Research On Topology And Control Strategy Of MMC And DC-DC Converter In New Power Electronic Transformer

As a new power equipment that integrates traditional transformers and power electronic technology,power electronic transformer(PET)has the characteristics of small footprint,highly controllability,higher efficiency and power factor,etc.Based on this,it is widely applied in smart grid,new energy power generation,electric traction and other fields.In order to improve the stability and realize circulating current suppression of the PET rectifier stage modular multilevel converter(MMC),as well as the soft switching and output voltage stability of the PET isolation stage DC-DC converter,this thesis takes the PET rectifier stage and isolation stage as the research objects and studies the control strategy of MMC converter as well as the topology and operation characteristics of dual active bridge(DAB)DC-DC converter.Firstly,the working principle of power electronic transformer,MMC sub-module as well as MMC is introduced and the mathematical model of MMC is established.Then the dynamic equations of MMC AC current and internal unbalanced current are shown.At the same time,several common modulation strategies are expounded.Secondly,in order to improve the stability of MMC converter and realize the circulating current suppression,the deadbeat current predictive control strategy which is based on AC current and internal unbalanced current dual-objective regulation is presented,the circulating current can be suppressed while improving the stability.In the meantime,the corresponding theoretical research is carried out and improvement ways are shown to deal with the trouble of time delay in practical application.Thirdly,in order to ensure the soft switching operation of the PET isolation stage,the topology of DAB converter is optimized,and a series resonance tank is introduced to form the dual active bridge series resonance converter(DABSRC).Then,the frequency domain model of DABSRC is shown and its operation characteristics are analyzed.According to the operation characteristics,reasonable values of relevant parameters are presented to realize soft switching operation.Meanwhile,in order to improve the stability of output voltage of the DABSRC,the small-signal model and closed-loop controller of the output voltage are established to improve the stability of the DABSRC.Finally,according to the theoretical analysis of this thesis,the MMC converter based on deadbeat control strategy,the DABSRC converter based on closed-loop control,and the MMC power electronic transformer are modeled in the Matlab/Simulink environment.The simulation analysis of PET rectifier stage,isolation stage and the whole system is completed.The simulation results show that the stability and dynamic response of MMC can be effectively enhanced,as well as the purpose of circulating current suppression and capacitance voltage balance can be achieved;the design method based on frequency domain analysis can ensure the soft switching of DABSRC under different operating conditions,and the closed-loop control can significantly improve the stability of the converter;PET can achieve the goals of isolation voltage and step-down,unit power factor with good operating performance.