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

Abstract

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.

Keywords

• Half-bridge submodule
• Hybrid modular multilevel converter (MMC)
• Loss optimization
• HVDC
• Reliability