A Modular Solid-State Transformer for 3 kV DC Grids: Design, Topology, and Performance Evaluation

The growing emergence of Distributed Energy Resources (DER) that operate natively in dc challenges the traditional logic of unidirectional power flow and the exclusive use of ac in distribution systems due to inefficiency and control problems. dc microgrids are seen as a promising alternative to address these challenges, and Solid-State Transformers (SSTs) could be a feasible solution to enable the development of such controllable hybrid dc/ac grids. This paper presents the design, modular topology, and experimental validation of a 150 kVA SiC-based SST. This SST follows a four-module input-parallel, output-series configuration, enabling the creation of a 3 kV MV dc microgrid from the LV ac electrical grid, allowing intrinsic voltage balancing among modules, straightforward scalability in both voltage and power, and easier maintenance. Each two-stage module, in turn, consists of an active front-end and an isolated dc/dc, which is composed of a dual full-bridge driving a series CLLC resonant tank at 88-90 kHz. Laboratory and field tests demonstrate continuous dc voltage regulation in a range from 1.2 kV to 3 kV, black-start capability and support of the ac grid through reactive power. The prototype achieves a power conversion efficiency of 96.4 % under nominal conditions while transferring 151.6 kW, with line current THD remaining below 1.8 %. The SST, along with the constructed MV dc microgrids, addresses the lack of real demonstrators that can validate the benefits of dc integration.