A New Control Scheme to Process Ripple Power Through Isolation Stage of the Three-stage Solid State Transformer

Three-stage solid state transformer (SST) consists of a multilevel AC-DC converter in stage-l followed by dual active bridge (DAB) isolated DC-DC converters in stage-2 and a three-phase inverter in stage-3. The output filters of stage-l are designed to filter both switching frequency based harmonics and second harmonic ripple generated due to single-phase power processing. As a result, the use of less reliable electrolytic capacitors in stage-l is inevitable. In this paper, a new control scheme is proposed for the SST to transfer the second harmonic ripple current from the output of stage-l to the output of stage-2 in each phase. When the three phases are connected at the output of stage-2, the second harmonic ripple currents which are phase shifted by $\frac{2 \pi}{3}$ radians add to zero. As a result, the capacitive filter requirement at the medium voltage DC (MVDC) bus is reduced, which is realized using reliable film capacitors. The operating principles along with detailed mathematical analysis and simulation results are presented.