The influence of cellulose insulation aging degree on its water sorption properties and bubble evolution

The paper concerns the issue of water in a oil-paper insulating system of a transformer. The author described the influence of Kraft paper ageing on its water sorption and desorption properties. The research was done for new and substantially aged paper. The paper ageing degree was described by means of the cellulose polymerization degree. Volume absorption and line sorption of new and aged paper were investigated. The research was done taking into consideration moisture distribution in the transformer's insulating system and possibilities to generate a particular kind of water desorption called the bubble effect. The article discusses this effect, which is dangerous for the transformer. The author refers to factors which affect the initiation temperature of the bubble effect. Moreover, the article presents research results of the bubble effect initiation temperature for new and aged paper as a function of water content in paper insulation. Formulae describing the initiation temperature of the bubble effect are given for some assumed conditions.