摘 要 随着我国的电网规模迅速增大，大容量的远距离输电问题使得直流输电成为了电网的发展趋势。模块化多电平换流器（modular multilevel converter,MMC）作为一种基于全控型器件的新型多电平拓扑结构的电压源换流器，拥有输出的电压波形质量高、谐波的含量少、易于扩展等优点。在电网的并网互联、提高电能质量、远距离输电等方面，模块化多电平换流器高压直流输电（modular multilevel converter based high voltage direct current，MMC-HVDC）已经展现出极其广泛的工程应用前景，而直流线路的保护就成为了急需研究的问题。目前，国内外学者的研究侧重于MMC-HVDC系统的拓扑结构、数学模型、控制及调制方面的问题，而对其本身直流线路的故障特性研究较少。因此，本文将主要对MMC-HVDC系统直流输电线路上可能发生的短路故障和其保护策略进行研究分析，具体内容结构如下： 分析研究了MMC-HVDC系统的拓扑结构、运行原理、数学模型、控制策略及调制方式，并且利用PSCAD/EMTDC电磁暂态仿真软件搭建了MMC-HVDC系统的仿真模型，仿真结果验证了所建模型的正确性。 通过研究MMC-HVDC系统直流侧发生故障时的故障附加状态网络，分别分析了区内故障和区外故障，分析结果表明，当直流线路发生区内故障时，电流故障分量的极性相同；当直流线路发生区外故障时，电流故障分量的极性相反。当直流线路发生区内故障时，单极故障时电压故障分量极性相同，双极故障时电压故障分量极性相反。并利用二阶微分法提取了故障时电压、电流的故障分量，得出了保护策略，利用前文所建仿真模型验证了故障分析以及保护方法的正确性。 通过对MMC-HVDC直流线路串联小电感的方法，通过故障附加状态网络对直流线路可能发生的短路故障进行了分析，并得出了保护策略，利用前文所建仿真模型验证了故障分析以及保护方法的正确性。

ABSTRACT With the rapid expansion of China's power grid, the trend of DC transmission development occurs when there is a problem of long distance transmission with a large capacity. As a voltage source converter with a new multilevel topology which based on fully controlled devices, Modular multilevel converter (MMC) has the advantages of high voltage waveform, low harmonic content, easy to expand and so on. Besides, the modular multilevel converter based high voltage direct current system (MMC-HVDC) has shown a wide range of engineering applications on interconnecting in the network, improving the power quality and long distance transmission. Therefore, the protection of DC line has become an urgent problem for studying. At present, the research of scholars at home and abroad focuses on the problems of topology, mathematical model, control and modulation of the MMC-HVDC system, but less research on the fault characteristics of its own DC line. Therefore, this paper will mainly study and analyze the possible short circuit fault and its protection strategy on DC transmission line of the MMC-HVDC system, and the specific content structure is as follows: The topology, operation principle, mathematical model, control strategy and modulation mode of the MMC-HVDC system are introduced in this paper, and the simulation model of the MMC-HVDC system is built by using electromagnetic transient simulation software PSCAD/EMTDC. The correctness of the model was verified by the simulation results. By studying the fault additional state network of the DC side of the MMC-HVDC system, the internal faults and external faults are analyzed respectively. The analysis results show that the polarity of the current fault component is the same when an internal fault occurs in the DC line, and the polarity of the current fault component is opposite when an external fault occurs in the DC line. When an internal fault occurs in the DC line, the polarity of the voltage fault component is the same when the single-pole fault occurs, while the polarity of the voltage fault component is the opposite when the bipolar fault occurs. The two order differential method is used to extract the fault components of the voltage and current from the fault and the protection strategy is obtained. The simulation model is used to verify the correctness of the fault analysis and the protection method. By using the method of connecting the small inductance of the MMC-HVDC DC line, this study also analyzes the possible short circuit fault of the DC line through adding the fault additional state network and obtains the protection strategy. The simulation model as noted above confirms the correctness of the fault analysis and the protection method.