Power Electronic Traction Transformer Second-order Harmonic Voltage Ripple Suppression And Startup Optimization Strategy

With the rapid development of high-speed railway trains,the bulky power frequency transformer has become one of the key factors restricting the further development of high-speed railway train traction performance and efficiency.The power electronic traction transformer(PETT),combining power electronic technology and electromagnetic coupling technology,has the advantages of light weight,multi-function,and high controllability,and is the future development direction of vehicle-mounted electric traction transformers.PETT performs AC/DC conversion through power electronic converters,realizes voltage conversion and power transmission through the medium frequency transformer,which can greatly reduce the size and weight of the transformer by increasing the operating frequency of transformers.With the further development of high-speed rail trains and power electronics technology,PETT is expected to become an alternative to the existing on-board traction substation system and will be one of the key technologies for high-speed railway trains in the future.This thesis takes the power electronic traction transformer as the research object,and studies the second-order harmonic voltage ripple suppression strategy and startup control strategy of PETT.The main contents of this thesis are as follows:Firstly,the topology of cascaded power electronic traction transformer is introduced.The topology and working principles of the cascaded PETT are analyzed in detail.The mathematical models of the PETT unit power module is derived and established.According to the mathematical model,the control strategy of the PETT unit power module is studied.These all lay a solid foundation for the subsequent analysis and research.Secondly,the principle that the single-phase rectification in the PETT generates the second-order harmonic voltage ripple on the high-voltage DC capacitor is analyzed in depth,and the expression of the second-order harmonic power and the second-order harmonic voltage ripple are derived.The topology structure,working principle and control method of the active filter device based on split capacitor are analyzed in detail.Aiming at the problem of the existing active filter structure based on split capacitor,an improved second-order harmonic voltage ripple suppression topology structure based on split capacitor,applying on PETT,is proposed,where one bridge arm of the DC/DC converter is utilized to take charge of the active filter control function,and corresponding filtering control strategy are proposed.In this thesis,the influence of the split capacitor parameters and capacitor voltage on the filtering performance and filtering characteristics is analyzed in detail,and the working state,wavefrom and characteristics of the DC/DC converter after adding the second-order harmonic voltage ripple suppression control are analyzed in detail,which proves the rationality of the proposed improved structure in principle.The improved second-order harmonic voltage ripple suppression topology structure based on split capacitor makes the number of components required to be added to the filtering device the least through device multiplexing.The secondary ripple voltage suppression control is decoupled from the rectification control,and has no influence on the rectification control and its improved control strategy.The operating frequency and control frequency of DC/DC converter are much larger than the frequency of the secondary ripple voltage,so taking into account the active filter control has a hardly effect on the normal operation and power transmission of the DC/DC conversion link.The simulation circuit is established using Matlab/Simulink simulation platform,and the feasibility and effectiveness of the proposed structure and control strategy were verified by simulation.Finally,the startup process of PETT was analyzed in depth,and the mechanisms of the inrush current generated by the AC/DC rectification link and the DC/DC conversion link during the startup process are studied in detail.And in view of the problems of the existing startup strategies,the starting optimization strategies are proposed.The constant current startup strategy is proposed for the startup process of the AC/DC rectification link,and the three-stage constant peak current startup strategy for the startup process of the DC/DC conversion link,and an overall startup strategy for the improved second-order harmonic voltage ripple suppression topology based on the split capacitor is proposed.The constant current startup strategy and the three-stage constant peak current startup strategy take the startup current as the control target to achieve complete control of the startup current during the startup process,which in mechanism suppresses the inrush current,and at the same time accelerates the startup speed and greatly shortens the startup time.The overall startup strategy for the improved topology of second-order harmonic voltage ripple suppression can achieve the split capacitor voltage balance during startup,suppress the inrush current,and shorten the startup time.The rationality and effectiveness of the proposed constant current startup strategy,the three-stage constant peak current startup strategy and the overall startup strategy are verified by simulation.