Pole-to-pole dc short circuit fault blocking capability is a major concern when modular multilevel converters (MMCs) are applied in high voltage direct current (HVDC) transmission systems. Traditional MMCs based on half-bridge submodules (HBSMs) cannot block the fault current, thus needing expensive direct current circuit breakers (DCCBs). There are MMCs based on improved submodule topologies that can block dc faults, but usually with significantly increased device cost. This paper proposes an improved HBSM-based MMC (HB-MMC) which needs fewer extra semiconductor devices. The improved HB-MMC is more competitive than other schemes when it comes to investment cost and loss. An H-bridge circuit composed of two director switches and two diode valves is installed around one arm of the HB-MMC, which will not change the direction of current during the normal operation, whereas it can change the direction of the fault current when a pole-to-pole dc short circuits fault is detected. After two director switches have been turned off, a reverse-biased voltage provided by one arm can block the fault current. The topology can be further improved to provide reactive power during faults. Simulation results are presented to validate the features of the proposed improved HB-MMC.

中文翻译：

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基于半桥子模块和H桥电路的改进的具有直流故障阻断能力的模块化多电平变换器

当模块化多电平转换器 (MMC) 应用于高压直流 (HVDC) 传输系统时，极对极直流短路故障阻断能力是一个主要问题。基于半桥子模块 (HBSM) 的传统 MMC 无法阻断故障电流，因此需要昂贵的直流断路器 (DCCB)。有基于改进的子模块拓扑结构的 MMC 可以阻止直流故障，但通常会显着增加设备成本。本文提出了一种改进的基于 HBSM 的 MMC (HB-MMC)，它需要更少的额外半导体器件。在投资成本和损失方面，改进后的 HB-MMC 比其他方案更具竞争力。由两个导向开关和两个二极管阀组成的 H 桥电路安装在 HB-MMC 的一个臂周围，在正常运行时不会改变电流方向，而当检测到极间直流短路故障时，它可以改变故障电流的方向。在关闭两个导向器开关后，一个臂提供的反向偏置电压可以阻止故障电流。可以进一步改进拓扑以在故障期间提供无功功率。仿真结果用于验证所提出的改进 HB-MMC 的特性。