Operational Flexibility of the Modular Multilevel Converter under Unbalanced Grid Conditions

This paper has developed a static synchronous compensator (STATCOM) based on the open-DC modular multilevel converter (MMC) which has one more degree of freedom than the cascaded H-bridge converter to deal with the unbalanced grid conditions. In addition to the negative sequence current and the zero sequence voltage, the DC circulating current is also available in the open-DC MMC so that there are three control freedoms for addressing the unbalanced grid conditions. The corresponding control scheme is thus designed, and the laboratory test results are provided as well to validate the proposed control scheme.