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With the growing fleet of a new generation electric vehicles (EVs), it is essential to develop an adequate high power charging infrastructure that can mimic conventional gasoline fuel stations. Therefore, much research attention must be focused on the development of off-board DC fast chargers which can quickly replenish the charge in an EV battery. However, use of the service transformer in the existing fast charging architecture adds to the system cost, size and complicates the installation process while directly connected to medium-voltage (MV) line. With continual improvements in power electronics and magnetics, solid state transformer (SST) technology can be adopted to enhance power density and efficiency of the system. This paper aims to review the current state of the art architectures and challenges of fast charging infrastructure using SST technology while directly connected to the MV line. Finally, this paper discusses technical considerations, challenges and introduces future research possibilities.

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The distinctive features such as fault-tolerability, modularity, power and voltage scalability have propelled modular multilevel converters (MMCs) as one of the emerging power converter topologies for marine applications. Pulse-width modulation (PWM) schemes are widely used to control the MMCs due to their good control capability and ability to operate at higher switching frequencies. However, the required number of carriers for gating signal generation drastically increases with the submodule count per arm in an MMC. Moreover, these carriers should be accurately synchronised with each other to generate high power quality waveforms. However, it is hard to establish accurate synchronisation between these carriers due to the sampling issues, memory limitations, and computational delays. In addition, the MMC demands reconfiguration of PWM carriers and control strategy for voltage scalability and retrofit applications. To avoid these issues, a simple PWM scheme using one carrier is proposed in this study. The main advantage of the proposed scheme is an easy extension to the n-level MMC with a wide range of output voltage levels. The effectiveness of the proposed PWM scheme along with the capacitor voltage balancing control is demonstrated through detailed simulations and verified experimentally under steady-state and dynamic conditions.