Loss optimization for bottom IGBTs of half-bridge submodules in a high-power hybrid modular multilevel converter

•Compared with the conventional control methods, a new control freedom i.e., the independent control of HBSM and FBSM output voltage enabling the flexible regulation of the charging and discharging behaviors of HBSMs and FBSMs, is proposed.•The optimal charging and discharging behaviors of HBSMs which can reduce the loss of the bottom IGBT in HBSMs, are derived in detail. When the modulation index is 0.827 and a unity power factor, the total loss of T2 can be reduced by 8.9 percent.•For the unidirectional power flow requirements e.g., off-shore HVDC and on-shore HVDC where the renewable energy sources distribute in the Gobi Deserts, the proposed method can save the bottom diode（IGBT） of HBSMs for the inverter (rectifier), which can reduce the converter’ cost. Power losses on the upper and bottom IGBTs in half-bridge submodules (HBSMs) are not even when hybrid modular multilevel converters (MMCs) work as inverters, which results in a large difference in their lifetimes. Since the system reliability depends on the device with the lowest lifetime, the safe and reliable operation of the converter will be seriously threatened by the reliability of the devices. To address this issue, the power device loss distribution problem is studied, and the effect of forward voltage drops and on-state resistances on device conduction loss are analysed. Furthermore, a control method based on the targeted charging behaviours of HBSMs is proposed to decrease the loss of bottom IGBTs. The optimal charging behaviours of HBSMs are explored by increasing the HBSM charge, changing its average switching function, and decreasing the variation number of the inserted HBSMs, sequentially. Moreover, the natural balancing mechanism between HBSM and FBSM capacitor voltages is analysed, and the potential change in HBSM topology is explored. Simulation and experimental results demonstrate the feasibility and validity of the proposed method.