Solid-State Transformer for EV Ultra-Fast Charging Stations with Series-Connected dc-buses for Multiple Charging Voltages

This paper proposes an ultra-fast charging station topology based on multilevel converters that allows obtaining three charging voltages (400 V, 800 V, and 1.2 kV) in order to manage demand from different types of electric vehicles. The proposed charging station is realized through a solid-state transformer configuration employing a double-star modular multilevel converter to achieve a filter-less and direct connection to the medium voltage grid. The three dc voltages are obtained through the series-connection of the low voltage isolated dc-buses within the solid-state transformer. In particular, the series connection improves the charging power distribution among the converter and mitigates the circulating power requirements to manage demand imbalances among the three dc-buses. The reduction in circulating power not only allows reducing losses on the converter when compared to existing topologies but also facilitates the use of lower-rated components. In addition, the proposed charging station exclusively integrates 400-V batteries as storage system, eliminating the need for additional dc-dc converters. The paper provides analytical expressions for circulating power, and numerical results from MATLAB-Simulink® to prove the effectiveness of the series-connection of dc-buses in reducing circulating currents.