Since HVDC transmission system has run in China, there happened transfer failure events caused by endergonic arrester damage in parallel with metallic return transfer breaker (MRTB) many times, a study method reflecting action mechanism of DC breaker is needed urgently. Ningdong ±660 kV HVDC system electrical characteristics before and after the transfer failure are obtained by comparative analysis of transfer success waveform and transfer failure waveform. Meanwhile, detail simulation model of Ningdong HVDC system with actual parameters and control protection function is built in EMTDC software. Simulation results reappear transient phenomena of switch transfer and fault, and verify that single column with smaller impedance is damaged due to absorbing higher energy because of inconsistent characteristic changes of arrester with four columns generated by long-term running and operation. Finally, the reason of blocking HVDC system is found out, which is bug of time delay of metallic return ground protection, by analyzing event logs of transfer failure for inverter station. According to analysis conclusion, the improvement suggestion during ground return transfer metal return is proposed. And for the future HVDC projections, the suggestion of system design of arrester in parallel with MRTB is also proposed.%在我国高压直流输电投入实际运行以来，在大地回线转金属回线过程中，多次发生由于金属回线转换开关(MRTB)并联的吸能避雷器损坏而导致转换失败的事件，迫切需要精确反应开关动作机理的研究手段。通过对宁东直流输电系统现场大地回线转金属回线转换成功的与失败的录波波形对比分析，得出转换失败前后的系统电气特征。采用宁东直流实际的主回路设备参数和控制保护功能，建立起以直流电磁暂态程序(EMTDC)为基础的详细模型，再现了开关转换和故障过程的暂态现象。不仅验证避雷器由于四柱并联，长期运行操作使每个柱特性变化不一致，较小阻抗单柱承受较大的能量而损坏，而且，通过对转换失败之后逆变站金属回线接地保护动作闭锁直流系统事件进行分析，得出由于逆变站金属回线接地保护软件策略缺陷，导致保护动作闭锁直流系统。根据分析结果，对宁东直流大地转金属回线过程提出改进建议，同时对后续直流工程的MRTB并联避雷器的成套设计提出建议。
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