A modular solid state transformer with a single-phase medium-frequency transformer

A number of topologies have been introduced for Solid State Transformer (SST); each has its own pros and cons. The three-stage topology, employing a multilevel converter as the first stage, is the most commonly adopted topology in the recent literature. In this paper, a three-stage SST topology is proposed which employs modular multilevel converters in both the first and second stages. The proposed topology: is easily scalable to higher voltage levels; requires only one medium-frequency transformer, simplifying the implementation and control; and provides high-voltage DC as well as high-voltage AC, low-voltage AC and low-voltage DC terminals.