Research On Three-Level APF With Consideration Of Reactive Power Compensation

Harmonic and reactive current are the two factors that lead to the decrease of power quality in power grid.People have used several devices to improve the power quality.The most widely used power electronic devices include active power filter and static var generator.Several key technical of three-level active power filter including the switching device modulation strategy,harmonic and reactive current detection algorithm,and APF control strategy are analyzed in this thesis.Firstly,the working principle of APF is briefly introduced,and the working mode of three level APF main circuit and diode neutral point clamped inverter is analyzed in detail.The various steps of the SVPWM algorithm,which are widely used and have obvious advantages,are described,including the determination of the location of reference voltage vectors,the NTV synthesis method and the classical seven stage switch sequence design.The three level NPC inverter has the problem of unbalanced neutral point.In this paper,the effect of each switch state on the neutral point voltage balance is studied,and the fundamental reason for the neutral point voltage shift is explained.In order to solve the problem of neutral point voltage offset,virtual space vectors are constructed,and a neutral point voltage balance control strategy based on virtual small vector is proposed.In view of the shortcomings of traditional SVPWM,the vector coordinate is transformed into gh coordinates,which realizes fast region determination and action time determination,avoids trigonometric function calculation and simplifies the calculation process.The switch sequence is redesigned to effectively reduce the vector mutation during switch state switching.The simulation and experiment prove the feasibility and effectiveness of the proposed method to improve the virtual space vector modulation.At present,the commonly used harmonic and reactive power detection algorithms are based on the instantaneous reactive power theory detection algorithms,including the p-q,i_p-i_q and d-q method.In this paper,the difference and connection between them are analyzed,and the extraction of the reactive current components of the fundamental wave should be paid attention to in the case of reactive power compensation.The simulation results show that the d-q detection method has the best detection effect and can adapt to the voltage distortion of the power grid.Finally,the mathematical model of three level APF in abc coordinates and dq coordinates is derived,and the decoupling control method of current d axis component and q axis component in dq coordinate system is discussed.The controller of the voltage loop is designed,and the quasi PR controller and the PI controller are used together to achieve the precise compensation of the specified subharmonic.The voltage loop adopts the PI controller and realizes the control of the DC side voltage U_dc by feedback.Finally,the simulation analysis of the APF system has been carried out to verify the research results of this paper.