DC Capacitor Voltage Balance Control Strategy Of Three-phase Cascade Power Electronic Transformer

Power electronic transformer is a kind of complex converter system which uses power electronic devices and high-frequency transformers to realize power transformation through power electronic transformation technology.In this thesis,a three-phase power electronic transformer applied in distribution network is studied,and the following work is carried out to solve the dc voltage imbalance between clustered and individual of its H bridge modules of the cascade rectifier:Firstly,the research status of power electronic transformer is introduced.At the same time,the typical topology and control method of power electronic transformer are introduced.On this basis,the three-phase cascade converter and the equivalent circuit of three-level H-bridge single-module switch are established,the circuit equation expression is derived,its working principle is analyzed,and its operating characteristics under phase shift control and voltage equal-voltage modulation are derived.Then,this thesis focuses on the solution of the unbalanced dc voltage problem of three-phase cascade rectifier in three-phase phase and single-phase internal module.In order to solve the problem of phase unbalance,an independent voltage balance algorithm with constant total power is proposed.In order to solve the problem of inphase unbalance,the method of real-time level calculation and voltage balance modulation of inverting carrier cascade sort is proposed.Thirdly,on the basis of the study,based on MATLAB/Simulink to establish the three-phase cascade 36 modules of the simulation model of the converter,the input stage of simulation at system startup and the dynamic characteristics of load change process,and verified in this thesis.At the same time,the effectiveness of the cascade h-bridge single-module capacitor voltage balancing solution studied in this thesis is verified.Finally,On the basis of theoretical research and simulation verification,a lowpower experimental verification platform composed of three phase nine modules is constructed.The control program design based on FPGA chip is completed,and the dynamic and static performance of the system is given.The correctness and feasibility of the phase and phase equalization strategy of the three-phase cascade rectifier under various working conditions are verified by experiments respectively.