A Triple Active Bridge (TAB) Based Solid-State Transformer (SST) for DC Fast Charging Systems: Architecture and Control Strategy

The isolated dual-active-bridge (DAB) based solid-state-transformer (SST) system is one of the feasible solutions to enable the grid-tied large scale photovoltaic (PV) power plants and electric vehicles (EV) charge stations. In this work, a triple-active-bridge (TAB) based SST system is proposed to integrate the colocated battery energy storage system (BESS) and EV charging stations through the high frequency ac link in the TAB, while a cascaded H-bridge (CHB) converter is utilized to directly interface with the medium voltage (MV) ac grid. The proposed novel TAB-SST charging concept not only eliminates the bulky line frequency transformer, but also can reduce the instantaneous active power extracted from the grid with the integrated BESS. In addition, in this work, a comprehensive control strategy is proposed for CHB converter to mitigate the DC-link voltage imbalance among the cells in each phase and also the cells across different phases. Furthermore, an enhanced power flow management strategy is presented for the TAB converter to optimize the integration between EVs and BESS, which can lead to economic system operation while achieving much smoother power profile from the grid. Both simulation and experimental validations are presented to verify the effectiveness and feasibility of the proposed system architecture and control strategy.