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Kraft transformer insulation paper aged in natural ester (vegetable oil) dielectric fluid was compared to identical paper aged in conventional transformer mineral oil. Sealed steel aging tubes containing copper, aluminum, Kraft paper, and dielectric fluid (mineral oil and natural ester) were aged at 150/spl deg/C for 500, 1000, 2000, and 4000 hours. The extent of paper degradation after aging was determined using paper tensile strength, paper degree of polymerization, and furanic compounds in the aged fluid. Water contents of fluids and paper were compared. Paper aged in conventional transformer oil degraded at a significantly faster rate than in natural ester dielectric fluid. Paper in mineral oil reached three criteria for IEEE end-of-life (50% retained tensile strength, 25% retained tensile strength, and degree of polymerization of 200) within the first 1000 hours. After 4000 hours of aging, paper in natural ester did not degrade to any of the IEEE end-of-life criteria. At 4,000 hours, the paper aged in natural ester retained about 55% of the original tensile strength and a degree of polymerization of about 280. Paper aged in conventional transformer oil degraded to the same values in about 315 and 390 hours, respectively-an order of magnitude faster. The reduced paper-aging rate in natural ester is primarily attributed to the fluid maintaining the paper in a very dry state.