A Three-Phase Cascaded Midpoint Clamped Multilevel Converter With Low DC-Link Capacitor

Multilevel converters can achieve high-voltage,high-power output and have many advantages in switching stress,harmonic quality and system efficiency.So multilevel converters are widely used in new energy power generation,high-power AC speed regulation and other fields.On the basis of summarizing the advantages and disadvantages of existing converters,this paper studies a three-phase cascaded midpoint clamped multilevel converter.The three-phase converter has the advantages of low voltage stress of switching devices and easy to realize modularity.In addition,compared with the traditional three-phase cascaded H-bridge multilevel converter,it can reduce the number of voltage sources on the DC side,so the volume of the converter is reduced.By analyzing the topology and working principle of the three-phase cascaded midpoint clamp multilevel converter,a hybrid modulation strategy of carrier stacking and carrier phase shift was adopted.On this basis,the midpoint potential pulsation problem of the DC side of the each module was analyzed,and the midpoint potential balance of each module unit of the converter was realized by monitoring the output current and the midpoint potential of each module and switching the favorable vector in real time.Based on the midpoint potential balance control strategy,the selection of DC-side capacitors for each module of the converter was theoretically analyzed.Subsequently,the uncontrolled rectifier circuit and the converter under study were combined into a frequency conversion system.By analyzing the distortion of the output voltage caused by the DC bus voltage fluctuations,a capacitor reduction strategy based on modulated wave reconstruction was studied.Based on this strategy,the high quality of output voltage can still be guaranteed when a small capacitor was used on the DC side.Then the realization method of the capacitance reduction strategy in the converter and selection of DC side capacitance was further analysis.Finally,the simulation verified the correctness of the theoretical analysis.Based on theoretical analysis and simulation verification,the experimental prototype of an uncontrolled rectifier circuit and three-phase cascaded midpoint clamp multilevelconverter was built.Through relevant experiments,the researched converter and related strategies were further verified.The experimental results proved the superiority of the studied converter and the feasibility of the research strategy...