The master's thesis deals with and analyzes the theoretical fundamentals for the measurement of partial discharges in pressed paperboard, which is immersed and impregnated with mineral oil and loaded with the electric field. In the master's thesis we presented the theoretical bases of the types of partial discharges, their causes in the transformer, and gave the basical knowledge for their understanding. Further on, numerical calculation of several configurations using the Finite Element Calculation Program (FEMM), with which we simulated the phenomena in the oil channel between two conductors with different configurations. For the experimental part, we made a test device in order to test different materials with different oil-channel width configurations while varying the strength of the electric field. On the basis of the numerical calculation by the FEMM program, experimental experiments carried out and then compared the obtained results with theoretical ones. As conclusion, we have provided a solution that allows us to test partial discharges in any material. With this configuration, we made a comparison between a 4 mm thick TRBD (transformerboard) plate and a 4 mm thick plate, which consisted of two 2 mm plates glued together we internally glued together from two 2 mm plates. Based on this comparison we concluded, which TRBD plate has the greater dielectric strength and is more suitable for further use.