Research On Control Strategy Of Modular Cascaded Power Electronic Transformer

Since the 21st century,the rapid development of power electronic technology and power electronic devices has provided the necessary prerequisites for the rise of power converters.As a new type of power converter,power electronic transformer combines power electronic technology with the principle of electromagnetic induction.While realizing voltage conversion,energy transfer and electrical isolation,it also has modularity and provides interfaces and harmonics for distributed power generation systems.Wave treatment,reactive power compensation and other functions,good power quality,high controllability,so it is widely used in smart grid,high-voltage direct current transmission,electric drive,etc.In some medium-and high-voltage large-capacity systems,in order to increase the voltage level,modular cascaded power electronic transformers are often used.This article takes the modular cascaded power electronic transformer with series input and parallel output as the research object.The structure consists of three parts: rectification,isolation and inverter.This article mainly starts from the first two parts of the modular cascaded power electronic transformer.In-depth research has been carried out on the capacitor voltage balance of the rectifier stage,the optimization of the return power of the isolation stage and the power balance.First of all,the problem of the capacitor voltage balance of the modular cascaded power electronic transformer rectifier stage-the cascaded H-bridge rectifier is studied.The topological structure of the cascaded H-bridge rectifier is introduced.The second-order generalized integration method and coordinate transformation are used to derive the DQ current decoupling control mathematical model in a two-phase synchronous rotating coordinate system.The reasons for the unbalanced capacitor voltage on the output DC side of the cascaded H-bridge rectifier are analyzed in detail,and the advantages and disadvantages of the two commonly used capacitor voltage balance control methods are compared and analyzed.Combined with the pulse compensation balance control,a DC side based on Kalman filter is proposed.Capacitor voltage balance control,while achieving the output DC side capacitor voltage balance,improves the steady-state performance of the system and has better robustness.Finally,compare the proposed capacitor voltage balance control strategy with the traditional capacitor voltage balance control strategy in MATLAB/Simulink.Secondly,research on the return power of the dual active bridge DC-DC converter in the isolation stage of modular cascaded power electronic transformer.Three phase shift control methods of dual active bridge DC-DC converters are introduced.According to the working principle of the dual active bridge DC-DC converter,the reason for the return power generation in the converter is analyzed,and the relationship between the transmission power,the return power and the phase shift angle is derived using the triple phase shift control as a representative.Combining genetic algorithm and nonlinear programming optimization,a triple phase shift control for the optimization of the return power of the dual active bridge DC-DC converter is proposed,which reduces the return power and improves the power factor of the transformer.Finally,the proposed control strategy is compared and simulated with the traditional single-phase-shift control and dual-phase-shift control strategies.Finally,the power balance problem of modular cascaded power electronic transformer isolation stage-cascaded dual active bridge DC-DC converter is studied.Analyzed the reasons for the unbalanced power of each unit module,introduced two traditional power balance control strategies,combined with fuzzy control,proposed a dual active bridge DC-DC converter output current sampling balance control strategy to reduce Complicated coordinate transformation and mathematical operations are implemented,and the proposed power balance control method is simulated and verified.Finally,the overall simulation design of the modular cascaded power electronic transformer is carried out to verify the correctness of the theoretical analysis of the modular cascaded power electronic transformer.