Paper materials are used as the insulation of power transformers. Over time these materials slowly degrade, until they reach a point when they no longer function effectively as transformer insulation, and the transformer reaches its end of life.
The aging rate of paper is affected by temperature, water, oxygen and acids. Investigations have been performed previously on Kraft and Kraft thermally upgraded types of paper. However, a different type of paper used in transformer insulation, Grade 3 presspaper (which contains cotton), has not been extensively tested and compared to Kraft paper. In these experiments, the aging rate of Grade 3 presspaper was studied and compared to previous studies of Kraft paper. Traditionally, the aging rate of paper has been studied in sealed vessels. The problem with this approach is that the chemical environment within the vessel will change during aging, and so the aging rate will be affected. In the experiment setup, the water and oxygen content was controlled to more accurately determine the aging rate. Relationships between the A-value (pre-exponential factor), water content of paper and oxygen content of oil were developed. Similar to Kraft paper, the life expectancy of Grade 3 presspaper is very near to that of normal Kraft paper and the aging rate of Grade 3 presspaper with the same water content increased with oxygen content in the oil. Life curves were developed based on the water content of the paper and the oxygen content of the oil.