Three-phase AC-DC solid-state transformer for low-voltage DC power distribution applications

A topology based on an interleaved modular multilevel converter is presented as a step-down solid-state transformer (SST) for low-voltage DC power distribution applications. Interleaving reduces the currents stresses through the legs of the modular multilevel converter, which is responsible for the voltage modulation across the submodules. The medium-frequency transformer provides galvanic isolation and operates at 10 kHz, allowing weight and volume reduction, while connecting the interleaved modular multilevel converter at the high voltage side to a full-bridge converter at the low voltage side. This topology operates as a dual-active bridge converter where the phase-shift angle controls the bidirectional power flow. The topology is suitable for a wide range of medium-voltage applications such as AC or DC distributed power systems, traction and renewable energy systems. The converter features, modulation technique and control scheme are presented. The topology validation is made through simulation considering a three-phase converter rated at 13.8 kV/60 Hz, four submodules whose switching frequency is 10 kHz, 11.27-kV/800-V, 10-kHz medium frequency transformer, output voltage of 800 Vdc and rated power of 100 kW.