| With the development of Chinese high-speed and heavy-load railways,the reliability of electric locomotives has become the focus of attention of the railway department.As the power source and key components of the locomotive,the on-board traction transformers’ main insulation is mineral oil-Nomex paper composite insulation,which determines the operating safety of the train to a certain extent.Partial discharge is considered to be one of the main cause of local deterioration of the oil-paper insulation,which may lead to a lot of hidden troubles.Exploring the partial discharge characteristics of the oil-paper insulation to the onboard traction transformer and its mechanism of insulation degradation can provide some theoretical basis and data support for the diagnosis and evaluation of the on-board traction transformers’ lifetime,which is of great significance to ensure the safe and stable operation of the locomotive.The in-turn insulation structure of on-board traction transformers was referred in the article.And the Nomex insulation paper and conventional mineral insulating oil was used as objects to design and carry out related tests.Based on the partial discharge test platform,the comprehensive test phenomena,spectrum changes the static trends and dynamic trends of characteristic parameters are integrated,which made the entire process of partial discharge divided into four stages.And it provided a certain criterion for the assessment of the severity of partial discharge.Based on the macro performance parameter test platform,the performance parameter changes’ rules of Nomex insulating paper at different stages are analyzed from the macro level.The results of study were showed in the article.It suggested that with the continuous development of partial discharge,the real part of the dielectric constant ε’ of Nomex insulation paper decreased first and then increased;the volume conductivity σ rose at low frequency;the tangent value of dielectric loss angle tanδ gradually increased at low frequency,and it changed unclearly at high frequency.Based on micro-feature analysis test platform,the changes in micro-morphology,surface functional groups and aggregate structure of the Nomex insulating paper at different stages were analyzed,and the changes in surface morphology,molecular chain breaks and aggregate state parameters during the development of partial discharge were discussed.And then the partial discharge damage mechanism in Nomex insulating paper was studied from the micro level.Research suggested that the micro-morphology of Nomex insulating paper changed during the development of partial discharge.Local defects and fractures appeared at the fiber crossover during the development stage.Local defects in the middle and late stages of the discharge developed along the direction of fiber extension.All three layers of Nomex insulation paper were broken down and severely damaged.Aramid pulp was more corroded than chopped fibers.The layer of Nomex insulation paper near the high-voltage needle electrode deteriorated the most.The surface functional groups of Nomex insulation paper changed with the development of partial discharge.The products containing new functional groups such as methyl groups appeared at the initial stage,which accelerated the process of partial discharge,and the proportion of products was reduced in the middle and late stages.And some new substances such as naphthalene were generated,which further affected the partial discharge.The aggregated structure of Nomex insulation paper changed during the development of the discharge.Deterioration occurred first in the amorphous region,and then gradually advanced to the crystalline region,causing the grain size of the insulating paper fibers to gradually decrease,until the main chain structures of aramid fibers were destroyed,and in the end their strength was completely lost and carbonized,thereby providing some data with regard to insulation characteristics for fault diagnosis of on-board traction transformers. |
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