Research On Control Strategy Of Power Electronic Traction Transformer For Electric Locomotive

Power frequency traction transformer is the most important component in the traction drive system of electric locomotive.Its large size,heavy weight and uncontrollable defects seriously restrict the development of electric locomotive in the future.Therefore,research and development to replace the power frequency traction transformer is of great significance to improve the transmission performance of electric locomotives.Power Electronic Transformer(PET),also known as solid state transformer(SST),is a new type of intelligent transformer that combines power electronic transformation technology with medium / high frequency transformers.It can realize voltage level conversion and energy transmission,and has the advantages of small size,light weight,electrical isolation,high power density,high reliability and other advantages.It is used in the traction drive system of electric locomotive,so it is also called Power Electronic Traction Transformer(PETT).PETT proposes that the power locomotive drive system is oriented towards high speed,high power,improving train comfort and reducing the overall vehicle Weight provides advantages.The power electronic traction transformer is composed of several power converter links,and the units are connected in series or parallel.Therefore,there are more main circuit alternatives.Based on the analysis of the typical main circuit topology scheme,this paper selects the PETT with input,serial,and output in parallel as the research object.The structure is divided into three stages(rectification stage,isolation stage,and inverter stage).This paper starts with the control methods of the two important links of the rectification cascade H-bridge rectifier and the isolation stage dual active full bridge DC-DC converter.The capacitor voltage balance control of the rectification stage,phase shift control of the isolation stage,power several key specific issues such as equilibrium have been studied in depth.First,a mathematical model is established based on the topology structure of a power electronics traction transformer cascade H-bridge rectifier.Detail of analyzed the modulation and typical control strategies,and the shortcomings of the modulation algorithm are summarized.Based on the analysis of the instantaneous power theory,a new control strategy for cascaded H-bridge rectifiers is proposed to achieve unit factor operation and improve current distortion.At the same time,a virtual moment of inertia is injected into the rectification stage,so that the converter can obtain the characteristics of a virtual synchronous machine,and further improve the dynamic performance of the system.Finally,the proposed new control strategy is compared with the traditional double closed-loop control strategy.Secondly,an in-depth analysis of the unbalance phenomenon of the DC capacitor voltage of the cascaded H-bridge of the rectification stage is performed.From the perspective of capacitor energy analysis,a method different from the traditional pulse-compensated DC capacitor voltage balance control method is proposed,which not only reduces the number of controllers used for the balance control algorithm,but also can quickly achieve the purpose of controlling the DC capacitor voltage uniformity.Finally,the proposed control strategy is compared with traditional pulse-compensated control.Next,the power-electronic traction transformer isolated dual active full-bridge DC-DC converter is researched,and the working mode of the dual active full-bridge DC-DC converter is deduced.Based on the phase shift control,the converter is established Transmission power model.Analyze single phase shift control and single side dual phase shift control,propose a single side double phase shift control strategy to optimize the return power,reduce the phenomenon of return power,and improve the loss caused by the traditional phase shift control.Finally,the proposed control strategy is compared with traditional phase-shift control.Finally,the power balancing principle of the isolated dual-active full-bridge DC-DC converter is analyzed,and the traditional power balancing control strategy is analyzed.A method to collect only the output currents of the dual-active full-bridge DC-DC converters is proposed.The balanced control strategy reduces complex coordinate transformations and mathematical operations.The overall simulation design of PETT was carried out in MATLAB / Simulink,which verified the correctness of the theoretical analysis of PETT in this paper.