Modular multilevel structure of a high power dual active bridge DC transformer with stepped two-level output

This paper addresses the issues of dc voltage matching and dc fault protection in potential super-grids. An approach for high power dc-dc conversion is proposed and analyzed. A front-to-front connection of modular-multilevel converters (MMC) forms a dual active bridge (DAB)-like structure. Near two-level operation is achieved by sequential switching of half-bridge chopper cells. This alleviates dv/dt stress exercised by two-level DAB configurations on the ac stage. The operating mode, switching patterns necessary are distinct from conventional MMC. Furthermore, common-mode currents, cell capacitance size, arm inductances are significantly reduced. The considered structure is shown to be useful for high power applications due to the low switching frequency, modularity, reliability and dc fault isolation capability. Various operation aspects are illustrated using simulations. A reduced-scale test rig provides an experimental proof of the concept.