One of the most critical faults affecting modular multilevel converter (MMC) based bipolar high-voltage direct-current (HVDC) transmission systems is the single-phase-to-ground (SPG) faults between the converter transformer and the valve. However, half-bridge (HB) and full-bridge (FB) based MMCs exhibit a different behavior following such a fault and, thus, converter protection should be addressed in a different manner for each configuration. For HB-MMCs, an SPG fault at the valve-side leads to a severe overvoltage on the submodule (SM) capacitors in the converter upper arms and to grid-side non-zero crossing currents. Although FB-MMCs only exhibit overvoltage, these are more severe than for their HB counterparts. To address this problem, this paper presents a protection strategy considering thyristor bypass branches placed in parallel with upper arms of FB-MMCs. By employing this configuration, the upper arm overvoltage in the faulted converter is mitigated and remote converters can be quickly blocked using their local protection schemes. For completeness, the effectiveness of the strategy is verified through time-domain simulations in PSCAD/ EMTDC. The studies in this paper demonstrate the effectiveness of the presented protection scheme for station internal faults occurring in FB-MMCs in bipolar HVDC systems.

中文翻译：

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在基于 FB-MMC 的双极 HVDC 系统中保护由换流阀侧单相接地故障引起的子模块过压

影响基于模块化多电平换流器 (MMC) 的双极高压直流 (HVDC) 传输系统的最严重故障之一是换流变压器和阀门之间的单相接地 (SPG) 故障。然而，基于半桥 (HB) 和全桥 (FB) 的 MMC 在发生此类故障后表现出不同的行为，因此，应对每种配置以不同方式解决转换器保护问题。对于 HB-MMC，阀侧的 SPG 故障会导致变流器上臂子模块 (SM) 电容器上的严重过压和电网侧非零交叉电流。尽管 FB-MMC 仅表现出过电压，但它们比 HB 对应物更严重。为了解决这个问题，本文提出了一种考虑与 FB-MMC 上臂并联放置的晶闸管旁路支路的保护策略。通过采用这种配置，故障转换器中的上臂过压得到缓解，并且远程转换器可以使用其本地保护方案快速闭锁。为完整起见，该策略的有效性通过 PSCAD/EMTDC 中的时域模拟得到验证。本文中的研究证明了所提出的保护方案对双极 HVDC 系统中 FB-MMC 中发生的站内部故障的有效性。该策略的有效性通过 PSCAD/EMTDC 中的时域模拟得到验证。本文中的研究证明了所提出的保护方案对双极 HVDC 系统中 FB-MMC 中发生的站内部故障的有效性。该策略的有效性通过 PSCAD/EMTDC 中的时域模拟得到验证。本文中的研究证明了所提出的保护方案对双极 HVDC 系统中 FB-MMC 中发生的站内部故障的有效性。