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Thermal management is one of the most significant challenges for reliability improvement of modular multilevel converters (MMCs). The thermal stress difference caused by the uneven loss distribution between top and bottom IGBTs/diodes will lead to lifetime difference of the IGBTs/diodes which will decrease the reliability of the MMC. In this paper, a thermal optimization strategy for MMCs is proposed, where the maximum thermal stress in the IGBT/diode of each SM can be effectively reduced through injecting optimum second-order harmonic current into the circulating current of MMCs, and accordingly the lifetime of MMCs can be improved by the proposed strategy. Detailed simulation results with PSCAD software and experimental results with 3kW MMC platform are provided to confirm the validity of the proposed thermal optimization strategy for MMCs. INDEX TERMSCirculating current injection, Modular multilevel converters, lifetime, thermal optimization.

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