Research On Stability And Parallel Control Of LCL-type Active Power Filter

With the increasing number of nonlinear loads and the increasing requirements for power quality,the problem of power grid harmonics has received more and more attention.Active power filter(APF)is an effective measure to control harmonic problems.This thesis takes three-phase three-wire LCL type APF as the research object,and research on the optimization of control strategies to improve system stability and modular APF parallel operation control strategy.Traditional quasi-PR controllers have phase lag characteristics.According to the open-loop frequency characteristics of the current loop,the problems existing in the parameter design process of the traditional quasi-PR controller are explored.The impedance analysis method is used to analyze the reasons for the instability of APF under the traditional quasi-PR control in the weak grid.To solve this problem,phase compensation is added to the traditional quasi-PR controller,the frequency characteristics of the controller before and after phase compensation is compared,the influence of each parameter of the quasi-PR controller with phase compensation on the control performance is studied,on this basis,the controller parameter design method is given,the quasi-PR controller with phase compensation can improve the stability of the system under weak grid is verified by simulation and experiment.This thesis analyzes the mechanism of the power grid voltage affecting the APF compensation current from the perspective of the closed-loop output admittance.The amplitude-frequency characteristics of the closed-loop output admittance with grid voltage proportional feedforward and without grid voltage feedforward are compared,and studies the high frequency interference amplification effect of proportional voltage feedforward.On the basis of studying the reasons why the traditional grid voltage full feedforward control strategy is prone to instability,a new implementation form of the grid voltage full feedforward is proposed to realize the grid voltage suppression of fundamental interference.The effectiveness of the full feedforward control strategy of grid voltage proposed in this thesis is verified by simulation and experiment.The stability problem caused by the resonance of the LCL filter is studied,the discrete domain mathematical model of the APF control system is derived,and the stability of the control system without damping is analyzed,the analysis results show that the method is less robust to the grid impedance.this thesis analyzes the stable conditions of the capacitive current proportional feedback active damping,the optimal damping coefficient is derived,the influence of the phase lag of the quasi-PR controller with phase compensation on the stability of the system is analyzed,through reasonable phase compensation,a strong robust control system that adapts to changes in grid impedance is constructed,the strong robustness of the capacitive current proportional feedback optimization method proposed in this thesis to the change of grid impedance is verified by simulation and experiment.In order to improve the redundancy and the reliability of the modular APF,the realization ideas,advantages and disadvantages of the four coordinated control strategies of centralized control mode,master-slave control mode,decentralized logic control mode and non-interconnected control mode are compared and analyzed.The operation characteristics of different capacity allocation schemes are analyzed,the characteristics of the truncated current limiting method and the capacity proportional current limiting method are studied.For these two methods,there are problems of secondary pollution and large amount of calculation,the cut-off proportional current limiting method is given,ensure that each APF module operates stably within its own capacity,the effectiveness of the current-sharing capacity allocation scheme,the cut-off current limiting method and the cut-off proportional current limiting method is verified by simulation and experiment.The thesis has 78 pictures,4 tables,and 83 references.