Research For A Ground's Auto-passing Neutral Section System Adopting The Cascaded H-bridge Multilevel Converter

Due to the perfect combination of energy saving, environmental protection, transport capacity, speed and efficiency, electric railway has been widely applied in high-speed railway and heavy-load freight transportation in China. However, there exist some neutral sections in the traction power supply system of electric railway with single-phase 25kV/50Hz voltage. When the electric locomotive runs on the line with loads, the neutral section may result in speed loss of train, the increase of energy consumption, trasient overvoltage between pantograph and train, the electric arc between the pantograph and catenary and so on. Even pantograph-catenary faults will happen in extreme condition. So it is necessary and significant to study the equipment and technology that can thoroughly solve the problems associated with the neutral section.Firstly, a ground's auto-passing neutral section system adopting the cascade H-bridge multilevel converter is proposed. Combined with the system topology and operation process of train through neutral section, the operation principle of the proposed system is discussed.Then, the control targets and the equivalent circuits of the auto-passing neutral section system at each stage are analyzed. Also, the adaptability of the proposed system is discussed according to the operation characteristics of high-speed EMU and freight locomotive. It is pointed out that the two core problems of the system are power control and phase adjustment. And modulation technique is the key to realize control target.For the H-bridge cascade multilevel converter, the operation modes of the H-bridge unit are analyzed in detail. The working principle of the nearest level modulation (NLM) is introduced. The equivalent characteristics and the maximum voltage resolution are analyzed emphatically. It is concluded that the maximum modulation error comes from the last module. Then, the effect of modulation error to the performance of the converter is analyzed.In order to overcome this shortcoming of NLM, an improved NLM is proposed. Namely,the function time of the last module is calculated based on the area equivalence principle to eliminate the modulation error. Lastly, the simulation results verify the effectiveness of the proposed algorithm.According to the working characteristics of the AC drive system and the necessary conditions for the harmless separation of the pantograph and the target wire, the direct power control is adopted in the proposed system. The parallel operation mode of the neutral section and the power supply arm under the locomotive traction and regeneration condition are analyzed, respectively. Based on the instantaneous power theory, the mathematical model of the single-phase grid connected inverter is described. And the power feed forward direct power control algorithm of the single-phase grid connected inverter is derived.Next, the effect of phase switching on the train traction drive system is analyzed in depth. Then, it is clear that the phase switching will certainly result in the power jump of the vehicle network system, and may lead to DC bias in the vehicle mounted traction transformer. The necessary condition for the elimination of DC bias is deducted. Then, in order to solve the problem of phase switching, phase shift at variable frequency,reconstructing voltage based on second order generalized integrator-phase locked loop and progressive phase are proposed. The corresponding implementation processes are introduced, respectively. Further, a comprehensive comparison among the three schemes is carried out.Finally, a simulation model based on Matlab/Simulink is established. The model is composed of the electric locomotive model, the contact net model and the cascade multilevel converter model. The effectiveness of the improved nearest level modulation is verified by simulation. The process of the train passing through the phase is simulated. And the feasibility of the proposed system is validated. Further, a hardware-in-the-loop experiment platform based on RT-LAB is built. The performance of the proposed system is confirmed by the experimental results.