changed. Therefore, it even causes resonance some times, bringing the resonance to be amplified. The passive power filter also has negative voltage regulation effect. All these factors restrain the application of the passive power filter (PPF). Compared the active power filter (APF) with passive power filter, the APF has the features as high level control and rapid response, and tracing and compensating each time of the harmonic, automatically generate reactive power for the required change. Its feature will not be influenced by the system, and free from the risk of harmonic amplification, and it has the advantages of small relative volume, light weight and etc. Along with the technology mature day by day, more and more attention has been paid and its application has been becoming wider and wider, so that the problems as harmonic pollution, low power factor and three phase load imbalance in the power system can be eliminated.
