Hybrid MMC AC Side Smooth Start-Up Strategy

Modular multilevel converters are currently the mainstream topology in flexible HVDC projects. Compared with half-bridge MMC, hybrid MMC has DC fault ride through capability, and has economic cost advantages over full-bridge MMC and has broad application scenarios. At present, the main problem of hybrid MMC is that it is difficult to start smoothly, and it is difficult for the full-bridge sub-module and half-bridge sub-module to achieve capacitor voltage balance during the AC side start-up process, and the converter is easy to generate inrush current when unlocked, threatening the system safety. In order to solve the above problems, a three-stage AC start-up scheme is proposed, which can ensure that the half-bridge submodule and the full-bridge submodule can reach the self-absorbed energy voltage during the charging process, and the proposed dynamic grouping and sorting algorithm can improve the calculation efficiency of the traditional sorting algorithm and accelerate the capacitor-voltage balance process. The proposed method is verified by ADPSS, an electromagnetic transient simulation platform independently developed by China Electric Power Research Institute.