Offshore wind farms pose the challenge of transmission of produced energy in an economic and effective way over long distances, justifying investments in High Voltage Direct Current (HVDC) systems. A solution to surmount the need for an expensive and large power transformer station is to connect each wind turbine to a DC/DC converter in which a Medium Frequency Transformer (MFT) is utilized to reduce the size of the station. Further, the DC outputs of the converters of the wind turbines can be connected in series to create a high DC voltage. In this solution, the MFTs are required to be insulated against a high DC voltage to ground in a limited compartment. This contribution presents a prototype of 125 kV 50 kW 5 kHz oil-paper insulated MFT, which has been manufactured and subjected to verification HV tests. The dielectric design of the prototype MFT based on finite element solution of non-linear Maxwell-Wagner equations is introduced and the acceptance criteria and practical design considerations are discussed.