Impact Of Aging And Moisture On Creepage Discharges On Oil Impregnated Pressboard

Creepage discharge were found in many power transformers which serviced a long time, aging and increase of moisture were respected as the reason. However, present theories and results cannot explain the phenomenon well, because test results show there no impact of aging on breakdown voltage of pressboard and moisture constant of failure power transformers was still less than the limit value. So, impact of aging and moisture on creepage discharge on oil immersed pressboard should be researched deeply, which is very important to indicate the function of aging and moisture in creepage discharge, comprehend the phenomenon of creepage discharge and prevent the accident of power transformer.To research the impact of aging on creepage discharge, various aged pressboard samples were prepared, breakdown voltage and creepage discharge tests were carried out, breakdown voltage, creepage discharge inception voltage, extension voltage, enduring time, breakdown channel length and white mark development velocity were analyzed and compared, microstructure in normal direction and layer direction of various aged pressboards were observed by microscope and SEM. The results show that creepage discharge in cellulose layer for aged pressboards has higher average developing velocity compared to that for un-aged pressboards, which is related to the increase of cavity density and size between cellulose layers induced by thermal aging process. This finding indicates that the thermal aging not only may decrease the mechanical stress of the pressboard, but also may decreases creeping discharge stress in cellulose layer.To find out the condition of partial high moisture area generation, moisture move between oil and pressboard in warming and cooling process was researched, moisture distribution in pressboard layer direction was analyzed and calculated. The results show the high moisture area will generate in oil in warming process and generated in pressboard surface layer in temperature cooling process.Various aged oil and pressboard sample were prepared, moisture constant in oil and pressbord were measured. The results show the aged pressboard has lower moisture absorption. This is related to the decrease of specific surface area of celloluse induced by the thermal aging process. This finding indicates that moisture in oil will increase in age power transformer.To research the impact of moisture on creepage discharge, various moisture constant oil samples and pressboard samples were prepared according the moisture equilibrium mechanism in different media, the impact of moisture constant and water form on creepage discharge were observed and analyzed. The results show increase of moisture in oil will down the creepage discharge inception voltage signally, but has no effect on creepage discharge development velocity; increase of moisture in pressboard will both down the creepage discharge inception voltage and increase the creepage discharge development velocity; Formation of pretreated water is the main reason of creepage discharge decelop repidly under lower electric stress and long distance condition.Current impulse signal, UHF signal, acoustic signal and dissolved gases generated by creepage discharge were measured, their trends were analyzed, severity diagnostic method for creepage discharge was present.