This paper describes a novel, model based, design optimisation methodology for high-power medium frequency transformers for medium-voltage high-power electronic applications, namely emerging solid state transformers. Presented procedure enables the designer to interactively select the most optimal design in a simple and intuitive way, while inherently offering flexibility in terms of various design alternatives, depending on the component availability. The core of the design algorithm is explained in detail together with all of the relevant modelling and technical challenges associated with realization of a prototype. A 100 kW, 10 kHz medium frequency transformer prototype has been designed according to the presented algorithm, practically realised and thoroughly tested in order to verify the theoretical design.