Study On Growth And Partial Discharge Characteristics Of The White Mark In Oil-paper Insulation

Power transformers are the key equipment in the power grid,and its reliable operation is closely related to the safety of the power system.As the main insulation material of large power transformers,the electrical performance of oil-paper insulation is related to the safe operation of the equipment.The“white mark”is a special white damage trace formed on the surface of pressboards by discharge.Its essence is gas channels formed inside pressboard pores through gas generation and expansion caused by large discharge energy or high temperature.Although both of the white mark and common black carbonized traces are discharge damage traces which will cause the decline of insulation performance,the biggest difference between them is that due to the re-impregnation of insulating oil,the white mark can recover itself and will gradually disappear after a certain period of time,so it is difficult to accurately detect even through the hanging cover maintenance.Therefore,it is necessary to study the factors influencing the white mark growth and the variation law of morphology,as well as the correlation between partial discharge characteristic parameters and white mark growth state,so as to explore effective means and methods of the early warning of the white mark defects.In this paper,the formation process of the white mark in oil paper insulation is simulated through the discharge damage test in the laboratory,and samples with different morphology of the white mark are obtained.By studying the digitalizing method of the white mark morphology image,the quantitative expression of the white mark state and development law is realized,and the influence of water content and electric field intensity on the initiation,growth and disappearance characteristics of the white mark is studied.Also,the change of electrical performance of oil-paper insulation before and after the white mark is formed is studied.Then,by studying the variation law of partial discharge characteristics and characteristic parameters during the evolution of the white mark,the discharge mechanism and the causes of different growth types are analyzed,and parameters which can characterize the initiation and development of the white mark are explored.Finally,the evaluation method of the white mark growth state is explored by using partial discharge characteristic parameters.The main conclusions are as follows:（1）The quantitative development law of the white mark growth morphology and the influencing law of moisture,electric field strength and other factors on the white mark are obtained.The results show that the higher the water content of the pressboard is,the lower the white mark initiation voltage is.When the water content of the pressboard is 1%and 2%,the growth area and speed of the white mark are relatively large,and the growth speed is fast under high electric field strength.The white mark reduces the power frequency breakdown strength of the pressboard.Although the white mark will gradually disappear due to the re-impregnation of oil after the voltage is not applied,the insulation performance of the pressboard cannot be completely restored in a short time.（2）The differences in partial discharge characteristics and causes of different types of the white mark are compared,and parameters characterizing the growth process of the white mark are obtained.The results show that the growth process of the white mark can be divided into two types,and the discharge morphology corresponds to partial discharge characteristics one by one.The discharge characteristics of white mark type I are successively presented as high energy intensive discharge,low energy discharge and small amplitude intensive discharge,while white mark type II are successively presented as high energy discharge and low energy sparse discharge.Whether the maximum field intensity E_max of the needle tip reaches the threshold is the key to the sustainable development of the white mark,and it is also the reason resulting in different growth types.For two types of the white mark growth,the difference in discharge repetition rate N is large,and it well reflects the severity of the white mark development.The discharge power P has a good correlation with the white mark area S.The initiation signs of the white mark are the abrupt changes of average apparent discharge quantity Q_ave,maximum apparent discharge quantity Q_max,discharge repetition rate N,discharge power P,ratio of positive and negative pulses numbers r and cumulative discharge energy E.（3）According to partial discharge characteristic parameters,the evaluation method of the growth state of the white mark is explored.The results show that if average apparent discharge quantity Q_ave,maximum apparent discharge quantity Q_max,discharge repetition rate N,discharge power P,ratio of positive and negative pulses numbers r and cumulative discharge energy E are set as characteristic parameters,the rationality of the stage division can be verified by hierarchical clustering and K-means clustering methods,the accurate recognition of the white mark growth state can be realized by using the classification learner,and the accurate estimation of the growth area of the white mark can be achieved by the artificial neural network.