在基于模块化多电平变流器(Modular Multilevel Converter, MMC)的柔性直流输电系统中，MMC阀侧单相接地(Single Phase to Ground, SPG)故障会导致MMC桥臂子模块过电压损坏子模块等严重后果，影响系统可靠运行。因此，本文分析了传统混联型MMC柔直输电系统阀侧SPG故障特性，提出了一种基于晶闸管环混联型MMC拓扑，以保护阀侧单相接地故障下的柔直系统。在所提基于晶闸管环混联型MMC中，每个桥臂由全桥子模块和半桥子模块组成，晶闸管支路连接三相下桥臂电感形成环路。所提基于晶闸管环混联型MMC在发生SPG故障时，通过触发晶闸管环，闭锁子模块，可抑制下桥臂电流，箝位MMC阀侧交流电压，关断MMC交直流故障电流。所提基于晶闸管环混联型MMC具有过电压低、电流自关断时间短、全桥模块占比低、功耗小等优点。本文通过PSCAD/EMTDC仿真和实验验证了所提保护方法的有效性。

In high-voltage direct-current (HVDC) systems based on MMCs, SPG fault on the MMC valve side may lead to serious consequences and affect the reliability of the system. Therefore, this paper analyzes the characteristics of SPG faults on the valve side of traditional hybrid MMC based HVDC system, and proposes a hybrid MMC topology based on thyristor branch loop to protect system under SPG faults on the valve side. In the hybrid thyristor branch loop based MMC, each arm is composed of full bridge submodule and half bridge submodule, and the thyristor branch connects the three-phase lower arm inductors to form a loop. In the case of SPG fault, the hybrid MMC based on thyristor branch loop can suppress the lower arm current, clamp the AC voltage on the MMC valve side, and shut off the MMC fault current by touching the thyristor branch loop. The proposed hybrid MMC based on thyristor loop has the advantages of low overvoltage, short current self-shutdown time, small number of full bridge submodule and low power consumption. In this paper, the effectiveness of the proposed topology and control is verified by PSCAD/EMTDC simulation and experiment.