Research On Control Strategy Of The Power Electronic Transformer In AC/DC Microgrid

Hybrid AC/ DC microgrid has the characteristics of both AC microgrid and DC microgrid,which can better meet the demand of AC load and DC load,reduce the link of power electronic transformation and improve the economy and reliability of the power supply system.In the current AC/DC hybrid microgrid topology,the hybrid microgrid with power electronic transformer as the interface can maximize the use of a variety of distributed energy and better achieve the centralized management of distributed power and load.Therefore,this paper proposes a hybrid AC/DC microgrid architecture based on MMC-PET,and the control strategies of MMC-PET under different operation modes of microgrid are studied.The specific contents are as follows:Firstly,the mathematical models of input stage,isolation stage and output stage of MMC-PET are established.Moreover,the carrier SPWM technique,the law of DC capacitor voltage fluctuation and internal circulating current of MMC are analyzed.In addition,the power transmission characteristics of DAB are studied.Secondly,aiming at hybrid AC/DC microgrid based on MMC-PET,this paper define four typical operation modes of hybrid AC/DC microgrid which are both grid-connected while interconnection,one grid interconnection,both off grid while interconnecting and both off grid while interconnecting.In each operation mode of hybrid AC/DC microgrid,the power balance relationships are analyzed.Moreover,the control strategies of MMC-PET in In each operation mode of hybrid AC/DC microgrid are studied.In grid-connected mode,the capacitor voltage equalization and current suppression of MMC and current sharing control of isolation stage are studied in detail.In order to verify the correctness of these strategies,the MMC-PET simulation model is established by using MATLAB/Simulink.And in off-grid mode,based on the droop characteristics of AC/DC microgrid,a control method isproposed when the output stage of MMC-PET is used as the hybrid microgrid interconnecting converter.Finally,the main circuit parameters of MMC-PET are designed.Furthermore,the effectiveness of the overall control strategy of MMC-PET in grid-connected and off-grid mode is verified by using MATLAB/Simulink.