The Deterioration Features And Evaluation Methods Of High-Voltage Bushing Insulation Of Traction Transformer

Traction transformer high-voltage bushing is one of the key equipment in the traction power supply system of high-speed railway,and its insulation performance directly affects the safe and reliable operation of the transformer and even the traction power supply system.Compulsory maintenance or equipment replacement of substations caused by insulation degradation failures will directly lead to electric multiple units blackout accidents affecting the normal operation of trains,and even produce a chain reaction of failures causing huge losses,so it is of great significance to accurately grasp the service status of the high-voltage bushing insulation.As the power transmission channel of the traction transformer,the insulation in the bushing bear strong impulse load.The bushing is generally operated with no load or light load,and the load can reach up to 3 times the rated capacity when only the electric locomotive passes through the power supply section of the bushing.And,the locomotive has operating conditions such as starting,braking,uphill and downhill,and over-phase.These factors make the traction load have impulse characteristics such as large amplitude changes,high overload coefficients and abrupt changes.In addition,there are also great differences in train operating density and capacity on different regions and different lines.This traction impulse characteristic causes the following problems: 1)Existing insulation life models are not suitable for status assessment and service life prediction of the bushing insulation under different impulse loads,so that some seriously aged bushings are still operating,and some bushings with good insulation conditions are forcibly replaced before reaching their design life.2)The impact load leads to frequent temperature fluctuations and makes the thermal aging rate of oil-paper insulation extremely uneven,and the difference in moisture absorption capacity of uneven aged oil-paper insulation makes the moisture distribution equally uneven,so the existing assessment methods are not applicable to the condition assessment of uneven oil-paper insulation bushing.3)Due to the short skylight time of high-speed railroad traction power supply system,the high-voltage bushing in alpine regions is cold-started at low temperature every day,however,there is a lack of systematic understanding of the electrical performance of bushing insulation at low temperature.In response to the above problems,the following work is carried out in this paper.1)An accelerated thermal aging test system for insulation that can simulate the effect of impulse load considering the characteristics of traction load was designed,the characteristics of the effect of impulse load characteristic parameters(load rate,duration time and time interval)on thermal aging of insulation were clarified,and an insulation life estimation method considering impulse load characteristic parameters was further proposed.2)The characteristics of the effects of different aging and moisture status on the electrical performance of insulation at low temperatures were clarified,the discharge development process under different typical insulation defects of the bushing was further explored,a computational finite element model of moisture diffusion and electric field distribution of the bushing were established and verified,and the transient process of moisture diffusion and the influence law on the electric field of the capacitor core of the bushing were obtained.3)The frequency-domain dielectric spectroscopy test results were reconstructed using the modified Haviliak-Negami and Dissado-Hill dielectric relaxation models and an proposed improved Debye model considering complex interface polarization,and an improved meta-heuristic algorithm was proposed for the model parameters’ optimal solution,and the reconstruction method was verified by real measurements.4)The dielectric characteristic parameters for distinguishing aging and dampness were extracted,an optimized support vector machine model was proposed for distinguishing aging and dampness status of the bushing,and a hidden Markov model-based method for locating moisture point in bushings was further proposed to determine the source of moisture,and the proposed method was validated using the modeled and physical bushings.