随着现代社会经济的快速发展，城市对电能的需求迅猛上升。在城市配网中采用闭环运行方式相比于开环运行方式而言，具有供电可靠性更高，设备冗余度更低的特点。同时相比于交流系统，通过直流系统实现配网闭环具有短路电流和合环冲击电流更小等特点，因此采用直流电缆闭环系统的城市配电网优势明显，且柔性直流电缆系统控制灵活，能实现有功无功独立控制。超导电缆不仅具有功率密度高、损耗小、体积小、阻抗以及电磁辐射极低等优点，而且超导直流电缆系统具有输送同等功率电能时工作电压更低等特点，因而可以降低换流站的设备成本。因此超导电缆应用于配电网中时，与传统直流电缆系统相比将在换流站设备上具有一定的经济优势。受到电网实际需求推动，超导电缆应用技术在2010年后已呈现出从单机设备到应用系统的发展新趋势。本文针对城市配电网应用领域，分析了基于传统柔性直流电缆系统和基于超导柔性直流电缆系统的两个配电环网方案及参数。通过比较换流站和直流电缆的一次投资总费用可以得出，基于超导柔性直流电缆系统的配电环网方案的一次投资成本更低的结论。此外，本文建立了基于高温超导柔性直流电缆系统的配电环网方案PSCAD仿真模型，在对方案参数和换流站的控制策略进行讨论后，针对严重的馈线三相短路故障情况进行了仿真分析，并得出配电环网系统方案中的关键负载的电压恢复时间远短于依靠联络开关恢复供电的开环配网系统的结论。由于高温超导电缆冷却系统的可靠性是决定超导电缆安全运行的重要因素，本文针对一套1kA的高温超导直流电缆实验室系统展开热分析，通过理论计算对高温超导直流电缆系统中的电缆终端和电缆本体恒温器的漏热进行了分析和评估，并通过辐射公式和经验公式对于恒温器辐射漏热的计算结果进行对比。此外，本文还通过流场和温度场的耦合场有限元数值仿真计算了电流引线分别在空载和通入1kA电流情况下的漏热，得出高温超导电缆输电系统中的主要漏热集中于电流引线上，需要针对电流引线重点优化设计的结论。
With the economic development of the modern society, the demands for power supply of urban areas have rapidly increased. Compared to the open-loop mode, the application of closed-loop mode in urban distribution network has higher reliability and lower equipment redundancy. And the use of DC cable systems in closed-loop schemes has smaller short-circuit current and closed loop impulse current as compared with AC system. So the urban closed-loop distribution network with DC cable systems has obvious advantages, and the flexible DC systems are able to control the active and reactive power respectively. The superconducting cables have the advantages of high power density, low loss, little volume, low impedance and extremely low electromagnetic radiations, and the superconducting cable systems are able to operate under lower voltages transmitting the same power, which reduces the equipment cost of the converter stations. Therefore, applying superconducting cables to distribution networks have certain economic advantages over conventional cables on converter station equipment. Driven by the actual demand of the power systems, the applied technology of superconducting cables has presented a new trend of development from single unit equipment to application system after 2010.In this paper, two closed-loop distribution schemes, which are respectively based on a conventional DC cable system and a superconducting DC cable system, are proposed and analyzed according to the application areas of urban distribution networks. By comparing the primary investment cost of DC cables and converter stations, it can be concluded that the closed-loop distribution scheme by a superconducting DC cable system is more economic. In addition, the paper also establishes the PSCAD simulation model of the closed-loop distribution scheme by a superconducting DC cable system. After a few discussions on the scheme parameters and the control schemes of the converter stations, a transient simulation of the scheme after a serious three-phase fault occurred on the feeder line is conducted, and a conclusion is obtained that the voltage recovery time of the critical load in the closed-loop distribution system is much shorter than the open-loop distribution network system, which relies on the tie switch to restore the power supply. Since the reliability of the high temperature superconducting cable cooling system is an important factor to determine the safe operation of the superconducting cable, the paper conducts thermal analyses on a 1kA high temperature superconducting DC cable laboratory system. The heat leak of terminal and cable cryostats are assessed with theoretical analyses, and comparative analysis is carried out between the heat radiation results calculated by the radiation formula and the empirical formula respectively. Besides, the heat leak of the current lead with no load and with the current of 1kA is also analyzed by finite element numerical simulation of the flow field and temperature field. The paper draws the conclusion that the main heat leakage in the high temperature superconducting cable transmission system concentrates on the current lead, and the design of the current lead need to be optimized with emphasis.
