Research On Harmonic Detection Method And Optimal Control Strategy Of Active Power Filter

With the widespread use of power electronic devices,the adverse effects caused by harmonics are becoming more and more serious.Because of the advantages of fast response,high compensation accuracy and high intelligence,active power filter has become one of the most effective methods to control power grid harmonics and improve power quality.In this paper,parallel active power filter is studied.Firstly,the working principle of APF（Active Power Filter）is analyzed,its mathematical model in three-phase coordinate system is established,and the filter capacity parameters are calculated based on this.The influence of DC capacitor and filter on the stability of APF system is discussed,and the capacitance voltage value,capacitance and filter inductance parameters are analyzed and calculated.Secondly,the improved FBD（F.Buchholz,M.Dpenbrock,FBD）detection method is used to calculate harmonic current.By analyzing the basic principle of traditional FBD detection method and discussing the errors caused by phase-locked loop and low-pass filter,the error is reduced by adopting non-phase-locked loop and adaptive module instead of low-pass filter.The simulation models of traditional FBD and improved FBD are built in Matlab,and the phase-locked loop is analyzed in terms of three-phase symmetrical voltage,three-phase asymmetry and The THD values of calculated DC active conductance and fundamental positive sequence current are compared and analyzed.Thirdly,the space voltage vector control method is used to track the command current.By analyzing the basic principle of the space voltage vector method,the space voltage vector method is used as the control strategy of the current inner loop and a simulation model is built in Matlab.The change of the three-phase tracking current with the command current is discussed,and the load current before and after APF compensation is compared and analyzed.Finally,the repetitive PI compound control is adopted as its control strategy to control the DC voltage,and the basic principles of repetitive control and PI control theory are analyzed.The influence of the change of DC capacitor voltage on APF system and the stability of APF system by two control algorithms are discussed.Then,the simulation models of repetitive PI parallel control and single PI control are built in Matlab,and the DC voltage stability is compared and analyzed.Results show that the improved FBD harmonic detection method reduces the error of PLL（phase-locked loop）and low-pass filter when the three-phase power supply voltage is asymmetric or distorted,and improves the calculation accuracy.Based on the improved FBD harmonic current detection,and the control strategy of voltage outer loop control with repeated PI parallel connection and space voltage vector control to track harmonic current,the THD value of the compensated grid current is lower,which improves the ability of APF system to dynamically compensate harmonic current and improve power quality.