Aging properties of monoesters and comparison with tri-ester and mineral oil

Two types of esters derived from vegetable oil, triglyceride (tri-ester) and monoester, have been implemented as insulation in the oil filled transformer. Tri-ester type is superior in flash point but inferior in viscosity and pour point compared to the monoester type. An accelerated aging test is an important test to assess a long time performance of an insulation system. Two kinds of monoesters, palm fatty acid ester (PFAE) and stearic/oleic/linoleic acids methyl esters (M182), and one tri-ester, Envirotemp (FR3), were thermally aged at the temperature of 120 °C for 60 days. The aging test was performed with the presence of a limited amount of air in oil sample by using as-received oil. The important properties like breakdown voltage, resistivity, and moisture content are evaluated at each 15 days interval. Experiment on mineral oil under the same condition was also conducted for comparison.