Aging of paper insulation retrofilled with natural ester dielectric fluid

The aging rate of transformer insulation Kraft paper is much slower in natural ester (vegetable oil) dielectric fluid than in conventional transformer oil. This study investigates the effect that replacing transformer oil with natural ester fluid (retrofilling) has on the aging rate of thermally upgraded (65°C rise) paper initially aged in transformer oil. Sealed steel aging vessels containing copper, aluminum, dried thermally upgraded Kraft paper, and dielectric fluid (transformer oil or natural ester) were aged at 160 and 170°C for 250, 500, 750, 1000, 1500, and 3000 hours. Half of the transformer oil systems were retrofilled with natural ester fluid after initial aging times of 750 and 250 hours at 160 and 170°C, respectively. Paper degradation after aging is determined using paper tensile strength and degree of polymerization measurements. After replacing the transformer oil with natural ester, the aging rate of the paper initially aged in transformer oil showed an abrupt change to the reduced aging rate for paper in a natural ester.