Assessment of moisture diffusion distance in pressboard insulation within transformer using Fick´s law

Pressboard insulation is one of the important dielectric media in a transformer. During operation of the transformer, moisture present inside the pressboard or produced as aging by-product starts migrating due to diffusion through the pressboard material. This moisture raises the dielectric dissipation factor and increases the risk of breakdown of the solid insulation. It also lowers the hot spot temperature range for bubble formation and accelerates the thermal aging of pressboard insulation. Thus it is essential to determine the distance diffused by the moisture so as to get an approximate idea of how fast it spreads. Fick´s laws of diffusion are well known equations of diffusion. In the present work a particular solution of the Fick´s law of diffusion is used to form an equation of the distance diffused in a certain time considering the environmental conditions within a working transformer. The results obtained from the equations using a typical daily load pattern are plotted in order to give an estimate of the actual distance that can be diffused throughout the day and also of the cyclic migration of moisture into and out from the pressboard. The proposed methodology will be helpful in monitoring the moisture location inside the pressboard insulation of a working transformer.