Active Power Filter（APF）is also parallel connected to the system. It can trace the load current wave forms and output the harmonic current for compensation by calculation. It introduces compensation current with a reverse phase to the power supply system by the control of IGBT. It can also provide current of leading or lagging phase to improve the power factor of the grid. APF can be equivalent to filter that gives a very low impedance path to the harmonics injected to the system. In this way the harmonics can be eliminated by short circuit to the harmonics. Benefits of harmonic control Secure the reliable operation of power supply and distribution system. After harmonic control, it can not only save the user’s energy consumption but also reduce the line loss of the grid. The reduction of the harmonic pollution may improve the quality of power system. Improve the reliability of electrical equipments and extend the equipment life. The harmonic control and reactive power compensation may avoid the over voltage and current of the devices caused by harmonics. The life of single phase devices may increases by 32%. The life of three phase device may increases by 20%. It is about 5% increased with the transformer. Improve the power factor and reduce the loss When the harmonic current is reduced, the original capacitor compensation device and other reactive power compensation device can be operated normally. APF can improve power factor and reduce the reactive current dramatically. About 6%~30% of the energy is saved by APF. Reduce the loss level and lower the transformer capacity rating. When the power factor increases and harmonic current goes down, it makes the average load current decline also. It is about 5%~10% capacity that can be saved by the utilization of APF. Improve production efficiency Effective control of the harmonics can save energy and reduces about CO2 emission by 3% which makes the power environment friendly. Application APF is widely used in Communications, Petrochemical and natural gas industries, Power industry, Metallurgical industry, Water treatment industry, Cement industry, Auto industry, Tobacco industry, Paper industry, Mine, Building industry etc.