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Recent studies have reported that thermal aging of insulation paper affects its moisture absorption property, which can affect moisture equilibrium in oil-paper insulation. In engineering practice, Fessler's model is one of the most widely used methods to estimate the moisture content of paper through measurement of moisture content of oil. However, this model lacks consideration of the thermal aging effect on equilibrium moisture. This study comprehensively investigated the effect of thermal aging on moisture equilibrium in oil-paper insulation. Based on the accelerated ageing and moisture measurements, the equilibrium moisture content of insulation paper with different aging and temperatures were obtained. The effects of aging condition and the temperature on equilibrium moisture are discussed and Fessler's model was modified accordingly. The results show that: 1) the decreasing of specific surface area is the dominant factor that affects the moisture equilibrium in oil-paper insulation, which is caused by thermal aging; 2) the modified Fessler model is reasonable and suitable to estimate the equilibrium moisture content of insulation paper with different aging. [ABSTRACT FROM AUTHOR]

Thermal aging of insulation paper has obvious effects on its moisture absorption property. The presented studies aim to investigating the effects of aging on moisture diffusion in oil-immersed insulation paper. Based on the experimental designs and preparation of experimental materials, the transient moisture contents of insulation paper with different aging status were obtained at different conditions. Then, the experimental results were discussed. Furthermore, the diffusion factors and equilibrium time constants were extracted based on the model of moisture diffusion in oil-immersed insulation paper, which are applicable to our studies. Finally, the effects of thermal aging on the diffusion factor and equilibrium time constant were analyzed. The results show that: 1) With the aging of insulation paper, the rate of moisture absorption increases, and the equilibrium moisture content of insulation paper decreases at the same temperature. Besides, the equilibrium time constant decreases with aging at the same temperature. 2) Considering the effects of aging, thickness of insulation paper and temperature, the empirical expression of diffusion factor was obtained and verified. [ABSTRACT FROM AUTHOR]