Control Of Three Level PWM Rectifier Under Unbalanced Power Grid

PWM rectifier is widely used in industrial,electric power and other occasions because of its characteristics such as less harmonic content on the AC side,adjustable voltage on the DC side,bi-directional energy flow and four-quadrant operation.In the locomotive traction,highvoltage transmission and other high-power applications,the two-level rectifier because of the capacity and voltage level restrictions can not meet the demand,so the three-level PWM rectifier can be used.Conventional control strategies,such as double closed loop control and direct power control,can make the three-level PWM rectifier work in the ideal state of unit power factor and sinusoidal AC current.The traditional virtual space vector modulation(VSVPWM)also has a good modulation effect on the midpoint potential fluctuation caused by the topology structure and modulation strategy,but the effect of conventional control and modulation strategy under unbalanced grid is not satisfactory.Therefore,it is of practical significance to study the control strategy and modulation strategy of three-level PWM rectifier under unbalanced power network.In this paper,the mathematical models of three-level PWM rectifier under balanced and unbalanced power grids are established respectively,and the power flow of the rectifier under unbalanced power grids is analyzed to prepare the foundation for the study of its control strategy.In the two-phase static coordinate system,the unit power factor of the rectifier and the sinusoidal current of the grid side were set as the control objectives,and the expected active power and the expected reactive power were obtained by eliminating the negative sequence current component and suppressing the active power fluctuation of the rectifier.The expected current is tracked without static error by using the compound control method of quasi-PR and repeated control.The control method is simulated by Matlab /Simulink,and the results show that the proposed method has a good control effect on three-level PWM rectifier under unbalanced power grid.In order to solve the problem of midpoint potential fluctuation of three-level PWM rectifier under unbalanced grid,the influence of each space vector on midpoint potential is analyzed,and the conclusion is drawn that the small vector and the medium vector affect midpoint potential fluctuation.The virtual small vector and virtual medium vector of traditional VSVPWM have no effect on the midpoint potential in a single period,which can control the midpoint potential of three-level PWM rectifier well in the balanced grid.However,the control effect becomes worse in the unbalanced grid,and the midpoint potential offset caused by the different capacitance value of DC side cannot be controlled.The improved VSVPWM in this paper reconstructs the virtual small vector and the virtual medium vector,and realizes the active control of the midpoint potential by introducing the duty cycle coefficient.The simulation verifies the correctness and stability of the improved VSVPWM.In this paper,an experimental platform of three-level PWM rectifier is built,and the control strategy and modulation strategy are verified based on the hardware-in-the-loop simulation platform d SPACE.The experimental results show that the control strategy and the modulation strategy are feasible and correct.The control strategy can achieve the control goals of sinusoidal current and unit power factor for the rectifier in unbalanced power grid.The two control sets can achieve no fluctuation of active power and symmetry of three-phase AC current respectively.The improved VSVPWM modulation strategy can effectively suppress the fluctuation of the midpoint potential at the DC side.