When metallic return line occurs transient fault, DC protection system will not request fault restart in existing protection logic in Xing-An HVDC Project, which reduces the reliability of DC system. This paper proposes a metallic return line longitudinal differential protection (87MLL) at SIMADYN D platform. This protection acts as the main protection for metallic return line earth fault. This paper uses EMTDC model to simulate earth fault at different power, different fault location, different resistance monopolar metallic return operation. The result shows that the protection has misstrip risk when fault resistance is large, the arc can’t extinguish during DC system restart when DC power is high. By comprehensively considering protection logic design scheme and EMTDC simulation result, we don’t suggest to add 87MLL in Xing-An HVDC Project.%兴安直流现有保护逻辑下，金属回线线路发生瞬时性接地故障后无故障重启逻辑，降低了直流系统的可靠性。提出了兴安直流SIMADYN D平台下增加金属回线纵差保护的程序修改方案，该保护作为直流金属回线线路接地故障的主保护，动作后果为启动直流线路故障重启逻辑。利用兴安直流EMTDC仿真模型，在单极金属运行方式下，开展不同功率水平、不同故障位置、不同过渡电阻的故障仿真。仿真结果显示该保护在过渡电阻较大情况下有拒动的风险，在大负荷条件下执行故障重启并不能有效地熄灭故障点电弧，故障重启失败后闭锁直流。综合考虑保护逻辑设计方案和EMTDC仿真结果，不建议兴安直流增加金属回线纵差保护。
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