| Summary: | In order to ensure the high-efficient and stable operation of the MMC-HVDC transmission system, it is necessary to analyse the failure mode and failure mechanism of MMC-HVDC in detail. First, this article briefly introduced the MMC-HVDC system working principle. Then, we deeply analyse characteristic investigation under internal AC bus fault including single-phase ground fault and two-phase short circuit fault. It is proved theoretically that when asymmetrical fault occurs in AC bus, negative sequence components will appear in the system and cause double frequency fluctuations in the active power. Finally, a simulation model of modular multilevel converter DC transmission is built in the PSCAD/EMTDC electromagnetic transient simulation platform, which validates the correctness of the theoretical derivation, and provides a basis for the research of system protection strategy.
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