I. Introduction
Four-Quadrant front-end converters represent today's railway standard solution for getting high-quality overhead-line currents or fulfilling other severe specification of reactive power.[1], [2]. Regenerative breaking also belongs to the modern requirements in an energy-oriented strategy, and is intensively used, especially in European alpine countries or in transalpine traffic. In that special context of long distance links from Northern- to South-Europe, another difficulty has been in the center of interest of developing engineers in traction for years, which is given by the different electric systems adopted in the different countries. Between the 50 Hz-ac systems operated at 25kV in France, to 3 kV-dc overhead lines in Italy, the most used system is the 16 Hz-15kV line of the German, Austrian, and Swiss railways. For that purpose, so called multi-current locomotives have been used, avoiding the changing at frontier stations and saving considerable efforts and time [3] [4]– [6]. When the connection of a propulsion system, generally based on a variable frequency asynchronous motor technology, is easily adaptable via the dc-circuit to a dc feeding system, the ac front end converter must be interconnected to the HV ac-line via an additional transformer. That component reaches large and voluminous dimensions, particularly when it is designed for a low frequency of 16 Hz. Multilevel chain with energy-exchange with a common dc-link via medium-frequency dc–dc converters.