Research On The LCL Filter Of Active Power Filter System

In recent years, with the increasing demand for electricity, the grid structure has become increasingly complex, leading to increasingly prominent power quality problem. Hence, harmonic issue need comprehensive management and compensation. As an effective means of harmonic compensation, Active Power Filter(APF) has attracted extensive attention. In APF system, turn-on and turn-off of the power switching devices generate a lot of switching times and octave harmonics, and LCL type output filter can filter out high order harmonic well, therefore, the study of LCL filter is particularly important to improve the performance and promote the application of APF. This paper takes three-phase three-wire APF as the research object, and mainly focuses on the parameters design and damping strategy of the LCL filter.As to three-phase three-wire APF system with LCL filter, different mathematical models in different coordinate systems were set up. By analysis and comparison, the appropriate mathematical model was selected, providing theoretical basis for the design of the control system. The inverter output voltage harmonic characteristic based on space vector modulation(SVM) strategy and LCL filter working principle were analyzed. Then, combined with APF output characteristic, the constraint factors of filter design and the relations between parameters were given, and the restricted conditions and steps of LCL filter parameters design for ideal grid was set. Finally, considering the influence of the grid impedance, parameters design steps established was corrected. By designing parameters and simulation combined with instances, the validity of parameters design was validated.Because of the inherent resonant characteristics of LCL filter, this paper emphasize on damping strategy of the LCL filter. In view of the passive damping method, firstly, the mechanism of harmonic suppression was analyzed from the perspective of the transfer function. Secondly, the impact of damping resistance introduction on system performance was analyzed from the system stability margin, high frequency harmonic attenuation ability and power loss three aspects. For the active damping method, active damping methods based virtual resistor and notch filter were analyzed, and the transformation relationship between virtual resistance method and passive damping method and the internal relation between virtual resistor and active damping method based on notch filter was expounded, center frequency offset of notch filter during discrete process was resolved by designing notch filter parameters in discrete domain. Finally, combined with the simulation result, the effectiveness of various damping methods was validated.Using three-phase three-wire APF system with LCL filter experiment platform, the paper study was verified. The experimental results proved that the parameters design of LCL filter, passive damping method and active damping method were effective. LCL filter can effectively prevent the high frequency ripple current from entering the grid, and has good harmonic compensation performance, and improve power quality of grid.