On Converter Fault Tolerance in MMC-HVDC Systems: A Comprehensive Survey

Since they were first proposed, modular multilevel converters have been strongly studied in the literature. Due to their high component count, some concerns regarding their reliability arise, and several fault-tolerance schemes have been proposed in recent years. This article presents a comprehensive survey on the converter fault-tolerance strategies for modular multilevel converter (MMC) high-voltage direct current (HVdc) systems based on the available technical literature. Some MMC-HVdc solutions adopted in industry are reviewed. This work proposes the analytical expressions to determine the maximum number of submodule failures in the MMC-HVdc system. Thereafter, the limitations of the main converter fault-tolerance schemes under submodule failures are described and critically compared. A case study is conducted based on 1000-MVA Xiamen MMC-HVdc project with ±320-kV dc-link voltage, aiming to evaluate the effect of the faulty submodules on the reliability and power losses of the converter. Finally, the remaining challenges and opportunities in the converter fault tolerance in MMC-HVdc systems are stated as well as the future directions of this field.