The UHF partial discharge measurement technique is under research in order to be used for the detection, identification and localization of discharging defects within a power transformer. However, it requires installed dielectric windows on the transformer tank for the placement of the UHF sensors, a feature not available in all transformers especially old ones that have a higher risk of failure. Therefore, it is necessary to develop UHF sensors that can be used in the case of transformers without dielectric windows. In this paper such a sensor in its early design stages is studied, which can be inserted in a transformer from one of its valves.@en

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