Advances in locomotive Power Electronic systems directly fed through AC lines

High-power converters with a medium frequency inductive coupling, commonly referred as Power Electronic Transformers, Medium Frequency Topologies, Solid-State Transformers (SST), or other similar acronyms, have been certainly one of the most important innovations expectations - for many years - in the field of electrical equipment of the railway rolling stock. The main purpose of these topologies is to replace the usual line-frequency on-board transformer on the AC powered rolling stock (25 kV - 50 Hz and 15 kV - 16 2/3 Hz), bulky, heavy and characterized by rather poor efficiency. Despite the extensive studies carried out until now, and the different experimental prototypes made, the realization of a system of this kind appears still difficult, mainly because of the high input voltage and the limited performance of the power semiconductor devices currently available. This paper, starting with the essential performance that a system of this kind must provide, presents a survey of the main solutions proposed until now, and introduces new possible addresses of research for new circuit topologies.