Moisture equilibrium in paper-oil insulation systems

Electrical equipment such as power transformers with a paper-oil insulating system have varying moisture levels dependent on ambient and operating conditions. The total water content is distributed between the paper and the oil in a ratio that is predominantly in favor of the paper. Small changes in temperature significantly alter the water content of the oil, but only slightly that of the paper. To predict these changes isotherms showing the relationship between the water content of the paper and of the oil may be used. These isotherms may be obtained experimentally, or more easily by combining known absorption curves for paper and oil; refinement and extrapolation to lower humidity range may be done with the help of theoretical relationships such as the Langmuir, Freundlich and the B.E.T. isotherms and Arrhenius type temperature dependence. Equilibrium curves constructed by these methods are presented and discussed.