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Monopole Operation of On-Load Faulty Valves Replacement of Modular Multilevel Converters (MMCs) in Bipolar MVDC Grids

This paper demonstrates monopole operation capability of MMCs used in bipolar MVDC grids, which can enable on-load faulty valves isolation and replacement. With the dedicated control scheme and the operation procedure, the mode transitions between bipolar operation mode, monopole operation mode and hot-swap of faulty valves can be implemented in a seamless manner without shutting down the MMC. Consequently, the MMC can virtually operate without interruption and deliver at least 50% of the rated power capacity even during the maintenance phase. Hence, the availability of the connected MVDC grids can be significantly enhanced. Validity of the proposed work has been verified by time-domain simulations.