Research On Control Strategy Of Power Electronic Transformer Based On NPC

With the development of intelligent power system,the performance of traditional transformer can hardly meet the demands of smart power grid.The rapid development of power electronics technology has promoted the development of power electronic transformer(PET).At present,power electronic transformer has been widely applied in various smart grids.Compared with traditional transformer,its working performance is more superior.Power electronic transformer can not only realize power transfer and voltage transformation,but also has isolation,self-protection,power grid interconnection and other functions,can achieve continuous adjustment of output,can solve many new problems in smart grid.Cascaded multilevel power electronic converter is the main power electronic transformer at present.It has many advantages in performance,such as high power density,high withstand voltage and high modularity.In order to improve the working performance of NPC power electronic transformer,the control strategy of NPC power electronic transformer composed of input,isolation and output is deeply analyzed in this paper.This paper firstly introduces the development process and research status of NPCS power electronic transformers,analyzes their topological structure and working principle in detail,and introduces the main development direction of current power electronic transformers.The mathematical model of power electronic transformer is analyzed,and the control strategy and characteristics of the corresponding module are analyzed.The difference between traditional transformer and power electronic transformer is analyzed.Secondly,by establishing the mathematical model of the cascade rectifier structure,the mathematicalexpression of the phase shift Angle between the cascade modules is given to determine the appropriate phase shift Angle of the cascade modules.In this paper,the two modulation modes of POD-SPWM and cps-spwm in the input stage are analyzed in detail.Then,the working principle of the isolated level DC/DC converter and the output level three-phase four-bridge arm inverter is introduced in detail,the working characteristics and power transmission characteristics of the dual-active bridge structure DC/DC converter in the isolated level are analyzed,and a voltage balance control strategy of the isolated level based on power feedback is proposed.A control strategy combining positive and negative sequence components of three-phase inverter and the fourth bridge arm is proposed.Finally,this paper builds the simulation model of NPC power electronic transformer in MATLAB/Simulink and verifies the control strategies of each module.The results show that the proposed control strategy achieves the control goal.