The Trap Variation Characteristics Of Nomex Paper Under Electrical Aging And Thermal Aging

With the development of high-speed and heavy-haul railways,it is significant to ensure the safe and reliable operation of trains.The on-board traction transformer is the key component in power conversion and transmission.The Nomex paper-mineral oil composite insulation system is the main insulation of the transformer.The quality of its insulation performance is related to the safety of the train.During the operation process of on-board traction transformer,Nomex paper are aged due to electrical stress and thermal stress,further leading to variation of trap parameters and charge transportation characteristics,the unevenness of electrical field,and even the occurrence of partial discharge(PD).The appearance of partial discharge may finally result in the insulation failure.The investigation of trap characteristics for Nomex paper in different electrical aging phase and aging phase provides important theoretical support for understanding the aging mechanism of Nomex insulation paper and improving the operational reliability of on-board traction transformers.In this paper,the experiment systems of accelerated thermal aging and thermal aging were designed and the samples were prepared separately based on the actual operating conditions of on-board traction transformer,the evolution of trap parameters in different aging stages was measured and analyzed.What’s more,in the electrical aging experiment,the multilayer oil-paper insulation model was designed,the trap parameters,partial discharge phenomenon,PRPD pattern,relative permittivity,electrical conductivity and microscopic morphology of Nomex paper during the aging process were recorded and analyzed.Then the effects of trap variation on the partial discharge characteristics and the charge transport characteristics during partial discharge were discussed due to the test results.Meanwhile,in the thermal aging experiment,the trap parameters,the surface charge accumulation and dissipation characteristics of Nomex paper during the thermal aging progress were recorded and analyzed.The influence of thermal aging on the charge transport characteristics of the insulating paper is discussed and the dissipation methods of positive and negative charges are analyzed.The research results show that the unaged Nomex insulating paper is dominated by deep traps.With the increases of applied voltage,the trap energy level of the first layer for multi-layer insulation paper increases,the trap energy level of the second and third layers of insulating paper decreases during the electrical aging process.A large number of shallow traps are generated in the second layer,when the applied voltage is 15 k V.The increase of applied voltage and bulk conductivity for the first layer during partial discharge progress lead to the increase of free charges quantity and the migration distance along the surface,it is one of the important reasons for the extension of creepage discharge branches.What’s more,the increase of trap energy levels results in the difficulty of charge de-trap,which may provide seed charges for more severe discharge.The existence of large number of shallow traps in the second layer brings about the free charges change rate and the charges quantity increase,which results in an extensive increase of discharge branches length in the second layer.In the thermal aging experiment,with the increases of aging time,the density of shallow hole traps increases,the density of deep hole traps decreases,the density of electron deep traps and electron shallow traps increase.The initial surface potential distribution of unaged Nomex paper remains bell shaped during its decay,accumulated charges mainly dissipated along the surface.Compared with the positive charge,it is easier for negative charge to accumulat and more difficult to dissipate.After thermal aging,the charge accumulation and dissipation characteristics in Nomex paper keep the same.While as for the Nomex paper in the first aging period,the amount of accumulated charges is the least and the charge dissipation rate is the lowest.With the increase of aging degree for insulation paper,the accumulated charge quantity and the charge dissipation rate increase,but the reasons for the increase of dissipation rate as to positive charge and negative charge is different.After positive corona charging,the migration along the volume is one important reason for the increase of dissipation rate.After negative corona charging,the migration along the surface of insulating paper is one important reason for the increase of dissipation rate.