Investigation on Ageing Parameters of a Thermally Accelerated Aged Paper-Oil-Insulation in a Hermetically Sealed System

This study is conducted to investigate the influence of a hermetically sealed system and as a consequence thereof the effect of missing oxygen from the ambient air on the relevant ageing markers of three different insulating liquids, to determine the correlation between those ageing markers and the condition of the paper insulation. In this regard, paper samples are impregnated with a non-inhibited and two different inhibited insulating oils having different oil components as well as different inhibitor contents, which are currently used. Although different papers from the past have shown investigations on ageing of oilpaper systems [1]–[4] usually oil was used, which is no longer available today and furthermore no simultaneous comparative study was carried out in the past, as it is presented in this paper. For this study, a hermetically closed system is reproduced, so that a permanent contact with the ambient air is avoided. The entire study extends over a period of 15 weeks at a temperature of 130°C. Accordingly, the study is done for a long-term thermal stress to obtain paper insulations with a wide spectrum of ageing status. The same measurement was also done for an open system, which will be presented in another paper. Oil sampling is performed routinely and parameters like the moisture content, the amount of acid as well as the interfacial tension (IFT) etc. are determined. Furthermore, the DP-value of the paper samples, impregnated with the different oils, is also measured to get information about the actual paper condition. The investigation shows that the oil types influence the ageing process of the insulation system and consequently the production of respective parameters. Moreover, the correlation between the considered oil parameters and ageing status of paper is evaluated and it shows that an assessment of the actual paper condition is possible.