A Converter Topology For Advanced Co-Phase Traction Power Supply System

Along with the social development needs, electrified railway has a growing proportion in the national economy,which is promoting the development of the economy, but the problem of harmonics, reactive power and negative sequence current brought by the public power grid is also becoming increasingly prominent. Advanced co-phase traction power supply system can completely cancel the electrical separation device to solve the power quality problems,which is a new type of intelligent traction power supply system. Along with the development of it, the demands of higher capacity and reliability of the power electronic converter is put forward. The cascaded converter can realize that input by the small power module and get the high voltage output, which helps the low voltage device be recommended to high voltage applications. The parallel converter can achieve power-synthesis, improve the output current level, the capacity and the reliability of the power supply system. In this thesis, cascaded-parallel hybrid converter utilized in advanced co-phase traction power supply system is studied, and feasibility of the system is also verified by theoretical analysis, simulation and experiment.On the basis of engineering practice, a cascaded-parallel hybrid converter structure and control system of that are designed for advanced co-phase traction power supply system. The emphasis of this paper is on the inverter part of the substation. The topology and working principle of single-phase three-level diode-clamped cascaded inverter is studied. The mathematic model of inverter is established, and on this basis double closed-loop control algorithm of single-phase cascaded inverter is studied.Combined with the research status of multi level modulation technology, this paper focuses on the multi-carrier SVPWM modulation strategy. The single phase three-level SVPWM (space vector pulse width modulation) modulation method is analysed and then the method of using CPS(Carrier Phase-shifted)technique to realize the cascade of multiple inverters is designed. Also, the harmonic characteristics of output voltage of cascaded converter is analyzed. This paper also analyzes the reasons and characteristics of the circulating-current of the inverter in parallel. The principle and implementation method of droop control is studied, which helps for design and implementation of parallel inverter system control strategy.On the basis of research mentioned above, by utilizing Matlab/Simulink simulation platform, single-phase inverter is modeled, simulated and verified, then, advanced co-phase traction power supply system substation based on cascaded- parallel hybrid converter is modeled. Simulation results of single-phase traction network side and harmonic characteristics of single-phase output voltage are analyzed when system is on the operating conditions of launch, loaded, variable loaded. Also, the characteristic of circulating current of parallel inverters in traction substation is analyzed. Simulation results indicate that the cascaded-parallel hybrid converter structure is workable. The output transformer of Substation can be canceled. Traction can meet the requirements of voltage and capacity.Meanwhile, the withstand voltage and power requirements of power electronic device is significantly reduced. The output voltage of substation can be controlled, and the steady state can be maintained in the normal operating range of the traction network voltage, the harmonic content is low. The system can achieve the power-sharing and small circumfluence, which means it has good reliability and redundant characteristic. The load fluctuation can be fast responsed, which means dynamic and static performance is good. Simulation results demonstrate correctness of theoretical analysis, feasibility and effectiveness of the cascaded-parallel converter system and its control and modulation strategy which are designed in this thesis.Finally, an FPGA based experimental low power platform is implemented. Experimental scheme, configuration and parameters of experimental system are also designed. With the experimental platform, experiments of single-phase cascaded converter and single-phase parallel converter are carried on, which further verifies the feasibility of the main circuit structure, control strategy and modulation strategy of this cascaded-parallel converter system.