Optimal design for hybrid MMC and its DC fault ride-through strategy

Modular multilevel converter (MMC) for bulk power overhead line transmission is a hot issue of flexible DC transmission study in China. Difficultly and costly to handle DC fault is the main challenge. This paper investigated a hybrid high voltage direct current (HVDC) system, in which the rectifier adopts line commutated converter (LCC) and the inverter adopts hybrid MMC composed of half bridge sub-module (HBSM) and full bridge sub-module (FBSM). On the premise of being capable of operating at low DC voltage with over modulation, clearing DC fault and successful voltage balancing of HBSMs, the required number of sub-modules and the more costly FBSMs were calculated. Moreover, an improved DC fault ride-through strategy was proposed which balanced the capacity voltage of FBSMs better and maintained the reactive power compensation during the DC fault period. Finally, the capability of DC fault ride-through, over modulation and the operation under low DC voltage with optimal sub-module count for the hybrid MMC were verified using PSCAD/EMTDC simulations.