Ageing of oil impregnated thermally upgraded papers

Investigation of the difference in the ageing process for several thermally upgraded and non-upgraded papers in transformer oil was performed. We observed that the level of nitrogen in the papers is crucial for the resistance towards hydrolysis. One of the papers in the experiment did not fulfil the upgrade specification with respect to the nitrogen level (data sheet). For hydrolysis of wet oil impregnated samples in argon atmosphere the upgraded papers seem to degrade less than non-upgraded papers. The paper with highest level of nitrogen degrades less than those with lower levels. The Insuldur process seems to be the best way of thermally upgrading the paper (Upgrade 1) and this also gives the highest level of nitrogen. For one of the upgraded samples (Upgrade 2) the nitrogen disappears completely after startup and this paper behaves as non-upgraded for the highest temperature. Dry oil impregnated paper (0.2% water content) under argon atmosphere does not seem to be hydrolysed and ages at a very slow rate. In the case of oxidation of dry oil impregnated samples (0.2% water content) in air there are initially no significant difference between non-upgraded and upgraded papers. However, the upgraded papers seem to have an improved performance after the water production from ageing becomes significant. The oxidation activation energy for non-upgraded paper is lower compared to hydrolysis and for upgraded paper this difference is smaller.