High voltage direct current transmission is an effective way of energy transportation from offshore wind farms to the grid. One of the most economical solutions to realize it is based on a series connection of the wind turbines. This technology requires implementation of a DC/DC converter incorporating a special transformer operating at high DC voltages with superimposed high frequency components. To design the high voltage insulation of such a transformer, the dielectric properties of the materials constituting the insulation are to be known in a wide range of variations of the electric field and temperature. In the present study, results of the measurements of field and temperature dependencies of the electric conductivities of a typical high-quality mineral transformer oil and oil-impregnated pressboard are reported. The measurements were performed at different combinations of voltage-temperature stresses reflecting the insulation’s operation condition. The obtained data are used for performance evaluation of the insulation system of a prototype of the transformer.