Drying of already impregnated oil-paper insulation

It is well known that oil-paper insulation is prepared, such that the paper installed in the high voltage apparatus is first dried with the influence of the high vacuum and elevated temperature. Then, under vacuum the paper is impregnated with oil. Sometimes the oil-paper insulation, after a short period under working conditions, loses its good dielectric properties because of a high moisture content. This is mostly because of penetration of atmospheric water in the apparatus. Hence it is interesting to know if it is possible to dry such an apparatus and how. In this paper we describe the experiments carried out on a model of the oil paper insulation with high water content in the paper. The insulation is dried under the influence of high vacuum and elevated temperature for some time. The results show that such drying on a model is possible. The needed vacuum, temperature and time for drying the model are presented and discussed. The experiments on a real apparatus are planned