Modular multilevel converter (MMC)-based high voltage direct current (HVDC) technique has made a great achievement in China, such that three- and five-terminal MMC HVDC systems have been constructed in NanAo and ZhouShan, respectively. Reduced DC voltage operation of HVDC systems under extreme weather conditions is a common practice used in most engineering applications, in particular for the applications using overhead transmission lines. In this paper, the control method of the HVDC systems based on hybrid MMC which is composed of half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs) under reduced DC voltage is illustrated. Because the system has the ability to continue operating with controllable active power and adjustable reactive power under extreme low DC voltage, the operating region of the system has also been analysed with the consideration of the maximum capacity of the connecting transformer at the AC side, the maximum current-through capability of IGBTs in the converter arms, the limit of the modulation ratio of the converter's output voltage and the capacitor voltage balance in SMs. Finally, a method aims to extend the operation region under a certain DC voltage by injecting circular current into the arms is presented and the feasibility of it is verified in PSCAD/EMTDC.