Cascaded Solid State Transformer Structure to Power Fast EV Charging Stations from Medium Voltage Transmission Lines

Electrical Vehicles (EV) are facing several issues, such as range anxiety, high charging times, and the development of related infrastructure. To minimize the charging time, a high amount of power is pumped into the battery resulting in fast charging. For this reason, the rating of commercial EV chargers installed in commercial charging stations is high (up to 400kW). These charging stations, when connected to the distribution side, possess serious issues. It may result in overloading the distribution system, and the immediate connection/disconnection of EV load may result in power quality issues on the distribution side. For these reasons, fast/superfast charging stations are usually installed on highways and fed from medium-voltage transmission lines. However, for connection, commonly, line frequency transformers are used, which are bulky and occupy significant space of the charging station. An improvement towards this is the utilization of Solid-State Transformer for integrating the transmission system to the fast charging system. Here, the line frequency transformer is replaced with a high-frequency transformer resulting in a lower footprint and compact charging system. In this study, an improved topology of Solid - State Transformer is suggested by implementing a high- frequency ac - link-based ac - ac converter and achieving a cascaded operation to connect this system to medium voltage transmission systems directly. Major benefits of this topology are (a) realization of fast charging, (b) high-frequency operation - the small size of components resulting in a compact system, (c) Soft switching operation is possible resulting in higher efficiency, and (d) isolated power supplies for feeding independent EVs.