A Modular Multilevel HVDC Buck–Boost Converter Derived From Its Switched-Mode Counterpart

This paper begins by presenting a generalized methodology for conceptualizing modular multilevel converter (MMC)-based dc-dc topologies, which is predicated on the concept of harmonic power balance. A compelling implication is that MMC-based variants of conventional switched-mode converter structures can be realized. As an example case study, this paper introduces a new dc-dc MMC for HVdc applications, which is derived from the classical buck-boost dc-dc converter. This new topology, which is revealed to be an alternative option to the well-known dual active bridge (DAB) converter with an intermediate transformer, offers buck-boost functionality and bidirectional dc fault blocking, using only two quadrant switching cells. Comparative analysis shows the proposed topology has lower operating losses and a lower total magnetics rating in comparison to an MMC-based DAB solution for dc stepping ratios around unity. A dynamic controller is developed that regulates the converter dc power throughput while maintaining balanced capacitor voltages. The converter operating principle, dynamic controller performance, and dc fault blocking are verified by simulation.