Solid-State Transformers: On the Origins and Evolution of Key Concepts

During the past two decades, solid-state transformers (SSTs) have evolved quickly and have been considered for replacing conventional low-frequency (LF) transformers in applications such as traction, where weight and volume savings and substantial efficiency improvements can be achieved, or in smart grids because of their controllability. As shown in this article, all main modern SST topologies realize the common key characteristics of these transformers-medium-frequency (MF) isolation stage, connection to medium voltage (MV), and controllability-by employing combinations of a very few key concepts, which have been described or patented as early as the 1960s. But still, key research challenges concerning protection, isolation, and reliability remain.