Continuous operation of LVDC source/load under DC faults in MMC-DC distribution systems

Abstract

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.