| Judging from numerous aspects of the railway such as its development history, construction plan and transit characteristics, the electric railway will still be a significant system of transportation in China’s future sustainable development. The research on the relevant technology of the electric railway will be of great importance in promoting the railway’s development. As being the power source of the electric railway, the traction power supply system plays a vital role in the railway system, especially in the rapid expansion of the high-speed and heavy-haul railway. At present, the problems of crossing neutral section, power quality and overloading ability in the current traction power supply system are becoming more and more serious. The co-phase traction power supply system which was transformed from the current system is able to efficiently improve the power quality and cut down half numbers of the neutral sections. However, not all of the neutral sections can be canceled and the whole traction network can’t be linked up completely.With the progress of the power electronics technology, the advanced co-phase traction power supply system(ACTPSS) based on three-phase to single-phase converter can realize such connection in the traction network. This power supply system is approved to totally remove the neutral sections, promote the power quality both in the power grid and the traction grid, effectively dispatch the electric traction network flow, achieve the energy recycling when locomotive brakes and adopt new power sources. It is a new ideal traction power supply mode. Considering the requirements of the high voltage and large capacity of the traction network, the ACTPSS which is based on diode clamped five-level three-phase to single-phase converter and its control theory are mainly studied in this thesis. The research on the theoretical analysis, simulation and experimental verification for the traction substations based on parallel connection of such converters, which are the kernel facilities of ACTPSS, are conducted.Firstly, the development and problems of the traction power supply mode and power electronics technology are analysed and discussed in the thesis. The necessity of the relevant research on ACTPSS is also pointed out. Then the diode clamped 5-level three-phase to single-phase converter is exploxed in depth. The topological structure and mathematical model of the 5-level three-phase rectifier and the H-bridge single-phase inverter are deeply analysed. The control strategies of the rectifier and inverter and the circuit parameter for the three-phase to single-phase converter are designed. What’s more, the harmonic characteristic of the converter when adopting the 45°SVPWM modulation algorithm is analysed for the first time and the reason why the DC-side voltage got a 2nd ripple harmonic in the single-phase inverter is given out.Secondly, the voltage unbalanced principle of the DC-side capacitors in the diode clamped multi-level converting circuit is discussed in the thesis. The operating principle, circuit structure and control strategy of the capacitive balancing auxiliary circuit are discussed, too. The capacitor balanced circuit and the single-capacitor balancing auxiliary circuit and its simplified circuit are proposed. Their control strategies are designed and the effectiveness of the circuits’structure and control strategies is verified through simulation and experiments. There are not any changes to the main circuit and its control strategy when adding the balancing circuit and these added auxiliary circuits could balance the voltage at random modulation depth and variety of loads.Thirdly, according to the current capacity configuration of substations, the parallel voltage source converter based traction substation model are proposed in the thesis. The principle caused the circulating current in the converters paralleled system and the circulating-current’s components are analysed. An improved droop current-sharing control strategy is presented. Also the analysis results of the circulating current, the effectiveness and reliability of the improved droop current-sharing control strategy are verified by simulation. As an example of three substations structure of ACTPSS, a wireless master-slave automatic current-sharing grid-connected control strategy is proposed. The given active and reactive power can be calculated via the dynamic characteristics of the voltage amplitude and phase of the traction network in this strategy. The control function of auto-distributing active power and compensating reactive power is realized in multi-substations in accordance with impedance characteristic of loads and the traction network.At last, the experiments are completed in this thesis. A three-phase to single-phase converter with voltage balanced auxiliary circuit experimental system is designed. And an experimental system for parallel multi-converters and grid-connected multi-substastions is studied. The experimental results indicated the voltage balancing effect of the auxiliary circuit, and feasibility of such converter based ACTPSS in paralleling and grid-connecting.Given all that, the ACTPSS based on three-phase to single-phase converter can completely cancel the neutral sections of the traction network, effectively dispatch the electric traction network flow, reduce the substation capacity and perfectly solve the power quality problems both in the traction grid and three-phase power grid. It is an efficient, green, safe and soft controllable system of the new traction power supply, and possesses vast potential development for future. |
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