Research On Detection And Predictive Models Of Thermal Aging Products Of Transformer Oil-paper Insulation

Transformers are the core equipments for energy conversion and transmission in power grid systems,and heat is the most important factor leading to the deterioration of transformer oil-paper insulation performance.It is of great significance for evaluating the aging state of oil-paper insulation and predicting the life of transformers to carry out the detection and mechanism research of oil-paper insulation thermal aging products.Firstly,this paper studies the accelerated thermal aging test of oil-paper insulation and degree of polymerization?DP?of insulating paper.Secondly,the contents of aging products of insulating paper are tested and analyzed for the relationships with aging time.Finally,by analyzing the relationships between aging products and DP,predictive models of oil-paper insulation thermal aging state and the degradation mechanism of insulating paper are studied.The results are as follows:After heating at 80?for 6h,the moisture in the insulating paper can be removed,and the water in the insulating paper stick can be removed by heating for 1d.the moisture in the oil is removed by heating at 60?for 24h.Oil-paper insulation thermal aging test devices are designed and fabricated.The R² values of the second-order kinetic models of DP and aging time are all above 0.97.The higher the temperature is,the faster the insulating paper ages.FTIR analysis confirms that the polar functional groups in the aged oil samples are derived from polar substances decomposed by insulating paper.The higher the aging temperature is,the higher the furfural content in the oil is.At the same temperature,the furfural in the oil gradually increases with the increase of aging time.There is no regularity between furfural content in the insulating paper and aging time.The contents of5-hydroxymethylfurfural?5-HMF?in the oil and insulating paper increase slowly at 80?,90?,and 100?,and the growth rate is relatively rapid at 110?.The methanol contents in the oil gradually increase with the increase of aging time.There are good linear relationships with the aging time,and the R² values are all above 0.90.The methanol formation rates at four temperatures are 0.0261,0.2655,0.2142,0.2192?g·L^-1·h^-1,respectively.The total sugar contents in the oil at 80?gradually increase with the increase of aging time.At other temperatures,the total sugar contents show a tendency of increasing firstly and then fluctuating.It is confirmed that the aged oil samples contain levoglucosan,D-mannose,L-rhamnose,D-galactose,D?+?-glucose,L-xylose,and D-fructose.No furfural and total sugar are detected in the actual transformer oil samples.The contents of 5-HMF are about 20?g/L,and the methanol contents are 0-320.98?g/L.The logarithms of furfural contents in the oil have good linear relationships with DP.There are no obvious regular relationships between furfural contents in the insulating paper and DP.The contents of 5-HMF in the oil and insulating paper have different linear relationships with DP in the different aging stages,and the R² values of the relational models are all above 0.90.The methanol contents in the oil increase with the decrease of DP at different temperatures,and the R² values of the models of methanol with DP are all above0.95,therefore,the contents of furfural,5-HMF and methanol in the oil and 5-HMF in the insulating paper can be used to evaluate the aging state of oil-paper insulation.Total sugar is only used to characterize the aging degree of oil-paper insulation during the premetaphase of aging.Cellulose of insulating paper is mainly degraded in the amorphous region in the premetaphase of aging,and the rate is faster.In the midanaphase,the crystal region is mainly degraded and the rate is slower.The degradation of low molecular weight substances generated in the late stage of aging is more difficult.The SEM analysis shows that during the aging process,the fibers on the surface of insulating paper are changed from a tightly packed,long and thick state to a severely aged state in which the fibers are twisted,the voids are large,and the number of fine fibers is sharply increased.In this paper,the relational researches between total sugar,methanol in the oil and DP provide new ideas for evaluating the aging degree of oil-paper insulation.