Abstract：Flexible HVDC transmission systems offer significant advantages in solving the challenges of large-scale renewable energy integration and long-distance power transmission, indicating a major trajectory for the future transformation and growth of power systems. Compared to traditional HVDC systems, flexible HVDC systems are weakly damped, and their fault current rise rate is rapid. If faults are not cleared promptly, converter blockages may occur, demanding higher requirements for quick fault identification. Current engineering practices typically install current-limiting reactors at both ends of flexible HVDC lines. However, with the development of flexible DC grids, a single converter station may have multiple outgoing lines. In such cases, concentrating the current-limiting reactors at the converter station outlet can effectively reduce construction costs and improve economic efficiency. Existing traveling wave protection schemes for multi-terminal flexible HVDC grids typically use the blocking effect of current-limiting reactors on high-frequency components to construct protection criteria. However, when current-limiting reactors are concentrated at the converter station outlet, there are no obvious boundary elements between adjacent lines, significantly reducing the adaptability of traditional protection schemes in boundary-less grid structures. Therefore, new protection schemes that do not rely on boundary elements need to be further investigated. To address these issues, this paper first analyzes the transmission characteristics of fault current traveling waves and constructs a composite transient energy with both amplitude and directional features. To further clarify the variation patterns of the composite transient energy's amplitude and directional characteristics with wave transmission, the paper qualitatively analyzes the waveform characteristics of fault currents at both ends of the line for faults occurring at different locations. For internal faults, the composite transient energy at both ends of the line has the same direction, and the amplitude difference is the largest when the fault occurs at either the near end or remote end of the line, while it is the smallest when the fault occurs near the midpoint of the line. For external faults, the composite transient energy at both ends of the line has opposite directions, and the amplitude difference is minimal. Based on these conclusions, this paper introduces an adaptive restraint correction function into the traditional ratio differential protection criterion. For internal faults, without changing the operating quantity, the adaptive correction function can reduce the restraint quantity, and the greater the energy difference at both ends of the line, the lower the correction function value, effectively enhancing the sensitivity of the protection. For external faults, the proposed adaptive correction function can effectively increase the restraint quantity to ensure reliability. Finally, extensive PSCAD/EMTDC simulation experiments verify the effectiveness of the proposed adaptive differential protection scheme based on composite transient energy for multi-terminal flexible HVDC grids. The simulation results show that the proposed scheme significantly improves the sensitivity of protection for near-end or remote-end internal faults compared to traditional differential protection and ensures reliability during external faults. The scheme does not rely on line boundaries and does not require threshold setting, offering good engineering applicability and being less affected by transition resistance and noise interference.
郑涛, 陈云飞, 马英, 李紫肖. 基于复合暂态能量的多端柔性直流电网自适应差动保护[J]. 电工技术学报, 2025, 40(5): 1440-1454.
Zheng Tao, Chen Yunfei, Ma Ying, Li Zixiao. Differential Protection for Multi Terminal Flexible DC Power Grid Based on Composite Transient Energy. Transactions of China Electrotechnical Society, 2025, 40(5): 1440-1454.
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2025, Vol. 40 
