Transformer insulating oil characteristic changes observed using accelerated degradation in consideration of field transformer conditions

To ensure soundness of highly aged transformers in their operations, it is important to identify how aging influences insulating oil in these transformers. The characteristics of insulating oil are roughly classified into electrical, physical, and chemical characteristics. In the past research on insulating oil in transformers, some of the characteristics showed a downward trend with increasing age. Among the electrical characteristics, the volume resistivity and the dielectric loss tangent showed a downward trend with increasing age, which is likely to reflect degradation that accompanies aging. The analysis of insulating oil components found some components that increased with age, which may contribute to degrading electrical characteristics of insulating oil. This study on field transformers gave results obtained through comparison of different samples in relation to the number of years in which they were in the field. Detailed evaluation of aging requires study on aging patterns using the same samples. In this paper, heat-accelerated degradation test was conducted on the same samples, and the patterns of changes in characteristics were studied. In addition, the influences of interactions between characteristics and water and between characteristics and pressboard were examined. The study using accelerated degradation test was conducted on one brand of mineral oil (α1). The obtained characteristics were the breakdown voltage, volume resistivity, interfacial tension, total acid value, and water content, i.e., electrical characteristics and characteristics considered to be correlated with electrical characteristics were extracted. As a result of examination, characteristic deterioration was evident for the water content, breakdown voltage, volume resistivity, total acid value, and interfacial tension in descending order of deterioration. The characteristics showed evident deterioration in the initial stage of accelerated degradation test and, thereafter, showed a saturation tendency with increasing age.