This paper describes the first results from experimental investigations which have been carried out to study the partial discharge phenomena in power transformer insulation. Using different types of modeled insulation defects and ageing tests it is aimed to organize a database of known defects, which will be used for recognition of discharges of unknown sources of defects. Moreover, the first results of the investigation of degradation processes of pressboard during ageing are described. The experiments were carried out using the classical IEC 60270 method and the UHF method. Phase resolved patterns of the defects were collected for statistical analysis. Different defects were discriminated using cluster analysis.@en