Negative-Sequence Current Control with a Hybrid Three-Level Modular Multilevel Converter

The new class of hybrid cascaded multilevel converters (HCMCs) brings numerous benefits to the field of voltage-source-converter high-voltage direct-current (VSC-HVDC) technology. HCMCs enable highly efficient and compact AC/DC power converter stations with a simpler physical implementation compared to the state-of-the-art modular multilevel converter (MMC) technology due to their tighter physical spacings of their semiconductor and capacitor components. A growing concern is whether HCMCs can provide negative-sequence current suppression control to AC grids during unbalanced operations, e.g., single-phase-to-ground fault scenarios. This paper demonstrates both normal and fault operation scenarios for an emerging HCMC converter, the hybrid three-level converter (H3LC), to verify its utility even when highly unbalanced grid conditions are incident. Throughout this paper, the H3LC is shown to reliably ride through highly unbalanced grid conditions while suppressing negative-sequence current.