Investigation on the Accelerated Ageing Behaviour of Oil-Paper-Insulation Using Different Insulating Oils

Inhalt:
The main goal of this research is the comparison of different actually used insulating liquids regarding their ageing be-haviour through which the pertinent ageing markers could be identified to estimate reliably the remaining lifetime of an oil-paper insulated transformer. Although different authors have investigated the ageing of oil-paper systems, usu-ally 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. Currently, the method of measuring the degree of polymerization (DP-value) is the most accurate technique to determine the condition of the paper insulation, thus with a low DP the paper gets fragile and brittle. However, for this measurement a paper sample is needed, for which the transformer has to be de-energized and opened with consecutive very high costs. Therefore, in order to reach an optimal lifetime of the transformer as well as giving recommendations about their future life expectation, it is necessary to observe the degradation of the cellulose during the operation time. Usually, for the determination of the condition of the paper insulation, oil samples are taken to measure different ageing markers, due to the fact that oil probes can be easily taken. Accordingly, in this paper an inves-tigation is conducted to measure different ageing markers, of different kinds of actually used oils during a long-term thermal stress (15 weeks at 130 °C) to examine how well the ageing markers are suitable for inferences about the current state of the paper insulation. Investigated are a non-inhibited and two different inhibited insulating oils. Furthermore, the DP-value and tensile strength of the paper samples, impregnated with the different oils, are also measured. For this pur-pose an open system is reproduced, so that a permanent contact with the ambient air is given. The same measurement is also done for a closed system, which will be presented in another paper. The obtained results show that the type of oil has an influence on the ageing rate and the ageing markers.