Research On Optimization Control Method Of DC-Link Capacitor Voltage For Three-Phase Shunt Active Power Filter

With application of a large number of power electronic devices in industrial production and daily life,a large number of harmonic currents have been injected into power grid,and thus a power quality problem has become increasingly prominent.Active power filter(APF)has been considered as a main device to control harmonic currents because of its high performance in harmonic compensation,and it has become a research hot-point of scholars at home and abroad.In this paper,three-phase three-wire shunt APF is taken as research object,the harmonic detection method and tracking control method of harmonic compensation current of shunt APF(SAPF)are studied,and an optimal control method of DC-link capacitor voltage is discussed.Working principle of shunt APF is analyzed and its mathematical model is established in different coordinate systems.Against problems of complex coordinate transformation and long delay time in three-phase i_p-i_q harmonic currents detection method based on instantaneous reactive power,an improved i_p-i_q harmonic currents detection method is adopted,which simplifies coordinate transformation and replaces low-pass filter with average link,an average link of its detection method reduces the detection delay caused by low-pass filter and improves the detection accuracy when the load changes abruptly.Against problem of optimal design of the loop width and tracking speed in hysteresis comparative control method of harmonic compensation currents,PI decoupling current control method in d-q coordinate system and SVPWM modulation of voltage reference vector are adopted.The control method has high tracking speed and high tracking accuracy for harmonic compensation,and switching loss of power devices is small.Based on the conventional PI control and sliding mode control,a sliding mode PI compound control with improved reaching law function of capacitor voltage on DC-Link is proposed.The compound controller divides motion of controlled variables into two motion segments,and control law function of sliding surface is obtained by sliding mode controller,and thus implementing online tuning of parameters of the PI controller.By analyzing relationship between DC-Link capacitor voltage fluctuation and harmonic compensation performance and that between power device loss and grid voltage,a droop controller is used to implement linear change of DC-Link capacitor voltage reference value with grid voltage fluctuation,and thus implementing optimal control of DC-Link capacitor voltage reference value.Simulation verification and experiment verification are carried out by using MATLAB/Simulink simulation software and developed prototype.simulation results and experiment results show that the improved i_p-i_q current detection method using the average link instead of low-pass filter,has low detection delay and high accuracy.PI decoupling control and SVPWM modulation have higher tracking speed and higher precision than hysteresis current tracking control does.The proposed sliding mode PI control with improved reaching law function has faster dynamic response,smaller overshoot,stronger robustness,and lower total harmonic content of compensated source current than the conventional PI control of DC-Link capacitor voltage.