With penetration and usage of renewable sources and loads based on power electronics increasing, the modular multi-level converter based direct current (MMC-DC) distribution system is getting broad attention. In MMC-DC system, DC/DC converters are employed to link the low voltage DC (LVDC) source/load and the medium-voltage DC (MVDC) distribution system. DC fault ride through capability at MVDC side is one of the technical difficulties, and has been the focus of previous studies. However, whether LVDC source/load could operate continuously under MVDC faults is still an open question. In this paper, key facilities, protection procedures and operating characteristics of LVDC source/load in two typical MMC-DC frameworks based on half bridge submodules (HB-MMC-DC) and full bridge submodules (FB-MMC-DC) are studied respectively. Analyses show LVDC source/load could operate continuously in HB-MMC-DC system with the help of DCCBs, but with voltage variation exceeding acceptable range, they fail to operate continuously in FB-MMC-DC systems. Therefore, a controllable storage unit is deployed at the LVDC side of the DC/DC converters to suppress voltage variation. The working principle and parameter design of the proposed solution are presented. Case studies are carried out in PSCAD/EMTDC for verification. Finally, comprehensive comparisons between the two MMC-DC systems are conducted.

随着基于电力电子设备的可再生资源和负载的渗透和使用的增加，基于模块化多电平转换器的直流（MMC-DC）配电系统受到了广泛关注。在MMC-DC系统中，DC / DC转换器用于链接低压DC（LVDC）源/负载和中压DC（MVDC）配电系统。MVDC侧的直流故障穿越能力是技术难题之一，并且一直是先前研究的重点。但是，LVDC源/负载在MVDC故障下是否可以连续运行仍然是一个悬而未决的问题。本文分别研究了两个典型的基于半桥子模块（HB-MMC-DC）和全桥子模块（FB-MMC-DC）的典型MMC-DC框架中LVDC源/负载的关键设施，保护过程和运行特性。分析表明，借助DCCB，LVDC源/负载可以在HB-MMC-DC系统中连续运行，但是当电压变化超出可接受范围时，它们就不能在FB-MMC-DC系统中连续运行。因此，可控制的存储单元被部署在DC / DC转换器的LVDC侧以抑制电压变化。介绍了所提出解决方案的工作原理和参数设计。案例研究在PSCAD / EMTDC中进行，以进行验证。最后，对两个MMC-DC系统进行了全面比较。在DC / DC转换器的LVDC侧部署了一个可控制的存储单元，以抑制电压变化。介绍了所提出解决方案的工作原理和参数设计。案例研究在PSCAD / EMTDC中进行，以进行验证。最后，对两个MMC-DC系统进行了全面比较。在DC / DC转换器的LVDC侧部署了一个可控制的存储单元，以抑制电压变化。介绍了所提出解决方案的工作原理和参数设计。案例研究在PSCAD / EMTDC中进行，以进行验证。最后，对两个MMC-DC系统进行了全面比较。