The Delta-Configured Modular Multilevel Converter

The modular multilevel converter (MMC) has become an attractive option for dc/ac conversion in many power system applications, in particular, for HVDC transmission. Due to its parallel arrangement of phase legs, the MMC is known to be a wye-configured architecture. In this paper, the equivalent delta formulation of the conventional wye architecture is introduced. The delta-configured MMC, enabled by series-stacking phase legs, is an alternative realization to the wye-configured MMC and has significantly different operating characteristics. The introduction of the delta MMC enables greater flexibility in converter voltage/current scaling for a given set of submodules, and may be an appealing option for dc/ac applications requiring a power tapoff from a high-voltage dc rail. The single-ended and differential delta topologies are derived, with the latter seen as the preferred topology due to its reduced energy-storage requirements. The principle of operation of the differential delta-configured MMC is described in detail. A dynamic controller is developed that regulates grid power injections while suppressing unwanted circulating ac currents. Comprehensive simulation results validate the differential delta MMC operation and the proposed control scheme.