Research On A Three-phase NPC Hybrid Cascaded Multilevel Converter And Control Strategy

Multilevel conversion technology has the advantages of using low-withstand voltage switching devices to achieve high-power and high-voltage level output.And it can improve output voltage current waveform quality and reduce output harmonics.In recent years,with the improvement of power output efficiency and the optimization of energy configuration,power conversion and transmission have shown a trend of large-scale,and multilevel conversion technology is increasingly sought.After analyzing and comparing the existing multilevel topology under the background of large-capacity high-voltage fan application,this paper introduces a new hybrid cascade rectifier composed of NPC full-bridge and NPC half-bridge structure,which can effectively reduce the number of cascaded modules and the modules are easy to expand.Firstly,this paper introduces the circuit structure and operation analysis of the three-phase NPC hybrid cascade converter in detail.Combined with the research status of modulation strategy of multilevel conversion technology,a modulation strategy combining carrier phase shift modulation and carrier cascade modulation based on carrier modulation is adopted according to the NPC converter substrate and cascade structure of the converter in this paper.Based on the above theoretical analysis,the equivalent switching model of each module is established.This paper clarifies the mechanism of small vector affecting midpoint potential,and the balance of midpoint potential of each module is achieved by changing the duration of redundant small vector action,which also lays a good foundation for subsequent voltage control.Secondly,this paper focuses on the voltage equalization control of the DC-side output voltage of the three-phase system composed of this topology under rectification application.As the number of modules per phase increases,wiring in traditional control and communication systems becomes more complex and the reliability and scalability of the system are limited.So this paper adopts a quasi-distributed in-phase voltage control scheme based on three-layer control for optimization.This method first applies the traditional double closed-loop control to the overall voltage control,and then adds the phase-to-phase control and equalization control module to correct the modulation ratio and fine-tune the output voltage,which reduces the processing amount of the main processor and the information interconnection line between the modules.And this paper realizes the modularity of the control scheme without increasing the complexity of the control algorithm.Finally,PSIM and MATLAB were used for simulation verification,and the design of software and hardware and the construction of low-power experimental platform were completed.The control strategy used is further verified through relevant experiments,and the simulation and experimental results show the feasibility of the topology and strategy used.