Moisture-Diffusion Behavior in Oil–Paper Insulation Based on Terahertz Time–Frequency Technology

Moisture content significantly impacts the electrical strength of oil-paper insulation. Acquiring moisture content accurately and promptly at different points in oil-paper insulation is crucial to assess the risks associated with oil-filled equipment. Moisture primarily resides in the insulating paper within oil-paper insulation, and the change in moisture content in pressboard is a dynamic process, especially after localized dampening. Besides the exchanges between oil and paper, the predominant movement of moisture occurs within different regions of the insulating paper. Previous studies have primarily focused on moisture exchange at the interface between insulating paper and insulating oil. Due to the lack of nondestructive testing methods, exploration of the migration and diffusion of moisture within the internal structure of insulating pressboard is limited. This aspect is pivotal for accurately assessing moisture distribution within the insulation, thereby directly influencing the evaluation of insulation risks. In this study, the rapid and nondestructive sensing capabilities of terahertz (THz) spectroscopy are used to quantify moisture content in solid insulation. The integral value of the absorption coefficient spectrum within the frequency range of 0.3–1 THz serves as a characteristic parameter for quantifying moisture content in insulating paper. A 2-D moisture-diffusion model for localized dampening in pressboard is established based on the Fick model. Using this approach, the diffusion behavior of localized dampening in new and aged oil-impregnated pressboard is demonstrated, and the diffusion coefficient within the pressboard is calculated. A relationship between this coefficient and moisture concentration is determined. Finally, the obtained diffusion coefficient is used to numerically deduce 3-D localized moisture-diffusion behavior within the pressboard.