| For the deficiency of existing traction power supply system, the advanced co-phased traction power supply system is proposed, which can completely abolish neutral section devices and better solve power quality problems. Moreover, the advantages of this system become more prominent with the constant improvement of power grade of power electronic device. In this thesis, cascaded converter utilized in output-transformerless advanced co-phased 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 three-phase to single-phase cascaded converter structure and control system of that are designed for advanced co-phased traction power supply system. The topology and working principle of three-phase three-level diode-clamped PWM rectifier is studied. The mathematic model of rectifier in dq coordinate system is established and double closed-loop dq decoupling control algorithm is studied combining with mathematic model. 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 voltage closed-loop control algorithm of single-phase cascaded inverter is studied. Mechanism of production of the second order ripple in DC voltage of three-phase to single-phase converter is analyzed, and the second order filter circuit is designed to restrain the second order ripple in DC voltage.With the research status of multi-level modulation technique, multi-level SPWM modulation strategy is mainly studied. Carrier disposition SPWM (CD-SPWM) and carrier phase shifted SPWM (CPS-SPWM) are deeply analyzed. Output harmonic characteristics of converter with different modulation strategy are compared. After comparing and analyzing the advantages and disadvantages of each modulation strategy, considering the characteristic of three-level diode-clamped converter, a proper modulation strategy is designed specifically for three-phase to single-phase cascaded converter system. Then phase disposition SPWM (PD-SPWM) is adopted for single module three-phase to single-phase converter. CPS-SPWM is utilized for three-phase to single-phase cascaded modules.On the basis of research mentioned above, by utilizing Matlab/Simulink simulation platform, three-phase to single-phase PWM rectifier and single-phase cascaded PWM inverter are modeled, simulated and verified, respectively. Then, advanced co-phased traction power supply system Substation based on three-phase to single-phase cascaded converter is modeled, and simulation results of three-phase grid side, DC bus and single-phase traction network side are analyzed when system is on the operating conditions of launch, no-load, loaded. Also, harmonic characteristics of voltage and current of three-phase grid side and single-phase output voltage are analyzed. Simulation results indicate that the system can work stably on different operating conditions. When operating condition is changed, system will react quickly with small overshoot and short settling time. When system is with load, power factor of grid side is 1. Harmonic content of voltage and current of grid side is low. Output voltage waveform of single-phase traction network is of high quality. And good dynamic response is obtained. Simulation results demonstrate correctness of theoretical analysis, feasibility and effectiveness of three-phase to single-phase cascaded 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 three-phase to single-phase PWM rectifier, single-phase cascaded converter and three-phase to single-phase cascaded converter system is carried on. Experimental results prove that the cascaded converter designed in this thesis can expand capability of power supply system and reliability of main circuit configuration, control strategy and modulation strategy is further verified.In conclusion, on the basis of theoretical analysis, structure, control and modulation strategy of three-phase to single-phase cascaded converter are designed for advanced co-phased traction power supply system. Simulation and experiment results prove that advanced co-phased traction power supply system based on three-phase to single-phase cascaded converter can abolish output step-up transformer and decrease demand for withstand voltage of power electronic devices. Output voltage level can be multiplied, which can decrease switching loss as well as switching frequency; If the number of voltage level is enough, harmonic content can be quite low, then the filter circuit can be canceled. Meanwhile, the number of converter cascaded can be determined by engineering requirement, which make it convenient to enlarge capacity. The system has a good dynamic response, and it fits various operating conditions. |
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