Method to evaluate the degradation condition of transformer insulating oil - establishment of the evaluation method and application to field transformer oil

To operate power transformers long-term, as well as ensuring their insulating reliability, it is important to study age-related decline in various insulating oil characteristics and the method used to evaluate the same adequately. Previous studies showed that age-related decline in insulating oil characteristics was caused by trace components in oil produced during oxidation degradation. To maintain and manage aged insulating oil rationally, a specific diagnostic method must be studied, based on measurement of the trace components in oil that cause degradation in these insulating oil characteristics. The present study evaluated the sensitivity to detect various components based on the amount of trace components produced in oil during oxidation degradation for field-aged insulating oil for 34 transformers in various degradation conditions. A study was also conducted to evaluate the degradation condition based on the ratio of trace components produced in oil. Consequently, the detection sensitivity levels were in the order of carbonyl value, saponification value, peroxide value, and total acid value. As the ratio of the saponification value - the so-called final product - increased, the degradation of insulating oil developed further. With the above study results, methods of evaluating the dissociation property in the preceding study and the breakdown voltage with the degree of water saturation taken into consideration were combined to establish a comprehensive method of evaluating aged oil. This was then applied to field-aged insulating oil as an example, whereupon the need to replace insulating oil could be evaluated.