Influence Of DBDS On Corrosion Process And Insulation Performance Of Copper Winding In Transformer Oil

The internal insulation of oil-immersed transformer is composed of oil-paper,which is directly related to the safety and reliability of transformer.The failure of oil-paper insulation is the main reason to result in the failure of transformer.Over the last decade,an increasing number of insulation failures have occurred induced by corrosive sulfur in the transformer oil at home and abroad,threatening the stable operation of oil-immersed transformer.Corrosive sulfur can react with copper to generate copper sulfide deposition on copper winding.Copper sulfide is a gray-black solid with high conductivity,which weakens dielectric strength of oil-immersed paper and reduces insulation resistance.With the growing trend of corrosion problems,this problem attracts great attentions,thereby giving rise to many studies the copper sulfur corrosion in oil-paper insulation.Mineral oil is refined from crude oil,which contains amount of active sulfur with corrosive property.Although numerous sulfur compounds are cleared in the process of refining,several trace inactive sulfur are still retained.These parts of the sulfur compounds exist as natural antioxidants in the mineral oil that can improve the stability of oil oxidation.In particular,dibenzyl disulfide?DBDS?is a high-efficiency antioxidant,which is widespread in a variety of oil brands.However,DBDS is also the main corrosive sulfur compounds,which results in the corrosive of oil.Domestic and foreign scholars have done a lot of research work on oil sulfur corrosion in oil,and several kinds of corrosive test standards for insulating oil before putting into operation have put forward.However,sulfur corrosion process is affected by many factors,leading to affect the accuracy of test standards,including inconsistent results for different test standards and corrosive oil through test.Meanwhile,the failure mechanism of oil-paper induced by Cu₂S deposition is still not clear.Thus,this paper mainly aimed at corrosive test standards,the several factors that affect the effectiveness of test standards were investigated,and the failure mechanism of winding insulation induced by Cu₂S deposition are further studied through analyzing the electrical properties of different corrosion degree windings.Previous studies mainly focused on the corrosive of DBDS and ignored the antioxidant of itself.In this paper,at first,for the corrosive and the antioxidant of DBDS,the effect of corrosive and antioxidant of DBDS on the properties of oil-paper was investigated.Secondly,there was not still unified whether copper conductors wrapped with paper for corrosive test standards,and few studies have considered the effect of electrical fields on copper sulfide formation.Thus,the effect of insulating paper and electrical fields on sulfur corrosion process were investigated,respectively.Finally,the effect of Cu₂S deposition on breakdown characteristic of oil-paper was investigated,the change trend of breakdown characteristic was futher analyzed,the failure mechanism of winding insulation induced by Cu₂S deposition was also preliminarily discussed.The main innovative achievements of this paper are as following:?1?The corrosive and antioxidant of DBDS was the main reason to result in the change of the properties of insulating oil.Under the experimental conditions of this paper,when the insulating oil contained<25 mg/kg DBDS,the DBDS antioxidant effect was dominant,the oxidation stability of the insulating oil was improved,the corrosive effect of DBDS was greater than its antioxidant effect when the insulating oil contained>25 mg/kg DBDS,the corrosive of oil was intensified.?2?Local enrichment of copper ions and acids in copper-insulating paper contact leading to the acceleration of copper sulfide formation was put forward.The metallic surface state was closely related to the occurrence of sulfur corrosion,the bare copper strip was not be corroded under nitrogen condition even when under high DBDS concentration in insulating oil.In order to improve the accuracy and validity of corrosive test standards,copper conductor should be covered with insulating paper to conduct corrosive test.?3?Electrolytic cells induced by electrical fields lead to the acceleration of copper sulfide deposition on the insulation windings was put forward.When the electrical fields was applied on the insulation windings,which lead to the generation of a large number of tiny electrolytic cells on the copper surface,and high amount of copper ions was promoted,not only resulting in the acceleration the corrosion process,but also increasing the aging degree of oil.Thus,electric field action should be put into corrosive test standards.?4?The effect of sulfur corrosion on the short-term breakdown strength of winding was not obvious,but it could significantly reduce the long-term electrical life of insulation.Cu₂S was mainly deposited on the innermost paper of winding,few deposits was found on the rest of paper layer.Thus,as a whole insulation,the insulation performance of winding did not significantly lower,which was not easy to be appeared breakdown.However,the corrosion degree of innermost paper was most serious,which lead to insulation performance degradation,conduction loss and heat increase,local overheating was easy to form,leading to further acceleration of sulfur corrosion and incidence of breakdown.