Multi-terminal flexible DC transmission is the primary means for transmitting renewable energy from deserts, gobi, and southeastern Tibetan regions. The online insertion and bypass of high- or low-voltage valves in modular multilevel converters (MMCs) is a unique issue for ultra-high voltage DC (UHVDC) systems, which does not exist in conventional HVDC systems. HVDC systems have only one valve per pole, so valve switching can be completed by simply blocking and disconnecting the corresponding sw