Three types of moisture diffusion mode in oil-paper insulation

The insulation system in transformer consists of cellulosic materials and processed mineral oil. Cellulose insulation materials degrade as moisture diffusion and thermal aging. So understanding the process of moisture diffusion is important. On basis of Fick´s second law, this paper introduces a reasonable model for transient moisture equilibrium in oil-paper insulation. Three partial differential equations were established to analyze the moisture diffusion in three types of oil-paper insulation. According to diverse boundary condition, supposing the diffusion coefficient as constant, the equations were solved by separation of variables. In the non-equilibrium moisture diffusion process, the diffusion coefficient is not a constant. The finite element method is applied to solve the equations. A series of surfaces for equilibrium and non-equilibrium moisture concentration distributing along thick direction in paper at regular intervals are obtained. According to the experimental data, fitted surfaces were plotted with genetic algorithm. Comparison the fitted surfaces with the result of finite element method, both of them prove the moisture diffusion following the exponential surface. The process of moisture diffusion is affected by the thickness of paper, the temperature, the self moisture concentration and the boundary condition.