A Multiport Solid-State Transformer by Modular Multilevel Converter and Quadruple Active-Bridge for Future Power Grid

With the increasing penetration of distributed energy sources, dc loads, and energy storage systems into the power grid, the demand for compact and highly efficient solid-state transformers (SSTs) is rapidly growing. This paper presents a modular multiport SST topology designed to deliver power to low-voltage ac (LVac) and low-voltage dc (LVdc) ports from medium-voltage ac (MVac) and medium-voltage dc (MVdc) ports. The proposed SST architecture employs a modular multilevel converter (MMC), a quadruple active-bridge (QAB) converter with a high-frequency transformer, and a two-level voltage source inverter. The MMC generates a stable dc-link voltage from MVac and MVdc inputs, while the QAB ensures galvanic isolation and voltage step-down to the LVdc level using fewer components. The final LVac output is achieved through an efficient two-level voltage source inverter stage. An advanced modulation technique is implemented to reduce total harmonic distortion (THD) and enhance the efficiency of the inverter. Simulation studies conducted in MATLAB/Simulink confirm that the proposed multiport SST maintains quality power across all ports and meets IEEE standards. Additionally, power losses and efficiency of the inverter are evaluated using the PLECS simulation platform. Results show a significant improvement, achieving 50.48% THD and 84.26% efficiency of the inverter at unity modulation index and 10 kHz operating frequency, outperforming conventional approaches. Experimental validation using a reduced-scale prototype further verifies the practical viability and effectiveness of the proposed SST system and modulation technique.