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Changes in hydrogen bonding of aged cellulose chains (dotted blue lines are hydrogen bonds). a Cellulose hydrogen bond distribution before aging, b aging causes -OH and -O- to break, and c cellulose hydrogen bonds decrease after aging
Source publication
Nondestructive evaluation of aging state of oil–paper insulation is an important means to prevent aging failure. A characteristic model based on terahertz-sensing hydrogen bond content is proposed in this study to evaluate the aging state of oil-impregnated pressboard nondestructively by analyzing its terahertz absorption spectrum. Oil–paper sample...
Citations
... Under the instantaneous high-impact force, the oil-paper insulation suffers tremendous mechanical stress, and the physical structure of the insulating paper is destroyed. Further, the insulating paper deforms and cracks, resulting in the failure of the oil-paper insulation [18]. However, temporary deterioration is a phenomenon whereby the insulation's performance decreases dramatically, caused by irregular operation and transient overvoltage. ...
... Considering that the water vapor pressure above the dilute solution changes linearly with the concentration of the dilute solution, P v (T) can also be calculated by the solubility of water molecules dissolved in the insulating oil S(T) and the moisture content of the insulating oil, as shown in Equation (18). ...
Oil–paper insulation is the critical insulation element in the modern power system. Under a harsh operating environment, oil–paper insulation will deteriorate gradually, resulting in electrical accidents. Thus, it is important to evaluate and monitor the insulation state of oil–paper insulation. Firstly, this paper introduces the geometric structure and physical components of oil–paper insulation and shows the main reasons and forms of oil–paper insulation’s degradation. Then, this paper reviews the existing condition assessment techniques for oil–paper insulation, such as the dissolved gas ratio analysis, aging kinetic model, cellulose–water adsorption isotherm, oil–paper moisture balance curve, and dielectric response technique. Additionally, the advantages and limitations of the above condition assessment techniques are discussed. In particular, this paper highlights the dielectric response technique and introduces its evaluation principle in detail: (1) collecting the dielectric response data, (2) extracting the feature parameters from the collected dielectric response data, and (3) establishing the condition assessment models based on the extracted feature parameters and the machine learning techniques. Finally, two full potential studies are proposed, which research hotspots’ oil–paper insulation and the electrical–chemical joint evaluation technique. In summary, this paper concludes the principles, advantages and limitation of the existing condition assessment techniques for oil–paper insulation, and we put forward two potential research avenues.
Terahertz spectral characteristics are closely related to the moisture content in the transformer oil–paper insulation, which has great potential in nondestructive testing. However, the difference in the thicknesses of oil–paper insulation samples affects the spectral characteristics and the quantitative relationship between spectral characteristic values and moisture content, limiting the practical application of the quantitative relationship. In this study, the quantitative relationship between the moisture content and absorption coefficient integral eigenvalues of the three thicknesses of insulating paper is first experimentally investigated to reveal the influence of thickness on eigenvalues. Theoretical analyses are also conducted to analyze the factors affecting eigenvalues. This study also proposes that the maximum detectable absorption coefficient of the transmissive terahertz and the surface scattering are the main reasons for the differences in the eigenvalues. Finally, the methods of determining the integration interval through the maximum absorption coefficient and subtracting the surface scattering are proposed to eliminate the effect of thickness on eigenvalues. Thus, these terahertz eigenvalues become universally applicable in assessing the water content of insulating paper samples with different thicknesses.
The terahertz spectrum of paper has been measured from room temperature to cryogenic temperatures. As the paper is cooled, the main absorption lines move to higher frequencies. The same behaviour is observed for two types of paper. The spectrum at base temperature provides the most direct comparison yet with calculations made using density functional theory, which assume the studied material to be at absolute zero temperature. These unique experiments should spur the development of better theoretical models of cellulose and cognate materials.
The additional moisture can be easily introduced into dielectric materials with the nano-doping, which could have adverse impacts on the insulating properties. Herein, samples with different moisture absorption are tested using Terahertz (THz) waves. The relaxation resonance integrated polarization model is developed to illustrate the water absorption effect by the nano-filler on the THz spectrum, which is related to the hydrogen bond (HB) state of water molecular clusters. Detection of moisture content is realized by the polarized resonance intensity in the proposed model. The different HB states of free water and bound water are effectively distinguished using the Debye parameters in the model, and the formation process of water involved in the HB network is analyzed. Meanwhile, the proposed model is verified by molecular simulation. This work provides an application-worthy method to distinguish nuances in moisture contents and distribution in nano-dielectrics, with advantages of non-destructiveness, rapidity and 2D visualization.
