Effect of sample thickness on quantifying moisture content in oil-paper Insulation by terahertz characteristics and the corresponding correction method

Abstract 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.