Research on AC line distance relay in the presence of modular multilevel converter based HVDC

In the AC/DC hybrid system consisting MMC-HVDC, it needs the distance relay zone 2 or stage III of higher AC line action to remove fault when the protection for lower level AC line which is directly connected with the MMC-HVDC cannot correctly remove fault. The changes of MMC-HVDC operation mode will result that the scope of higher line distance relay zone 2 is narrowed. It has a great probability to cause the miss operation of protection. At this point, it can only rely on distance relay zone 3 to operate which may lead to fault clearing time extended. This paper carries out research on this issue, deeply analyzes the effects of MMC-HVDC operation mode on AC line distance relay. An adaptive distance relay based on the measurement information on MMC-HVDC common connection points is developed. This adaptive distance relay can reduce the adverse impact on the AC line distance relay and ensure the correct operation of the protection device. In PSCAD/EMTDC, we establish an AC/DC hybrid system consisting MMC-HVDC to verify the proposed method and analysis.