既有电气化铁路牵引供电技术，在实现高速和重载交通运输中，还存在以下较为突出的问题：机车过分相导致的机车运行速度损失、牵引动力不足和车网之间牵引供电设备可靠性降低；单相电力牵引负荷造成的电力系统负序、谐波等电能质量问题等。柔性交流牵引供电技术可完全取消牵引供电系统中的电分相环节，实现接触网全线贯通供电，并对既有牵引供电系统存在的电能质量问题进行良好的治理。针对现有柔性交流牵引供电系统的核心装置——静止功率转换器（SPC）成本高、接线复杂等问题，提出了一种基于四相模块化多电平变流器（MMC）和三相变压器的新型主回路拓扑方案，在五相MMC的基础上，通过使负载电流回流到输入侧，实现桥臂复用，从而减少一相结构，节省功率器件和电容成本，分析了其运行原理并给出相应控制策略，计算了该方案所需的功率器件成本，与传统的五相MMC方案相比，可节省约33%。基于PSCAD/EMTDC仿真平台与动模试验平台试验验证了该新型拓扑的有效性。分析了柔性交流牵引供电系统的系统级控制方案，并与传统的异相供电方式进行对比，论证了柔性交流牵引供电技术的经济性和应用价值。建立了SPC、接触网与机车的阻抗模型，并进一步建立了完整的系统构网模型，通过传递函数与稳定性判据分析了影响系统稳定性的主要因素，重点分析了SPC控制延时对稳定性与稳态误差的影响，提出了虚拟电容控制方法增大控制延时的稳定边界。提出基于北斗卫星同步和光纤高速通信的稳态协同控制策略，建立了该控制策略下系统的小信号模型，分析了稳态协同控制的动态响应速度，以及通信延时对控制稳定性的影响。与无通信的下垂控制进行了对比，证明其在稳态与暂态控制性能方面均具有明显优势。针对系统中SPC或牵引所故障退出等大信号扰动导致短时电流过载率过大的问题，提出了暂态协同控制策略和电压松弛控制方法，可有效减小电流过载率，节省功率器件成本。基于PSCAD/EMTDC仿真平台、硬件在环试验平台、动模试验平台与大兴机场线轨道交通专线进行了全方位的试验验证，证明了所提出控制策略的有效性。
The existing electrified railway traction power supply technology still has the following prominent problems in the application of high-speed and heavy-duty transportation: the loss of locomotive speed, insufficient traction power and reduced reliability of traction power supply equipment between trains and networks caused by neutral sections, and power quality problems such as negative sequence and harmonics in the power system caused by single-phase electric traction load. Flexible AC traction power supply technology can completely eliminate the neutral sections in the traction power supply system, achieve full line connection of the catenary network, and effectively manage the power quality problems in the existing traction power supply system.In order to solve the problems of high converter cost and complicated wiring in the existing flexible AC traction power supply system, a new converter topology scheme based on four-phase MMC and conventional three-phase transformer is proposed. On the basis of five-phase MMC, bridge arm multiplexing is realized by returning the load current to the input side, thereby reducing one phase and saving the cost of power devices and capacitors. The operating principle is analyzed and the control strategy is given. The power device cost of this solution is calculated, it saves about 33% compared with the traditional five-phase MMC solution.The system-level control scheme of the flexible AC traction power supply system is analyzed and compared with the traditional power supply method to demonstrate the economy and application value of the flexible AC traction power supply technology. In view of the problems such as control accuracy, response speed and stability faced by SPC constant voltage grid-forming operation, the impedance model of SPC, catenary and locomotive was established, and a complete system grid-forming network model was further established. The main influencing factors of system stability are analyzed through the transfer function and impedance ratio criteria, focusing on the impact of SPC control delay on stability and steady-state error, and a virtual capacitance control method is proposed to increase the stability boundary of the control delay.A steady-state cooperative control strategy based on Beidou satellite synchronization and optical fiber high-speed communication was proposed. A small-signal model of the system under this control strategy was established, and the dynamic response speed of steady-state cooperative control and the impact of communication delay on control stability were analyzed. Aiming at the problem that large signal disturbances such as SPC or traction station failure in the system cause excessive short-term overload currents, a transient cooperative control strategy and a voltage relaxation control method are proposed, which can effectively reduce the current overload rate and save the cost of power devices.Based on PSCAD/EMTDC simulation platform, hard-in-the-loop test platform, prototype test platform and Beijing Daxing International Airport Express, the full range of tests are carried out to verify the effectiveness of the proposed control strategy.
