Research On Improved Repetitive Control Strategy Of Shunt APF

The control performance and efficiency of industrial equipments have been greatly improved due to the extensive use of power electronic components,but it also brings a lot of harmonic pollution to the power grid.The influence of harmonics on power quality should not be underestimated.Active power filter(APF),as a power electronic device,suppresses harmonics by generating compensation current to offset harmonics in the power grid.Compared with passive power filter,APF has the advantages of high compensation efficiency,fast response,low maintenance cost and dynamic compensation.Consequently APF is the most widely used harmonic compensation device at present.Firstly,this paper introduces the research status of APF from the aspects of the main circuit structure,harmonic current detection technology and current tracking control strategy.It focuses on the basic working principle of repetitive control,analyzes the improvement method of repetitive control,and determines the research goal of this paper.Based on the basic working principle of shunt APF,this paper establishes the mathematical models based on a-b-c and d-q coordinate systems,and designs the parameters of APF system.The design method of proportional integral controller is analyzed,and the reason why the excellent compensation effect cannot be achieved is pointed out.The traditional repetitive control is embedded into the equivalent controlled object,and the compensation accuracy is improved compared to the PI control method.However,the simulation results show that the repetitive controller cannot compensate specified harmonics,and the compensation effect will be affected when the grid frequency flutuates.In this paper,the filtering structure based on DFT is obtained by analyzing the Discrete Fourier Transform(DFT)algorithm,and an improved repetitive controller based on DFT is proposed.The specified harmonics can be compensated through the proposed control method,and the semi-delay control structure can further acceletate the dynamic response speed of the system.Aiming at the problem of frequency adaptability,two improved algorithms based on Lagrange fractional delay and Virtual Variable Sampling are discussed.Then,a DFT repetitive controller based on VVS is constructed.The theoretical and simulation results show that the controller not only has the selectivity of harmonic compensation,but also has the frequency adaptability.In this paper,the system model of shunt APF is built on the real time digital simulator platform.The main circuit parameters,signal conditioning and protection circuits are designed,and the system software control program based on DSP is compiled.The experimental results show that the DFT repetitive control strategy based on VVS,can realize the specified harmonic compensation and adapt to the grid frequency fluctuation.