Experimental determination of the diffusion coefficient of water in transformer solid insulation

This paper presents the methodology used and the results obtained in the determination of diffusion coefficient of moisture into non-impregnated transformer insulating paper. In order to determine the diffusion coefficient, drying curves of paper samples were obtained from thermogravimetric experiments. The parameters of the diffusion coefficient were found by applying an optimization process based on genetic algorithms. The error function between measured and simulated curves was determined, and the parameters giving the best agreement between measured and estimated values were obtained. As a result, a new equation for the diffusion coefficient of moisture into non-impregnated insulating paper is proposed, depending on average moisture concentration, temperature and insulation thickness. The proposed coefficient, validated by good agreement between the experimental drying curves and those obtained by simulation using the diffusion coefficient, can be used for the determination of the time required to dry a power transformer in a factory.