Research On Modulation Of Three-phase Buck Rectifier With Low THDI And Low Voltage Stress

Three-phase Pulse Width Modulation(PWM)rectifier is widely used in order to suppress the harmonic of the input current of the converter because of the distortion of the input current caused by the traditional uncontrolled rectifier and phase-controlled rectifier.The essence of three-phase Buck PWM rectifier is Buck rectifier,which takes inductance as energy storage component.Compared with three-phase PWM Boost rectifier,its output voltage range is wider and its reliability is higher,and there is no inrush current at startup,it is more suitable for data center system,superconducting energy storage system,grid-connected power generation system and other applications.It is significant to study a three-phase Buck rectifier with low THDI,low voltage stress and high efficiency.Firstly,this paper introduces the definition of space current vector and the calculation of vector action time,and analyzes the advantages and disadvantages of two methods of sector division and three methods of vector distribution.Finally,Switching Loss Optimized(SLO)Space Vector Pulse Width Modulation(SVPWM)are selected as the Modulation strategy of the traditional three-phase Buck rectifier.The operating mode and input current of the traditional three-phase Buck rectifier are analyzed.Because of the common-mode voltage and parasitic capacitance,the input current contains low-order harmonic and high-frequency ripple.It is proved by simulation and experiment that the three-phase Buck rectifier with high power density and high frequency has two disadvantages,one is high THDI,the other is the high voltage stress of the switch.In order to reduce THDI,in Chapter 3,a three-phase Buck rectifier with low THDI is studied.A common-mode filter is added to the output side of the traditional three-phase Buck rectifier,so that the high-frequency ripple does not return to the input side,thus,the quality of input current can be improved.But the voltage stress of the switching device in this topology is the same as that of the traditional topology,which bears the input line voltage.In order to reduce the voltage stress of the MOSFET in the three-phase Buck Rectifier Bridge,in chapter 4,the three-phase Buck rectifier with low voltage stress is studied.The freewheeling diode of the conventional three-phase Buck rectifier is replaced by two diodes in series,and the common point of the two freewheeling diodes are connected to the common points of the input filter capacitor.In order to reduce the voltage stress,an improved SLO modulation strategy is proposed,which can ensure the voltage stress of each MOSFET to be zero or input phase voltage,therefore,the MOSFET bears the input phase voltage stress.However,this structure does not change the current path,so it has the disadvantage of poor input current quality.Combined with the topology of chapter 3 and Chapter 4,a three-phase Buck rectifier with low THDI and low voltage stress is proposed in chapter 5.The freewheeling diode of a conventional three phase Buck rectifier is replaced by two diodes in series and the output capacitance is replaced by two series capacitors,and connects the common point of the freewheeling diode,the common point of the output capacitor,and the common point of the input filter capacitor.In order to suppress the influence of low frequency fluctuation on input current,a small capacitor is connected in series between the output capacitor and the input capacitor.With the improved SLO modulation strategy,the voltage stress of the switch is lower than the peak voltage of the input phase and the quality of the input current is better.Finally,the simulation and experimental results show that the proposed topology has low THDI,low voltage stress and high efficiency.