Diffusion Behavior of Free Water and Bound Water in Insulation Paper Based on Langmuir Model

Moisture in oil–paper insulation is a threat to the safe operation of transformers. Consequently, the diffusion, migration, and equilibrium behavior of water molecules in oil–paper insulation has always been a research hotspot in this field. Water molecules exist in both free and bound states in oil–paper insulation. Currently, the Fick model is commonly used to describe the diffusion behavior of free water molecules in oil–paper, without considering the behavior of bound water molecules, resulting in significant deviations between the calculated and measured results. In this study, the Langmuir model was introduced to investigate the diffusion behavior of free and bound water during the drying process of insulation paper, revealing that the adsorption and desorption coefficients in the model could characterize the conversion process of free and bound water molecules in cellulose under different conditions, predict the drying behavior of insulation paper at a given temperature and pressure, and guide the drying process conditions of active part insulation in transformers.