A Multilevel-DC-Link Converter With Single-Device Modules in Series for VSC-HVDC Application

Voltage-source-converter-based high-voltage dc transmission (VSC-HVdc) is widely applied in the fields such as offshore wind power transmission, and modular multilevel converter (MMC) is a common converter for it. The large capacitors and numerous devices in MMC limit its technical economics. In comparison, the multilevel dc-link converter with single-device-modules in series (abbreviated as MDC) proposed in this article may have advantages in terms of volume, as well as the potential in cost and efficiency. The single-device modules (SDM) solve the problems on both voltage sharing and floating supply. The current model of MDC is analyzed, and the commutating process in MDC is discussed in detail. The SDMs perfectly match with the characteristics of the quasi-soft switch in MDC, maintaining energy balance during operation without overvoltage or undervoltage. Finally, a high-power MDC prototype of 6.2 MVA is innovatively implemented and comprehensively tested to verify the theoretical commutation analysis and feasibility of the proposal. The research is helpful for promoting the application of MDC in VSC-HVdc, thus further improve the competitiveness of VSC-HVdc.