Research On Control Technology Of LCL-Type Active Power Filter

Active damping and compensation current control are two key technologies of LCL-type active power filter(APF). To improve the stability and compensation accuracy of the APF system, these two issues must be solved well. This paper is focus on the active damping and compensation current control techniques for the LCL-type active power filter. The main research contents are summarized as follows:1、The operation principle of three-phase active power filter and the cause of the digital control delay are analyzed. Then the mathematical model is established according to the filter topology, which is the basis for the stability analysis of the system.2、The stability of LCL-type APF with different resonant frequency is studied under the control delay. In order to ensure the stability of the system, the capacitor voltage differential is used as the active damping. Based on this active damping method, the effects of damping delay on the resonant frequency and stability of the system are analyzed. It points out that the resonance frequency of the system can be changed when the damping delay exists. And when the damping is negative feedback, the open loop system can avoid the unstable poles, which is conducive to the stability of the system and reduce the design difficulty of the controller. Because of the derivative element can not be directly implemented in digital system, the lead-lag network is used to approximate the pure derivative element, and the frequency characteristics of the network in different discrete methods are compared.3、The stability of compound control which combined with repetitive control and PI control is researched. Based on the characteristics of 6k±1 harmonics generated by three-phase rectifier, the repetitive control kernel is improved, and the performance of the proposed system is analyzed comprehensively. Because of fractional delay in the improved repetitive kernel can not be implemented directly in the digital controller, the IIR digital filter is studied to approximate the desired characteristics of the fractional delay which helps to improve the performance of the system under fixed and variable grid frequency.4、System simulation model is built based on Matlab/Simulink platform. Based on the simulation result, it proves that the active damping designed of capacitor-voltage derivative can ensure the stability of the system, and the combined control system can effectively improve the compensation performance. Finally, on the basis of the original engineering prototype, the theory is verified by experiment, and the experimental results are consistent with the theoretical analysis.