Solid-State Transformer (SST)-Based High-Frequency MV Propulsion Drive With New Modulation Technique

The solid-state transformer (SST) technology offers attractive features for high-frequency marine propulsion drives. Existing solutions based on Si-IGBTs face limitations due to high switching losses at high frequencies. Conversely, SiC-MOSFET-based SST solutions, while promising for these applications, currently pose high costs. This article proposes a cost-effective three-stage [low voltage (LV)/dc-dc/medium voltage (MV)] SST-fed MV propulsion drive with a new hybrid modulation technique. It cleverly combines both Si and SiC devices in the MV stage to achieve a balance of performance and cost. The MV stage uses a series connection of a silicon-insulated-gate bipolar transistor (Si-IGBT)-based cascaded H-bridge (CHB) inverter and an SiC-based two level voltage source inverter (2L-VSI). The CHB inverter is operated at the fundamental frequency of the motor, supporting the fundamental voltage component across all operating speeds. Meanwhile, SiC 2L-VSI operates at a higher frequency (40 kHz) to inject the required harmonic components that ensure sinusoidal motor currents (THD <5%). Through this approach, the drive achieves a higher effective switching frequency and a substantial 49.48% reduction in semiconductor losses at the MV stage. A pulsewidth modulation (PWM) rotation logic is also implemented to address unequal power sharing among the cells. The proposed technique is validated experimentally using a seven-level CHB-based SST-fed field-oriented controlled (FOC) induction motor drive prototype, demonstrating the practical feasibility and performance benefits of the approach.