Research On The Mechanism Of Typical Partial Discharge For Mining Dry Type Transformer Nomex Insulation During Aging Process

This thesis is one of the main research contents of the National Natural Science Foundation of China “Nomex insulation aging mechanism for mining dry type transformer”(51577123),which aim in study the development law and mechanism of typical partial discharge of Nomex insulation paper in different aging stages.As the main insulation material of mining dry type transformer,Nomex insulation paper is excellent physical,chemical characteristics and electrical stability.However,among many failures of mining dry-type transformer,the insulation faults caused by partial discharge still accounts for a large proportion.In addition,the insulation material of transformer would suffer from the long-term effects of the combined aging stresses,resulting in the change of its physical and chemical characteristics,which further intensifies the production of partial discharge.Therefore,the evaluation of the aging status of Nomex insulation paper takes great significance to ensure the safe and reliable operation of mining dry type transformers by studying the variation law of typical partial discharge spectra and characteristic parameters and exploring the characteristic quantity of partial discharge.At present,there are a large number of scholars at home and abroad have carried out research on the electrical diagnosis technology based on the partial discharge characteristics,which is one of the hot topics of transformer aging status evaluation.However,there are few reports on the variation of typical discharge characteristics of Nomex insulation paper for mining dry type transformer during hygrothermal aging.In this paper,taking the Nomex insulation paper of mining dry transformer as the object,the hygrothermal aging was finished,the surface discharge and corona discharge signals in Nomex insulation were collected periodically.Combined with electric field simulation and theoretical analysis,the partial discharge mechanism of Nomex insulation paper and the influence of aging factors on the partial discharge characteristics were analyzed.In addition,considering the effect of underground humidity,the variation law of the partial discharge characteristics with aging time under different relative environmental humidity was studied.The research is as follows:Taking the Nomex insulation paper with thickness of 0.05 mm and diameter of 90 mm as the object,the hygrothermal aging test was established,the sealed reactor for high temperature and high pressure was used as the hydrolysis reaction vessel,and the aging temperature was controlled by an electric blast drying oven.Considering the aging stress of the mining dry transformer insulation system,combined the IEC60811 standards,the values of various aging parameters were determined.Accelerated aging tests were carried out on Nomex insulation paper with temperature of 160°C,180°C,200°C and initial moisture content of 3%,5% and 7%.The defect model of surface discharge and corona discharge of Nomex insulation paper were made,and the partial discharge tests were carried out by traditional pulse current method.Taking the Nomex insulation paper with different aging period as the object,high frequency current sensors were used to collect discharge signals under different applied voltages,PRDP spectra,two-dimensional spectra and discharge characteristics of Nomex insulation paper were obtained by LabVIEW during the development of discharge from initial discharge to breakdown,and the discharge development process was analyzed.The results show that with the increase of aging degree,the initial discharge voltage of Nomex insulation paper specimen did not change significantly,but the number of partial discharge increase significantly,which can be used as a characteristic to evaluate the aging condition of insulation.Based on the measurement principle of electrostatic capacitance method,the two-dimensional surface potential measurement platform of Nomex insulation paper was built,and the surface potential distribution of Nomex insulation paper under different environmental humidity and different aging period were measured.Scanning electron microscope was used to observe the microstructure changes of Nomex insulation paper in different aging periods,and it was found that there were more physical and chemical defects on the surface of Nomex insulation paper after hygrothermal aging,and then the influence of aging insulation paper on the development of partial discharge was analyzed.Combined with the characteristics of underground humidity,environmental humidity factor was added on the basis of studying the effect of aging state on typical discharge of Nomex insulation paper.A test platform was built to measure partial discharge in different relative humidity environment,the partial discharge tests were carried out in the sealed box with adjustable humidity,in which only the relative ambient humidity was changed by 30 %,50 % and 70 %,respectively.The partial discharge characteristic spectra of surface discharge and needle-plate discharge under different ambient humidity were extracted,and the development rules of typical discharges of Nomex insulation paper under different ambient humidity were discussed.The results show that with the increase of relative humidity,the initial discharge voltage in Nomex insulation paper decreases,the average discharge decreases slightly,but the number of partial discharge increase significantly,which will provide a theoretical basis for the aging mechanism research and state evaluation of the Nomex insulation of mining dry transformer.