Study on the Diffusion Behavior of Moisture in New and Aged Oil-paper Insulation by Terahertz Imaging Technology

The moisture content of oil-paper insulation and its distribution state are related to the breakdown risk and aging rate of oil-filled equipment. The water content testing technique, represented by Karl Fischer, is only able to obtain the average moisture content in insulation samples. Because it is damaging to the sample, it is not possible to continuously observe the dynamic diffusion and equilibrium processes of moisture in the oil-paper insulation. Benefiting from the sensitive, fast and nondestructive characteristics of terahertz wave response to polar water molecules, this paper achieves the observation and quantitative analysis of water diffusion behavior in oil-paper insulation by introducing terahertz time-domain spectral imaging technique. The results showed that the diffusion rate of moisture in the aged insulating paperboard was lower, and the moisture was more likely to be lost into the oil, mainly due to the cleavage of cellulose chains and the reduction of hydroxyl groups. The method can be applied to further study the diffusion coefficient of moisture in oil-paper insulation under different conditions.