Tsuyoshi Amimoto

Influence of Diverse Compounds on Electrostatic Charging Tendency of Mineral Insulating Oil used for Power Transformer Insulation

Some cases reported recently indicate flow electrification that becomes obvious due to aging deterioration of insulating oil and cellulose insulation in aged oil-immersed transformers. It is supposed that the inclusion of minor components influences the aging deterioration of mineral insulating oil, but details including the aging mechanism have not been clarified. This paper examines the influence of various compounds on the electrostatic charging tendency (ECT) of insulating oil. Mineral insulating oil mainly consists of hydrocarbon compounds, but it also contains extremely small amounts of sulfur compounds, nitrogen compounds and oxygen compounds. Sample oil was prepared by adding various compounds to synthetic oil suffering from small aging changes of the ECT. After heating the sample oil, the ECT was measured, and the influence of various compounds was examined. Aging changes of the ECTs were small in pure hydrocarbon compounds. On the other hand, increases by aging were detected in the ECTs of the sample oils to which impurities such as sulfur compounds and nitrogen compounds were added. In particular, outstanding increases were detected in the ECTs of sample oil to which sulfide compounds and sulfoxide compounds classified as sulfur compounds were added. It has been estimated that sulfur compounds influence the increase by aging of the ECT of mineral insulating oil. It has been also estimated that the increase by aging of the ECT of mineral insulating oil starts from sulfide compounds contained in new oil and that the ECT increase process triggered by sulfide compounds progresses by the production of sulfoxide compounds and compounds of high ECT.

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.

Hydrogen generation by adding passivator for suppressing copper-sulfide deposition in transformers

Generation of hydrogen from oils with the addition of passivators such as 1, 2, 3-benzotriazole (BTA) and Irgamet® 39 for suppressing copper sulfide deposition was investigated by heating tests. Large amounts of hydrogen were generated from the oils with the addition of Irgamet® 39 compared to the oils with the addition of BTA. BTA is suitable in terms of less generation of hydrogen. For Irgamet® 39 added oils, the amounts of hydrogen increase with an increase in oxygen concentration. Irgamet® 39 was consumed by thermal decomposition and oxidation. Consumption by oxidation of Irgamet® 39 is considered to be affected to the hydrogen generation. Amounts of hydrogen increase with increase in heating temperature for a certain period of time. After conducting heating test, Irgamet® 39 was remained in oil. Total amount of hydrogen, which is defined by sum of the amount of hydrogen generated by consumed Irgamet® 39 and the amount of hydrogen generated by residual Irgamet® 39, are considered to be independent of heating temperature. Effects of additives such as dibenzyl disulfide, which is one of the prime compounds that cause the copper sulfide deposition, and 2, 6 ditertiary-butyl paracresol as antioxidant on hydrogen generation were significant. Consequently, in case of adding Irgamet® 39 for the purpose of suppressing copper sulfide deposition, installation of a conservator system and rubber bag which reduces the dissolution of oxygen into the oil is effective to minimize the amount of hydrogen.