Accelerated aging of aramid insulation in various dielectric liquids

Aramid insulation and other transformer construction materials were aged in mineral oil, high molecular weight hydrocarbon, natural and synthetic esters, and silicone dielectric liquids. The initial aging sequence used aging temperatures of 130, 150 and 170 °C for 1000, 2000 and 5000 hours in all liquids but natural ester. A subsequent sequence used temperatures of 210, 230 and 250 °C for 500, 1000, 2000 and 5000 hours using all liquids but mineral oil. We find that the decrease in aramid tensile strength is greatest in synthetic ester and least in silicone. The aramid dielectric strength has the greatest decrease in synthetic ester and least in high molecular weight hydrocarbon and silicone. Synthetic and natural ester liquids degrade significantly at 210 °C and higher. The dielectric strength of silicone decreases significantly at 210 °C, as does its flash point at 230 °C. The synthetic ester acids were found to be highly corrosive to core steel.