Impacts of ultra-fast charging of electric vehicles on power grids: State-of-the-art technologies, case studies, and a proposed improvement using a solid-state transformer

The transportation sector is presently undergoing a gradual shift from the usage of conventional, fossil fueldriven vehicles towards electricity-powered electric vehicles (EVs). However, despite having numerous advantages over conventional vehicles, the wide-scale adoption of EVs has been inhibited by a few factors such as high cost and short driving range. Another factor is the long and slow charging time associated with EV batteries which may take up to several hours. One solution to overcome this obstacle is the use of ultra-fast charging (UFC) of EVs. UFC of EVs brings the charging time down significantly such that it becomes comparable with the refueling experience of conventional vehicles in gas stations. However, UFC comes with its share of negative impacts on the electric grid. This paper, therefore, presents a comprehensive technical review of the UFC of EVs, including the state-of-the-art EV charging standards, the existing architectures of EV charging stations, and the power converter topologies employed in EV charging stations. The impacts of UFC on the electric power grid are also discussed. Finally, a detailed case study, exploring the feasibility of installing three UFC stations in the city of Wollongong, NSW, Australia using real-life data is carried out via simulation in MATLAB/Simulink. The study is performed by quantifying power quality assessment metrics such as average voltage, average current, peak demand, current, and voltage Total Harmonic Distortion (THD) of the transformers powering the UFC stations. The simulation results conclude that the currents drawn by the UFC stations greatly exceed the rated values of the distribution transformers powering them. As a result, the existing grid infrastructure requires significant upgrading to facilitate the UFC of EVs with the conventional method. However, when the same study is repeated by replacing the conventional, bulky low-frequency transformers with the more compact solid-state transformer (SST), much better performance is obtained by limiting the drawn currents by the UFC stations within safe limits.