Study on degradation causing components of various characteristics of transformer insulating oil

To operate power transformers long-term, as well as ensuring their insulating reliability, adequate consideration must also be paid to the age-related decline in various insulating oil characteristics. This decline is considered attributable to the generation and increase of trace components in oil over time, which were not present when the oil was new. In this paper, to identify the components causing such degradation, various components detected in actual field aged transformers were added to the base oil to measure a range of characteristics (water content, acidity, interfacial tension, breakdown voltage, dissipation factor, and volume resistivity). The influence of these additives on various characteristics of insulating oil was evaluated to investigate the components responsible for the decline. When organic acids were added, the acidity value declined, as well as a significant decline in volume resistivity for decanoic acid among organic acids. For methyl ethyl ketone, which is a kind of ketones, a degradation of the characteristics was observed in the dissipation factor and volume resistivity, showing the same trend as that in the investigation results of the actual transformer oil. It emerged that phthalate compounds, or those derived from rubber material among the aromatic series, influenced the dissipation factor and volume resistivity in the presence of water. Consequently, the influence of various components detected in actual transformer oil on various characteristics of insulating oil was clarified.