Aging effect on physicochemical characteristics of insulating oil in service-aged transformers

Insulating oil samples were collected from a total of 37 transformers, varying significantly with different manufacturers, manufacture years of 1935 to 1985, and voltage classes of 146.7 kV to 157 kV. The impact of increasing age on the physicochemical properties of oil and the relationships among them were evaluated. Most characteristics, such as moisture content, dielectric strength, furan concentrations were found to deteriorate with increasing age. Interfacial tension and total acid values were used as effective indexes for trend monitoring to identify aging statuses and detect abnormalities at an early stage. Furthermore, the oils were reclaimed. Good correlations were found between interfacial tensions and total acid values.