The Study Of Three-phase Four-wire Three-level Active Power Filter

APF has been widely used in harmonic control of power grid because of its advantages of high compensation bandwidth,strong anti-interference ability,strong bearing capacity and fast dynamic response.Among them,the three-phase four-wire topology has stronger ability to restrain the central line current than the three-phase three-wire topology.More and more enterprises recognized this as an opportunity to study the three-phase shunt active power filter in the laboratory.In this paper,the mathematical model of three-phase four-wire system is derived from three-level operation analysis and switch function model,on the basis of the modulation strategy,the filter inductor is designed according to the requirements of the system for harmonic compensation bandwidth,the DC side capacitor is designed according to the limitation of the system for DC side voltage fluctuation,compensation capacity,RC resistance capacitance circuit and overcurrent protection circuit are optimized according to the requirements of the system for experimental safety performance.The double loop control strategy is theoretically deduced,PI + repetitive control strategy is proposed according to the requirements of the system,the influence of dead zone setting on compensation performance is quantitatively analyzed and the dead zone suppression control strategy is given.The effectiveness of the strategy is verified by simulation.From the view of the influence of compensation current on the voltage fluctuation of DC bus,the conventional DC side grading ring is improved on APF harmonic compensation and response ability.The feedback is introduced into the grading ring regulation process,which makes the system response faster and improves the rapidity and steady-state accuracy of the system on the DC side upper and lower capacitor voltage balance.Build Matlab/Simulink simulation and experimental platforms,the harmonic compensation experiment and the optimization design experiment of grading ring with unbalanced load have achieved good results,which verified the feasibility APF research method.