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An accelerated aging test is an important test to assess a long time performance of an insulation system. This paper deals with the investigation results on thermally aged natural esters insulating liquid used for transformator. Two kinds of monoesters, palm fatty acid ester (PFAE) and stearic/oleic/linoleic acids methyl esters (M182), and one tri-ester, Envirotemp (FR3), were aged at the temperature of 120 C for 60 days. An as-received oil was used as a sample for aging test, so that the presence of a limited amount of air, and thus oxygen, in oil sample could not be avoided. The important properties like appearance, moisture content, acid value, volume resistivity, and breakdown voltage are evaluated at each 15 days interval. Experiment on mineral oil under the same condition was also conducted for comparison. It was observed that the color of the FR3 was changed from light-green to dark-green. PFAE, M182 and mineral oils which were initially transparent became yellow-green, light yellow and brown, respectively. A sludge formation was observed in the mineral oil, whereas the increase in acidity was found in the PFAE and M182 oils. The moisture content of aged PFAE and M182 oils decreased whereas that of aged FR3 and mineral oils increased. The volume resistivity of all tested oils decreased with aging time, whereas their breakdown voltage showed a different behaviours. The breakdown voltage of M182 and mineral oils increased and decreased, respectively, whereas that of PFAE and FR3 relatively unchanged.

Properties of Thermally Aged Natural Esters Used as Insulating Liquid