The service life and operation safety of power transformers depend largely on the condition of a transformer insulation system. The transformer can be in service until its insulation system is able to withstand all kinds of service stresses. In this thesis, main transformer insulation (in a form of oil-barrier insulation system) is in focus. Due to a fact that well-proven and widely accepted breakdown theory in liquids has not yet been found, permissible el. stress values in transformer insulation are derived from experimental studies. One of the basic, and the most often applied, experimental results related to AC permissible stress in oil-barrier insulation are Kappeler’s results, published more than 50 years ago. Kappeler performed research in homogeneous el. field, with paper insulated electrodes, in degassed and gas saturated oil, for oil gap widths between 0,5 mm and 6 mm. Nowadays, results of Kappeler research are extrapolated up to 100 mm and more, despite the fact that validation of this extrapolation, according to the authors’ knowledge, is not published in the literature. The aim of this thesis is to present a concept for experimental research of dielectric stress in oil-barrier insulation system. In other words, Kappeler results are checked and extended to the larger oil gap widths (up to 30 mm). The insulation model concept with wrapped paper insulation is defined. In presented model geometry “early PD inception“ problem is not detected, which means the concept is suitable for further oil-barrier insulation research. Also, procedure for high voltage testing of insulation models is proposed as well as statistical analysis approach for experimental research with a small number of insulation models. High voltage testing procedure is based on test censoring (testing is stopped at predefined stress level without PD inception or breakdown registered). Furthermore, experimental results uncertainty is estimated by performing detailed analysis of related influencing factors (uncertainties of model geometry, high voltage testing procedure and voltage measurement). Finally, experimental verification is done by high voltage testing of two model prototypes. Obtained results confirm that proposed model concept is appropriate for experimental research of oil-barrier insulation permissible stress.