| The application of power electronic equipment is becoming more and more widespread due to the rapid development of power electronic techniques.While the rectifier brings convenience to industrial productions and daily life,it also absorbs reactive power and brings harmonic current pollution to the power grid.Therefore,it is very important to study rectifiers with unity power factor and low input current harmonics.Because the three-level rectifier has low input current harmonic content and small output voltage ripple,it is widely used in high-power applications.As a three-phase three-switch three-level rectifier,the VIENNA rectifier has the advantages of simple structure,high power density and low switching stresses.Therefore,the study of the VIENNA rectifier has important research significance.However,due to the limitation of the topology,the VIENNA rectifier has the issues of neutral-point voltage unbalance and low-frequency oscillation.The neutral-point voltage unbalance will cause the capacitor to withstand unbalanced voltage and affect the normal operation of the VIENNA rectifier.The low-frequency oscillation of the neutral-point voltage will increase the stresses on the DC side capacitor,reduce the life of the capacitor,and increase the output voltage ripple,which affects the quality of electricity.In this paper,a novel control strategy is proposed for these issues,which can solve the neutral-point voltage unbalance and the neutral-point voltage low-frequency oscillation issues synchronously.In this paper,the working principle of the VIENNA rectifier is analyzed firstly,and the small signal model of the VIENNA rectifier is established in different coordinate systems by using the state space averaging method.Then,the PI controller of voltage outer loop and current inner loop is designed.Secondly,two control methods based on SPWM modulation are presented,and a control method to achieve the neutral-point voltage balance is given.In this paper,three kinds of SVPWM modulation realization methods are presented,and the sector judgment and action time calculation in each realization method are derived in detail.The reasons for the unbalanced neutral-point voltage and low-frequency oscillation under SVPWM modulation are analyzed in this paper.A method of adjusting the action time of redundant small vectors by the dynamic adjustment factor is proposed in this paper,which can solve the neutral-point voltage unbalance and the neutral-point voltage low-frequency oscillation issues synchronously by making the amount of neutral-point charge generated by the basic vector in a switching cycle offset with the amount of neutral-point charge generated by the deviation of the upper and lower capacitor voltages.PSIM and MATLAB software are used for simulation,and the effectiveness of the control strategy is verified by simulation in this paper.Then the hardware and software designs of the VIENNA rectifier are carried out,and the experimental prototype is built in this paper.Finally,experiments are carried out,and the effectiveness of the control strategy is further verified. |
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