New single-stage bidirectional three-phase ac-dc solid-state transformer

High-power applications with low-voltage (LV) dc loads, for example, fast charging stations for electric vehicles (EVs), are typically supplied from the medium-voltage (MV) grid. Aiming for low volume, MVac-LVdc solid-state transformers (SSTs) that provide galvanic separation with medium-frequency transformers (MFTs) are thus considered, which are conventionally realized as two-stage systems that consist of an ac-dc power-factor-correction (PFC) rectifier and an isolated dc-dc converter. This letter extends a new single-stage isolated bidirectional PFC rectifier concept to MV levels, resulting in an SST that performs MVac-LVdc conversion in a single converter stage and, unlike many modular SST concepts, employs only a single MFT. The SST's primary side operates modular-multilevel-converter (MMC) bridge legs, whose input voltages contain large ac components, in the quasi-two-level mode to minimize the cell capacitors. The secondary side employs a standard LV three-phase rectifier, and a dual-active-bridge-(DAB)-like modulation strategy allows power flow regulation and ensures sinusoidal grid currents. The concept is explained and validated using detailed circuit simulations of an exemplary 1-MW system operating between a 10-kV three-phase grid and an 800-V dc output. The component stresses are evaluated, and an efficiency of 98.1% and a power density of up to 0.6 kW/dm3 are estimated. This letter extends a new isolated bidirectional three-phase buck-boost power-factor-correction rectifier concept to medium-voltage input, resulting in an solid-state transformer (SST) that performs MVac-LVdc conversion in a single converter stage and, unlike many modular SST concepts, employs only a single medium-frequency transformer. The SST's primary side operates modular-multilevel-converter bridge legs in the quasi-two-level mode to minimize the cell capacitors, while the secondary side employs a standard LV three-phase rectifier, and a dual-active-bridge-like modulation strategy allows power flow regulation and ensures sinusoidal grid currents.image