A Current-Slope-Based DC line Protection for MMC-MTDC Grid

The multi-terminal high voltage direct current (MTDC) grid based on modular multilevel converter (MMC) has been an important part of the power system, but due to its unique structure which is low-impedance, the fault current increases very fast and seriously threatens the safety of the equipment. So protection methods are needed to detect and discriminate the fault line, so as to ensure the reliability of the power system. This paper studies the fault characteristics of multi-terminal flexible DC transmission system under various fault scenarios, according to theoretical analysis, when an internal fault occurs, the fault current direction on both ends of the fault line is the same; however, the direction of the fault current on both ends of the healthy line is opposite. Then a current-slope-based fault line selection method can be proposed. By comparing the slope of fault current at both ends of the line, the fault line can be effectively identified, which can be used in the flexible DC transmission systems as a backup protection. Finally, the simulation model of Zhangbei four-terminal flexible DC grid is established by using PSCAD/EMTDC software, and the simulation is carried out to verify the effectiveness of the proposed protection principle.