7.2 kV Three-Port Single-Phase Single-Stage Modular Soft-Switching Solid-State Transformer with Active Power Decoupling and Reduced DC-Link

Abstract

This paper proposes a single-stage modular medium-voltage soft-switching solid-state transformer (M-S4T). The 7.2 kV 50 kVA M-S4T consists of five stacked 1.44 kV 10 kVA single-phase converter modules using 3.3 kV SiC MOSFETs and diodes in reverse blocking mode. The proposed converter has some unique features. First, the M-S4T is a medium-voltage stacked converter with reduced dc-link. Second, the M-S4T has a three-port structure (MVAC, LVAC, LVDC) for renewable, storage integration, or EV fast charging. Third, the M-S4T is a single-phase medium-voltage converter with active power decoupling, i.e. using a buffer port for 120 Hz power absorption to avoid large dc-link passives. According to the literature, though already applied to low-voltage converters for improved power density, the reduced dc-link and active power decoupling strategies have not been used in medium-voltage converters before. With high-frequency isolation, reduced dc-link, and a 120 Hz active power decoupling port, passive components in the MS4T can be minimized - which often dominate the size of medium-voltage converters. Also, the M-S4T is electrolytic capacitor less and high reliability is achieved - critical for utility applications like SST. Moreover, the 7.2 kV M-S4T has low EMI through low dv/dt, and high efficiency (~97% expected at full power) over the full load range with ZVS technique. Finally, the transformer of the M-S4T is designed to pass 55 kV basic-insulation level (BIL) and 60 kV high potential (Hipot) dielectrics withstand test - major challenges for SST. The topology, design, operating principle, controls, and simulation results are presented. These concepts are finally verified with experimental results at 2 kV peak of a single converter module and 1.6 kV peak of two stacked modules.