建设“全国联网”的过程中，线路持续增加，多馈入直流输电技术在交直流电网互联、多个电源供电以及受电多落点等方面，展现出的优势尤为显著；混合直流输电技术运用在多馈入系统中可提高受端运行方式的灵活性、扩大输送容量，充分发挥直流输电的经济性和灵活性。目前在混合多馈入受端交流线路中，如何快速、可靠地识别系统故障成为了亟待解决的主要问题，针对这一问题，本文进行了深入的分析与研究，主要工作概括如下：

(1) 分析了换流器LCC (Line commutated converter)、VSC  (voltage source converter)和MMC  (modular multilevel converter)的拓扑结构，构建了相关的数学模型，制定了不同的控制策略，在PSCAD仿真平台搭建了混合直流多馈入输电系统，并验证了该模型的正确性。

(2)提出了一种基于消除直流馈入分量的电流差动保护新方法。该方法深入分析了采用分布参数模型的受端线路电流差动保护中故障分量判据和全电流判据的动作特性。为应对区内高阻拒动和灵敏度下降问题，提出了消除直流馈入分量和负载电流的电流差动保护改进判据。通过仿真验证，改进后的方法能够准确区分区内外故障，并具备高灵敏度、强耐过渡电阻能力以及不受分布电容影响等优势。

(3)提出了一种基于电压波形综合距离相似度的新型纵联保护方法。该方法通过提取测量电压和测量电流用于计算参考电压，并对测量电压和参考电压的波形进行拟合优化；随后运用欧式距离、动态时间扭曲算法和熵权法处理电压波形得到综合距离相似度，基于计算结果实现故障区域识别。仿真结果表明，该方法在准确区分区内外故障并实现可靠动作方面表现突出，具备可抵御高过渡电阻、耐受噪声干扰和不需严格对时等优势。

(4)将所提出的保护方法与当前已有的方法进行全面对比，凸显了本文所提保护方法的独特优势。此外，将本文所提出的差动保护新方法和电压波形相似度方法进行了详尽的对比分析，明确了它们各自适用的具体场景并构建整体保护方案。

In the process of building the "national network", the number of lines continues to increase, and the multi-feed DC transmission technology has shown particularly significant advantages in the interconnection of AC and DC power grids, multiple power supplies and multi-drop points. The application of hybrid HVDC technology in the multi-feed system can improve the flexibility of the receiving mode, expand the transmission capacity, and give full play to the economy and flexibility of HVDC. At present, how to identify line fault quickly and reliably in a hybrid multi-feed receiving AC lines has become the main problem to be solved urgently. In view of this problem, this paper has carried out in-depth analysis and research, and the main work is summarized as follows:

The mathematical models, topological structures, and control strategies of LCC  (Line Commutated Converter), VSC  (Voltage Source Converter), and MMC  (Modular Multilevel Converter) converters have been analyzed. A simulation model of a hybrid DC multi-infeed transmission system was built in PSCAD and the correctness of the model was verified.

(2) A new current differential protection method based on eliminating the DC-input component is proposed. The fault component criterion and the full current criterion in the current differential protection of the receiving line based on the distributed parameter model are analyzed in detail. To deal with the problems of high resistance rejection and decreased sensitivity of internal fault, an improved criterion of current differential protection is proposed to eliminate the DC input component and the load current. Simulation results demonstrate that the improved scheme can identify between internal and external faults and operate correctly, featuring high sensitivity, strong resistance to transition resistance, and immunity to the effects of distributed capacitance.

(3) A novel pilot protection method based on voltage waveform integrated distance similarity is proposed. By extracting measured voltages and currents for calculating reference voltages and optimizing voltage waveform fitting, the method then employs Euclidean DTW (Dynamic Time Warping) distance and entropy weight method to process voltage waveforms and obtain integrated distance similarity. Fault area identification is achieved using the calculated results. Simulation results indicate that the proposed method can distinguish between internal and external faults and operate correctly, featuring resistance to high transition resistance, tolerance to noise interference, and no strict requirement for synchronization.

(4) By comparing the two protection methods proposed in this paper with other methods, the advantages of the protection methods proposed in this paper are demonstrated. Additionally, a comparative analysis is conducted between the two proposed protection methods to distinguish their respective application scenarios and formulate an overall protection strategy.