A comparative study of thermal aging of transformer insulation paper impregnated in natural ester and in mineral oil

Aging of two kinds of transformer insulation paper in natural ester is compared to that in conventional transformer oil (mineral oil). Sealed aging vessels containing copper, insulation paper (Kraft paper and thermally upgraded paper), and dielectric fluid (mineral oil and natural ester) were aged at 90, 110, and 130°C for 399, 371, and 329 days. Under the same aging condition, both the degree of polymerization (DP) and the leveling‐off degree of polymerization (LODP) for natural ester aged paper were higher than the ones for mineral oil aged paper, and the micro surface of natural ester aged paper was destroyed less seriously than that of the mineral oil aged paper. The aging time to reach LODP for natural ester aged paper was only 1/4 to 2/3 of that for mineral oil aged paper. When water content in natural ester for natural ester aged Kraft paper samples reached the maximum, acids concentration in natural ester increased abruptly. Water content in natural ester decreased in the whole aging process for natural ester aged thermally upgraded paper samples. Six physical and chemical mechanisms for the slower aging rate of natural ester aged paper are proposed. Evidence for transesterification between natural ester and paper was presented with infrared spectra instrument. Although the acids in natural ester were far more than in mineral oil, the natural ester can significantly slow the thermal aging of the paper. Three mechanisms of this abnormal phenomenon were presented. Copyright © 2009 John Wiley & Sons, Ltd.