| The detection and evaluation of the insulation status of transformers' oil-paper is an important measure to ensure the normal operation of the transformer.Moisture will reduce the insulation strength,accelerate the aging of the insulation,and seriously affect the insulation life.Therefore,the moisture content of the transformer oil paper insulation is an important indicator of the insulation state of the transformer.Currently widely used moisture detection methods,such as Karl Fischer Titration(KFT),Polarization Depolarization Current(PDC)and Frequency Domain Dielectric Spectroscopy(FDS)are all There are shortcomings such as time-consuming and complicated operation,which is not conducive to the actual engineering of the site.Therefore,it is necessary to develop a fast,direct,and non-destructive in-situ detection method of transformer oil-paper insulation moisture,which is suitable for engineering environments such as factory inspection,offline inspection and online inspection.Terahertz time-domain spectroscopy technology is a highly potential non-destructive testing technology,which is sensitive to changes in moisture and can be used for moisture content detection.Tera Hertz(THz)waves(electromagnetic waves of 0.1?10THz)have good penetrability to dielectric materials and non-polar liquids,while water is a polar liquid and has strong absorption of terahertz radiation.At the same time,the terahertz pulse is at the picosecond level,which can realize fast and non-destructive in-situ detection of the oil-paper insulation water content.This paper builds a set of Terahertz Time-Domain Spectroscopy(THz-TDS)test platform suitable for transformer oil-paper insulation system,and prepares oilimpregnated paperboard with a moisture content of 0.452?4.671% and uneven moisture distribution.Oil-impregnated cardboard,insulating oil with moisture content of3.7?25.8ppm,and insulating oil samples with different degrees of aging;use the built terahertz test platform to obtain the time-domain response waveforms of oil-impregnated cardboard and insulating oil with micro-water content;calculate the samples The relative frequency domain optical parameters and dielectric parameters of,and the corresponding molecular simulations have been carried out.For oil-immersed insulation board and special hydrogen bond network composed of water,in this paper,the oiled paper insulation dielectric spectrum analyses affected by the hydrogen bonding interaction with oil-paper insulating layer in the characterization of the dynamic polarization behavior of water molecules,established considering the effect of water involved in hydrogen bonding integrated polarization relaxation resonance model,to describe the paper insulation and moisture of the complex dielectric constant.Then the relationship between polarization parameters and moisture content is analyzed.Terahertz spectral scanning imaging was used to draw the moisture distribution image of oil-impregnated cardboard with uneven moisture distribution.For the insulating oil containing micro-water,the water molecules in the insulating oil system are used to resonate in the terahertz frequency band.Based on the peak height of the resonance characteristic peak,the micro-water content in different insulating oil samples is analyzed and calculated,and the test results are completed.Quantitative analysis;finally used this method to test insulation oils with different degrees of aging.The main results obtained in the thesis are:(1)The terahertz time and frequency domain response curves of oil-impregnated cardboard containing micro water are studied.As the water content of oil-impregnated paperboard increases,the peak value of the time-domain waveform decreases,and the peak time is delayed.The time-domain peak ratio/peak time difference(1/ps)can be used as a characteristic value for quickly identifying the micro-water content of the oilimpregnated paper,but it cannot be used to characterize oil immersion.The polarization characteristics of the paperboard;the terahertz complex dielectric spectrum of the oilimpregnated paperboard with micro water is further calculated,and a comprehensive relaxation resonance model is established.The proposed model includes the slow Debye relaxation process and the fast Debye relaxation process This theoretical model is consistent with the experimental results of the resonance polarization process of intermolecular stretching.The amplitude of the resonance term As represents the overall resonance polarization strength of the hydrogen bond network,which can indirectly reflect the water-cellulose hydrogen bond and water-There is a good linear relationship between the number of water-hydrogen bonds and the bond energy and water content.As the characteristic quantity of water detection,its linearity is better than that of time domain peak ratio/peak time difference;the water content of time domain and frequency domain are used to realize oil For the two-dimensional imaging of the moisture distribution in the immersed insulating paperboard,the resonance polarization intensity As is overall better than the peak ratio/peak time difference characteristic quantity.(2)The polarization behavior of water molecules in oil-impregnated paperboard in the terahertz frequency band is studied.It has been discovered that the water in the oilimpregnated paperboard system containing micro-water participates in two stages and two types(water-water hydrogen bond and water-cellulose hydrogen bond)in which the hydrogen bond network changes with water content.In the first stage(water content <4%),the number of water-cellulose hydrogen bonds continues to increase,resulting in an increase in the polarization of the rapid relaxation process and an increase in the disorder of the hydrogen bond network,while the number of water-water hydrogen bonds is less,The polarization intensity of the slow relaxation process increases slowly;in the second stage(water content>4%),the water-cellulose hydrogen bond is basically saturated,the polarization intensity of the rapid relaxation process is basically unchanged,while the water-water hydrogen bond The number of ions began to grow rapidly,the polarization intensity of the slow relaxation process rapidly increased far more than the polarization intensity of the fast relaxation process,and the disorder of the hydrogen bond network decreased.The water-cellulose hydrogen bond will enhance the fast relaxation polarization and increase the disorder of the network;the water-water hydrogen bond will enhance the slow relaxation polarization and reduce the disorder of the network.In addition,both hydrogen bonds will enhance the strength of resonance polarization.The relationship between the number of different types of hydrogen bonds and the water content in the oil-paper insulation system and the proportion of different types of water participating in the hydrogen bonding were calculated using molecular simulations.Combining the experimental results and theoretical calculations,it is found that although the water-cellulose hydrogen bonds are quantitatively Dominates,but the water-water hydrogen bond has a greater effect on the relaxation polarization.Terahertz technology can provide experimental support for molecular simulation.(3)The time and frequency domain response curves of insulating oil with different moisture content are studied.The study found that as the moisture of the insulating oil increases,the time domain peak value of the sample decreases and the peak time delay is consistent with the test results of the oil-impregnated paperboard;the terahertz absorption spectrum of the insulating oil sample containing slight water is calculated and found to be 0.51,1.19,1.48 and 2.27 THz,there are obvious absorption peaks,and the absorption peak intensity of 2.27 THz is the highest;the theoretical absorption characteristic peaks of the insulating oil system containing slight water are calculated by molecular simulation,and the vibration simulation results are also at the above frequencies.There is a characteristic absorption peak,which is consistent with the test data;the characteristic absorption peak at 2.27 THz is the highest,and it is less affected by the degree of water dispersion,and also has a good positive correlation with the moisture content.Therefore,2.27 THz is used as the oil The characteristic peaks of micro-water detection have good linearity;the influence of aging factors on the water content test of insulating oil is studied,and insulating oil samples with different degrees of aging are obtained through thermal aging tests,and terahertz testing is performed on the aging insulating oil Insulating oils with different aging degrees have characteristic peaks of micro-water content around2.27 THz.The water content of insulating oil samples with different aging degrees is calculated by using the 2.27 THz characteristic peaks.The error between the terahertz test results and the coulomb method measurement results within ±2ppm. |
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