The thermal aging property of oil-paper insulation is a key factor affecting the service life of transformers. In this study, nano-Al2
was added to insulating paper to improve its anti-thermal aging property and delay the aging rate of the insulating oil. The composite paper containing 2% nano-Al2
had the highest tensile strength and therefore was selected for the thermal aging test. The composite and normal papers were treated with an accelerated thermal aging experiment at the temperature of 130 °C for 56 days. The variations of the degree of polymerization (DP) and tensile strength of the insulating papers with aging time were obtained. The characteristics of the insulating oil, including color, acid content, breakdown voltage, and dielectric loss were analyzed. The results revealed that compared with a plain paper, the composite paper maintained a higher DP, and its tensile strength decreased more slowly during the aging process. The oil-impregnated composite paper presented a lighter-colored oil, less viscosity changes, and a considerably lower quantity of thermal aging products. In addition, nano-Al2
can effectively adsorb copper compounds and keep part of the acid products and water away from the thermal aging process. This characteristic restrained the catalysis of copper compounds and H+
in the thermal aging reaction and reduced the thermal aging speed of both the insulating paper and the insulating oil.
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