Study On Oil Sulfur Corrosion Resistance And Mechanism Of Copper Winding With Grain Boundary Characteristic Distribution Optimization

Oil immersed transformer is one of the core equipment of power system,and its stable,safe and reliable operation is an important guarantee of power grid security.Recently,the failure of insulation system caused by oil sulfur corrosion occurs frequently all over the world,which seriously threatens the safe and stable operation of power grid.At present,in a large number of studies,the ways to delay sulfur corrosion of copper winding of transformer mainly include refining and removing sulfur of insulating oil,changing oil and adding metal deactivator,etc.,but it needs to be studied in economy,safety,effectiveness and influence on oil paper insulation performance.In this paper,grain boundary optimized copper winding（GBCD winding）is obtained by Grain Boundary Characteristic Distribution optimization,and thermal aging tests are carried out under different temperature,concentration of corrosive sulfide dibenzyldisulfide（DBDS）and different atmosphere conditions,respectively,to explore the oil sulfur corrosion resistance of two kinds of copper winding and its influence on oil paper insulation characteristics under different influence factors;The effects of AC and DC electric fields and field strength on winding corrosion and oil paper insulation are studied emphatically.Finally,the special grain boundary ratio,the connectivity of grain boundary network and the state of grain clusters of the two kinds of samples are analyzed by using the method of polarization curve measurement of molecular dynamics electrochemical test,combined with the grain boundary characteristic distribution of the original samples and GBCD samples.On this basis,the micro morphology of GBCD samples after corrosion is compared,and the anti intergranular corrosion mechanism of GBCD samples is obtained.The main conclusions are as follows:（1）GBCD winding shows excellent oil sulfur corrosion resistance under all factors（temperature,DBDS concentration and reaction atmosphere）.The increase of temperature,concentration of DBDS and the presence of oxygen will accelerate the oil sulfur corrosion process.The original sample is greatly affected by it,the Cu₂S deposition was obvious on the copper winding and the surface of insulating paper.The insulation performance of oil paper is obviously affected and gradually deteriorated.The optimized sample shows good oil sulfur corrosion resistance under different influence factors and the oil sulfur insulating properties are not significantly affected.（2）GBCD winding shows excellent oil sulfur corrosion resistance under the electric field.The results of electrothermal test show that electric field can accelerate the reaction rate of oil sulfur corrosion,and the effect of DC electric field is more obvious than that of AC electric field.At the same time,with the increase of electric field strength,the oil sulfur corrosion reaction is intensified.Compared with the original sample,the samples treated by Grain Boundary Engineering are less affected by electric field,there is less cuprous sulfide deposition on winding surface,and the oil paper insulation performance is good.（3）Grain Boundary Characteristic Distribution optimization improves the content of special grain boundaries in GBCD samples,breaks the network connectivity of random grain boundaries,and improves the intergranular corrosion resistance of GBCD samples.Electrochemical experiments show that the corrosion potential of the samples treated by Grain Boundary Characteristic Distribution optimization is larger than that of the original samples,and the corrosion current density is only about one third of the original samples.It shows that GBCD samples are not easy to react with the original samples,and the reaction rate is smaller,showing excellent resistance to intergranular corrosion.Based on the analysis of the Grain Boundary Characteristic Distribution,it is found that GBCD samples have higher proportion of special grain boundary,the connectivity of random grain boundary network is obviously interrupted and there are a lot of large-size grain clusters in the crystal.The corrosion morphology also shows that GBCD samples can show excellent intergranular corrosion resistance.