| Power transformer is the core equipment of power transmission in power grid,the safe and reliable operation of power system is closely related to the insulation state of power transformer.Oil-paper insulation structure gradually exposes various insulation aging problems with the increase of transformer operation time under the combined action of heat,electricity,moisture and oxygen,etc.Therefore,studying the aging characteristics of oil-paper insulation under combined electrical and thermal stresses can understand the aging mechanism of oil-paper insulation better,providing reference for mastering the aging processes and failure aging states of transformer insulation in actual operation.The main work of this paper is as follows:(1)In order to study the aging characteristics of oil-paper insulation under combined electrical and thermal stresses,the experimental system of electro-thermal aging of oil-paper insulation is designed and built,which combine partial discharge of oil paper insulation and thermal aging,taking mineral oil-paper composite insulation as the research object,the electro-thermal aging test of oil paper insulation was carried out,four types of oil-paper insulation aging test are designed,the test conditions are corona discharge at 140?(Thermal-Corona discharge,abbreviated as TC),surface discharge at 140?(Thermal-Surface discharge,abbreviated as TS),thermal aging at 140?(Thermal,abbreviated T),surface discharge at room temperature 25 ?(Surface discharge,abbreviated as S).(2)In the corona discharge at 140?,the variation rules of some characteristic parameters(including gas content,acid value,furfural content of transformer oil and polymerization degree(DP),breakdown voltage of insulating paper)with the increase of aging time are studied,these rules are compared with those in thermal aging at 140?.The results show that the thermoelectric superposition effect can promote the deterioration of insulating paper and transformer oil in the corona discharge at 140?,the average degradation rate of insulating paper under the condition of TC is 2.33 times higher than that under the condition of T,the breakdown voltage of insulating paper first decreased and then increased with the increase of aging time under the condition of TC,but there was no obvious changes under the condition of T.The variation rules of gas content in oil with the increase of aging time is different under the conditions of TC and T.(3)Based on Ansoft electric field simulation,the reason of the difference of surface discharge inception voltage of oil-paper insulation under the condition of TS and S is analyzed.In the surface discharge at 140?,the variation rules of some characteristic parameters(including gas content,acid value,dielectric loss and volume resistivity of transformer oil and breakdown voltage of insulating paper)with the increase of aging time are studied.These rules are compared with those in surface discharge at normal temperature.The results shows that the acid value in oil increases and the volume resistivity decreases under the condition of TS,which is not obvious change in the condition of S.The content of gas in oil under the condition of TS is obviously larger than that under the condition of S,the variation rules of gas content in oil with the increase of aging time is different under the conditions of TS and S.The dielectric loss of oil increases and the breakdown voltage of insulating paper decreases with the development of the surface discharge under the conditions of TS and S.(4)According to the data of aging test obtained in the study above,the factor analysis of characteristic parameters of electro-thermal aging under two different conditions is carried out respectively,which can reduce the characteristic parameters from multidimensional to two-dimensional,so that the common factors among various characteristic parameters can be filtered out,and the aging diagnosis of oil-paper insulation characteristic parameters can be more convenient. |
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