Research On Control Strategy Of Modular Power Electronic Transformer

In recent decades,new energy sources such as wind power generation and solar power generation have developed rapidly,but traditional power transformers can not provide good grid-connected ports and output high-quality power for fast-developing new energy because of their own characteristics,so it is more and more difficult to adapt to the development of modern power systems.With the development of power electronics technology and the successful development of high voltage and high power switching devices,power electronic transformer has become a good choice to solve the above problems.In this paper,a three-stage power electronic transformer topology with modular multi-level converter structure is proposed,in which three-phase half-bridge modular multi-level converter is used for high-voltage stage and low-voltage stage,and single-phase H-bridge modular multi-level converter is used for intermediate stage.The structure of power electronic transformer can increase or decrease the number of sub-modules at all levels according to the actual needs,realize the flexible voltage rise and fall,and expand the application scope of power electronic transformer.Then,the working principle and mathematical model of three-stage structure of power electronic transformer are deeply analyzed.On this basis,a modular multi-level converter controller based on backstepping method is designed for high voltage stage.Then,two modulation methods of carrier phase-shifted PWM modulation and near-level approximation are introduced,which are internal sub-modules of modular multi-level converter.A capacitor-capacitor balance control method based on mapping strategy is proposed for capacitor-capacitor voltage control,and a hysteresis control strategy is proposed for the suppression of inter-phase circulations in modular multi-level converters.For the intermediate isolation stage of single-phase H-bridge modular multi-level converters proposed in this paper,a virtual orthogonal quantity is constructed,and a control strategy based on direct power control method is designed.Finally,a simulation model is built in MATLAB/Simulink,and the three-level model and its control strategy,sub-module capacitor voltage balance control and circulation suppression strategy are simulated and analyzed.The simulation results prove the effectiveness of the proposed strategy and results.At the same time,the main circuit parameters and some hardware and software of MMC PET are designed,which lays a foundation for the prototype.