Moisture equilibrium curves are widely used to estimate the moisture content of insulation paper in field transformers. The thermal aging effects on moisture equilibrium curves of mineral oil-paper and a new aging-resistant mixed oil-paper insulation system were studied. The purpose was to assess the moisture content accurately, particularly in transformers used for long-term operations. Thermal aging experiment was conducted at 130℃ for 80 days, and moisture equilibrium curves at five different aging degrees and three different temperatures were constructed. Results showed that with deepened aging degree, moisture equilibrium curves shifted toward the abscissa axis of the rectangular plane coordinate system. The mass fraction of moisture content in oil and paper was calculated at each aging degree. Comparative results revealed that the moisture absorption capacity of insulation paper had an age-related decreasing tendency, whereas the moisture absorption capacity of insulating oil had an age-related increasing tendency. The moisture equilibrium curves of mixed oil-paper insulation system at 85℃ of 5 different aging degrees are shown in Figure 1.
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