Study And Application Of Dielectric Spectrum For Insulation Of Traction Transformer In High-speed Railway

In recent years,the high-speed railway of China is developing rapidly,and the domestic operating mileage accounts for more than half of the total operating mileage in the world.The reliability and safety of equipment in high-speed rail system are important foundations for the rapid development of high-speed railway in China.As one of the important equipment in traction power supply system,the traction transformer bears the task of power conversion,thus its insulation status has important effects on the reliability and safety of traction power supply system.Therefore,how to estimate insulation status of traction transformer accurately has drawn extensive attentions in the industry.In addition to the on-line monitoring for the operating status of traction transformer,the estimation for the insulation status of traction transformer mainly depends on off-line maintenances and periodic testes,which can be conducted during in the maintenance time.However,on the basis of tested results,the insulation status of traction transformer can be estimated roughly without accurate diagnostic results.Especially,when the tested results occur visible change,the insultion is seriously deteriorated.Since the frequency domain spectrum method is of many advantages such as low test voltage,lossless and portable,rich insulation information,and strong anti-interference ability,it has been recognized its high research and application values by scholars.In field application,when the frequency domain spectrum method is used to estimate the insulation status of traction transformers,there are the following restrictions: 1)due to the limted maintance time,it cannot be achieve a temperature equilibrium between the insulation and eniviroment,thus the insulation temperature would be time-varing when the test is conducted;2)since the traction load is implused,the insulation aging of traction transformer has obvious characteristic of non-uniform,however,the effects of non-uniform aging on the oil-paper insulation are not clear.For improving the effectiveness and accuracy of the evaluation of insulation status,the following studies were conducted based on the frequency domain spectrum method: 1)on the basis of modeling the motions of charge carriers in oil gap,the distributed characteristics of charge carriers were analyzed,and the dielectric equation for oil gap and oil-immersed insulation paper were obtained and verified;2)the characterstics of frequency spectrum tested in time-varing temperature conditons were studied,a method to modify the tested ruslts in time-varing temperature conditions was proposed and verified,moreover,the relationship between the dielectric parameters and the complex conductivity was deduced,on the basis of generalizing and verifiying the temperature effects on the frequency spectrum of complex conductivity,a method to translate the tested results to referecnce temperature was proposed and verified;3)the effects of non-uniform aging on the frequency spectrum were studied,and the main factor was obtained,moreover,the effects of non-uniform aging on the moisture estimation of insulation were also analyzed;4)the moisture estimation of oil-immersed bushing at reference temperature was studied by making experimental modles,conducting experiments and analyzing experimental results,meanwhile,the effects of temperature on the test results were also studied,and a quantitative moisture evaluation method of oil-immersed bushing for field application was proposed and verified.