中压直流配电网具有潮流可控、供电质量高、便于清洁电源接入等优势，是电力系统的重要发展方向之一。然而，直流故障的快速清除和故障后的系统快速恢复是限制其广泛工程应用的技术瓶颈。兼顾技术经济性，不采用直流断路器，项目提出具备故障自清除功能的新型模块化多电平换流器MMC，该新型MMC包括AC/DC和DC/DC两类。首先，分析两类新型换流器的交直流耦合特性并揭示其故障清除机理，以解决直流故障的快速清除难题。然后，分析新型换流器在故障清除阶段的暂态响应特性，研究新型故障定位方法，基于新型换流器注入“小扰动”激励法，研究永久性故障和瞬时性故障的预判方法，以解决故障清除后健全区和故障区的分阶段快速恢复难题。最后，搭建直流配电网动态模拟实验平台并评估新方案，对上述理论进行验证和反馈校正。项目可突破直流故障清除和故障后系统快速恢复的双重难题，为电网节省数以亿计的设备投资，推动直流配电网广泛工程应用。

Medium Voltage DC Distribution Grids based on Modular Multilevel Converter (MMC-MVDC) has obvious advantages of high power flow controllability, high quality of power supply, flexible access to distributed generations, quick response and so on, and it is regarded as one of the most promising solutions for smart grid and energy internet. However, fault current elimination and fast recovery method of MVDC grids after short-circuit faults at direct current (DC) side have been the technical bottlenecks constraining its wide industrial application. Considering technique and economic, instead of direct current circuit breakers, novel modular multi-level converters, including AC/DC and DC/DC, are proposed, and they have the DC fault clearance capability. First, by establishing mathematical model of MVDC grid based on novel MMCs, the AC and DC coupling characteristic of novel MMCs will be analyzed so that the fault clearance mechanism could be revealed. According to the theoretical analysis and fault clearance mechanism, control strategy and hardware parameters will be determined to ensure the fast DC fault clearance of MVDC grid. Then, based on transient response characteristics of novel MMC, a proper method to locate the fault point will be proposed to divide the healthy area and the faulty line; based on a “small disturbance” excited by novel MMC, a method to recognize the permanent fault and non-permanent fault will be studied. As a result, the healthy area and the faulty line could be rapidly recovered in steps. Finally, dynamic simulation experimental platform of MVDC grid will be built to verify and correct the above theory and key strategies. Considering technique and economic, the research will break through technical bottlenecks of fault current elimination and fast recovery after the fault in MVDC grid.