Nowadays, direct current (DC) technology is increasing its presence in alternating current (AC) power systems. This trend is enabled by the progress in energy conversion through semiconductor devices and power electronics. Therefore, it is possible to imagine DC power distribution networks in future energy systems. However, in order to enable this evolution, a highly efficient, reliable and compact conversion principle / device is needed which will be used for DC power transformation. Research of new conversion topologies has been increased recently through concepts of solid-state transformers (SSTs). The core stage of this topology is the medium frequency transformer (MFT), since it provides the galvanic isolation. So far, many designs have been proposed, followed by developed prototypes and using different core materials (silicon steel, nanocrystalline, ferrites) in combination with different types of conductors (Litz wire, foil, hollow) and insulation materials (air, oil, water, solid). One of the tasks of the thesis was to develop tools for MFT design and optimization which are experimentally verified on a laboratory scale prototype of the device. Thereby, the broad design space is explored together with interdependencies between magnetic and insulation materials used for the implementation of high-voltage high-power MFTs. At the same time, characterization tests are carried out in order to determine core, dielectric, conduction losses as well as thermal properties of such devices.