Improving the determination of water content of power transformer insulation paper near the end of its functional life

One of the transformer's parameters which has a significant impact on the residual life of a fault-free power transformer is the condition of the paper insulation on its windings. Kraft and Kraft thermally upgraded paper is mostly used as insulation around the copper turns of the winding. Over time, chemical reactions involving water degrade the paper causing it to become brittle. The paper's theoretical end of functional life is when the chemical reactions have reduced the tensile strength to 25% of its original value. Consequently, in order to effectively determine the reliable life expectancy of the winding paper, its water content must be known. Many utilities use cellulosic adsorption isotherms to calculate the water content of transformer paper insulation from the water activity of the oil. However, a problem is that this method becomes less precise as the paper degrades because the adsorption isotherms change, and thus the water content of the paper derived from the water content of oil is no longer accurate. When this inaccuracy becomes significant, should the water activity, or the water content of paper, be used to estimate life remaining? In this article, a discussion is given on why the rate of degradation of paper is dependent on water activity, and the potential inaccuracies when using only water content of paper.