Investigation On Space Charge And Dielectric Properties Of Nomex Insulation Based On Multi-Factor Aging Dry-Type Transformer

This article is an important part of the National Natural Science Foundation of China’s general project"Research on the insulation aging mechanism of Nomex in mine dry-type transformers"（project number:51577123）.Mine dry-type transformers are the core of underground power supply in coal mines.If an accident occurs,it will not only cause huge economic losses,but also seriously threaten the personal safety of underground personnel.The aging and failure of the insulation in the transformer is an important cause of the accident.Due to its excellent thermal stability and electrical properties,Nomex insulation paper has become the first choice for mine dry-type transformer turn insulation materials.Under the influence of the complex environment in coal mines and the explosion-proof shell structure of mining electrical appliances,Nomex insulating paper,which is used as the winding insulation material of dry-type transformers for mining,is often affected by factors such as high temperature and humid environment and overvoltage,and its aging process is more frequent.Accelerate aging under the combined action of aging factors.At present,the frequency domain dielectric spectroscopy based on the dielectric response theory is not only a non-destructive insulation diagnosis technology,but its outstanding features such as convenient operation and large amount of information have become the focus of research in the field of transformer insulation diagnosis.Therefore,the research on the frequency domain dielectric response characteristics of Nomex insulation aging of mine dry-type transformers is a problem that needs to be solved urgently,and the relationship between the dielectric properties of Nomex insulation paper and the space charge characteristics,and the influence of dielectric aging on the internal space charge of the material It is also an issue that needs to be studied.Therefore,the research on the space charge and dielectric properties of Nomex insulation based on multi-factor aging mining dry-type transformers in this paper has important theoretical value and engineering significance.At present,scholars at home and abroad in the field of oil-paper insulation have conducted a lot of research and achieved fruitful results in the aging mechanism of transformer paper insulation and state diagnosis methods.However,there are few reports on paper insulation of dry-type transformers for mines,especially the aging mechanism and dielectric response evaluation methods of dry-type insulation are not perfect.Therefore,based on the combined aging test of electricity-heat-moisture,this paper takes Nomex insulation paper as the research object,and conducts in-depth research on the mechanism of each aging factor.On this basis,an exploratory analysis of the frequency-domain dielectric properties of Nomex insulation paper in different aging conditions is carried out by combining the microscopic morphology and the characteristics of the dry insulation structure.In addition,the influence of temperature on the frequency-domain dielectric test results is also analyzed.;Through the measurement of DC space charge,the internal space charge accumulation,dissipation and trap characteristics of Nomex insulation paper in different aging states are obtained,so as to analyze the internal aging information;during the above measurement experiment,the laboratory scanning electron microscope and high-power optical microscope are used to observe The microscopic morphology of the samples in different aging states is analyzed,and the influence of the changes in the microstructure during the aging process on the dielectric properties and space charge properties is analyzed.The specific research contents are as follows:A multi-factor aging test platform with the combined action of electricity,heat and moisture was built,and the stress intensity and aging plan were determined according to the IEC60811 standard and related regulations.Under the conditions of voltages of 1.3k V,1.6k V,temperatures of 160°C,180°C,200°C,and initial moisture content of 3%,5%and 7%,the Nomex insulation paper was subjected to a multi-factor combined aging test.At room temperature（27℃）,the DC charge characteristics of Nomex insulating paper in different aging states were measured.According to the space charge dissipation curve of the sample,the charge dissipation curve was calculated and drawn according to the total amount of internal space charge,and the trap of the sample was calculated.Energy level,the space charge behavior inside Nomex insulating paper is further discussed.Research shows that with the increase of aging time and aging stress intensity,the charge injection phenomenon in Nomex insulation paper becomes more obvious,and the trap density inside the sample becomes larger,but the trap energy level inside the sample does not change significantly.The frequency domain dielectric spectrum test was performed on Nomex insulating paper,and the changes of frequency domain dielectric parameters of Nomex samples under different aging conditions and different measurement temperatures were studied.The study found that the real and imaginary parts of the complex permittivity of Noemx insulating paper increased significantly after the measurement temperature increased.This is because the temperature provides the activation energy for carrier migration and enhances the dielectric of the sample.The frequency domain dielectric properties of Nomex specimens under different aging conditions are obviously different,that is,the evolution law ofε_rand tanδof specimens under different stress intensity conditions in the frequency domain of10^-2～10⁶Hz in the whole aging stage is not completely consistent.At the beginning of the aging stage,theε_rand tanδcurves of the samples under different aging conditions in the low frequency increased,and the evolution law was consistent with the aging degree of the samples.In the later stage of the aging stage,the frequency domain dielectric test results of the Nomex sample with severe deterioration showed thatε_rand tanδdecreased significantly in the test frequency domain.Combining the surface micromorphology of Nomex sample to further analyze the frequency domain dielectric properties,the analysis shows that:the microstructure of Nomex insulating paper,such as the morphology of the pulp matrix,the bonding state of the pulp matrix and the chopped fiber,etc.The migration and aggregation behavior of bound charges in the polarization process,and the introduction of air into the microscopic air gap structure of the degradation reduces the overall equivalent capacitance of the paper.The above factors comprehensively determine the frequency domain dielectric properties of the Nomex sample.