Roth DC collection and transmission grids have enjoyed a lot of popularity lately. In order to successfully take advantage of the benefits offered by DC systems, reliable connection between two networks of different voltage levels has to be obtained. This paper presents high step-down ratio, isolated DC-DC converter intended to connect high/medium voltage grid with a low voltage DC grid. Proposed converter combines two Modular Multilevel Converters in series at high/medium voltage side with conventional Six-Step Converter at low voltage side, interfaced by means of Three Winding Three-Phase transformer. Basic operating principles, along with design and sizing rules, are presented and supported by simulation results.