Ryuichi Nishiura

Elucidation of formation mechanism of by-products of copper sulfide deposition on insulating paper in oil-immersed transformer

Copper sulfide deposition on cellulosic insulating materials in oil-immersed transformers is evaluated by heating tests stipulated by IEC 62535. Although this method is suitable for evaluation of the corrosivity of unused insulating oil, the subject remains in diagnosis of the insulating oil used. It is because dibenzyl disulfide (DBDS), which is the prime compounds that cause copper sulfide deposition, is consumed by the reaction with copper during operating of transformer. Therefore, it is necessary to detect the existence of DBDS in insulating oil diagnosed as non-corrosivity by IEC 62535. The proceeding paper [6] reports that Bibenzyl (BiBz) and Dibenzyl sulfide (DBS) will be formed in the insulating oil as by-products of copper sulfide formation between copper and DBDS in oxygen-poor atmosphere which imitated the closed-type transformers. These by-products are considered to be the indication of copper sulfide formation. However, the influence of oxygen and addition agent (e.g. 2,6-di-tert-butylp- cresol) on the mechanism of the by-product formation is unclear. The purpose of this study is to elucidate the influence of oxygen and DBPC in by-products formation for development of diagnosis. It became apparent that benzyl alcohol, benzaldehyde and benzoic acid generate as by-products of copper sulfide formation in the air atmosphere which imitated the open-breathing transformers. Furthermore, it became apparent that the reaction product of benzyl radical and the radical of DBPC generates as by-product of copper sulfide formation in the insulating oil containing DBPC. These by-products are detectable even if DBDS disappears. Therefore, it is thought that diagnosis of the open-breathing transformer is possible by detecting these by-products.

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.