Water migration from solid insulation to oil has been a concern to oil-impregnated facilities. Scholars have carried out a large number of experimental studies and obtained the balance curve between moisture in oil and in paper , which can be used to estimate the water in paper insulation through the measurement of moisture in oil. However, with the development of paper-making technology in recent years, the absorption characteristics of insulating paper have changed greatly. On the other hand, the absorption capacity of insulating paper after aging could decrease significantly. Drawing on earlier rules in such cases would lead to intolerable errors. This paper presents a quantitative method to obtain the rule of water balance between oil and paper efficiently, which can be used in both engineering and laboratory conditions. Firstly, the mechanism of moisture migration in oil-paper system was clarified, based on which the method of diffusion potential was proposed to quantitatively describe the water diffusion trend of insulating paper and insulating oil. By setting up the experimental platform, the moisture content in insulating paper under different vapour pressures and temperatures was obtained, and a mathematical formula was established to describe the quantitative relationship. Since the difference in water solubility of mineral insulating oils was small , the mathematical relation between water vapor pressure and temperature and water content of insulating oils in standard IEC-60422 could be referred. The temperature control equation of moisture content in insulating paper and oil was obtained according to the pressure balance rule. Finally, the equation to describe the variation of moisture content in paper with oil moisture, time and temperature was established. 

moisture,oil-paper insulation,equilibium curve