Exploiting buck–boost duality in dual active bridge modular multilevel converters to achieve high DC step ratios

Abstract

A previously unidentified duality between buck and boost configured dc-ac modular multilevel converters (MMCs) is firstly revealed. Armed with this insight, a new dual-active-bridge (DAB)-MMC is proposed for high-voltage dc (HVDC)-to-medium-voltage dc (MVDC) power conversion that utilizes cascaded buck and boost dc–ac stages to obtain high dc step ratios. Single-phase and three-phase variants are presented. When compared with the conventional DAB-MMC solution for the same dc step ratio, both the single-phase and three-phase topologies offer reduced MVDC side transformer winding current stresses, while the three-phase topology also yields reduced MVDC side MMC submodule current stresses. The former is achieved by having the freedom to design the transformer with a lower turns ratio, and the latter is achieved due to the inherent paralleling of submodules on the MVDC side of the converter. Analysis of the three-phase topology reveals its low-voltage side transformer winding current stresses can be reduced by a factor of 3.27. A generalized mathematical model of the proposed buck–boost DAB-MMC is derived and used to propose a dynamic controller for both the single-phase and three-phase topologies. Real-time simulations obtained from a real-time digital simulator system that incorporates an FPGA-based controller for the valve firing controls validate the proposed buck–boost DAB-MMC operation and dynamic controls.