Control strategy for hybrid LCC-C-MMC HVDC system under AC fault at rectifier side

This paper focus on the new hybrid HVDC system, whose rectifier side is traditional line commutated converter (LCC) and inverter side is made up of the series connected combined modular multilevel converter with clamp double sub-modules (C-MMCs). However, when the AC grid fault occurs at rectifier side, the DC voltage of LCC will drop; at this time, the DC voltage of C-MMC can not be decreased much since the modulation range is limited. Thus, the transmitted power may interrupt if the rectifier DC voltage is lower than the inverter side. To solve this problem, the paper proposed a new low DC voltage ride through control strategy. When the fault is very serious, which means the DC voltage will drop much, the converter unit at high voltage terminal is switched off to make the system operate under the half-voltage mode, then, the healthy converter unit is controlled to transmit power continuously. During the process, one arm is bypassed, and the other two arms are blocked. Finally, the PSCAD/EMTDC results show, the proposed control strategies achieve excellent operation characteristic under 10% and 50% ac voltage drop