Evaluation of paper insulation of power transformers based on the content of 2-furfural in mineral oil corrected to a reference temperature of 20ºc

The reliability of power transformers operation largely depends on the insulation system condition, especially on the condition of paper insulation. Determining the condition of paper insulation of transformers in use cannot be made by direct measurements. The most common way of assessing the paper insulation condition is by analyzing the content of 2-furfural dissolved in the insulating oil and by applying correlations between the degree of paper polymerization and the 2-furfural content in the insulating oil. The concentration of 2-furfural in the oil depends on the moisture content of the insulation system and the oxygen and acid content in the oil. In order to gain a better insight into the cumulative content of 2-furfural produced by paper degradation, excluding changes in the 2-furfural content caused by migration between paper and oil at different operating temperatures, dependence of the degree of polymerization of kraft paper to 2-FAL content in the oil, corrected to a reference temperature of 20°C, for a moisture content in cellulose of 1-1.5% and 3-4% is presented in this paper. This diagram has been verified on 4 transformers according to available data from the database of the Electrical Engineering Institute Nikola Tesla. In the future, this diagram should be completed for moisture content of 2.5% in cellulose insulation.