The use of paper probes to monitor the drying of large power transformers

Some problems associated with the drying of large power transformers are studied in this paper. It is shown that the conventional end-of-drying criteria cannot be used as dryness criteria when moisture is deeply embedded in the insulation. The solution proposed here consists in introducing into the drying transformer paper probles that are representative of the transformer insulation, and in monitoring their humidity profiles remotely and continuously during the drying. The presence of unfavorable temperature distributions in the transformer insulation and the deleterious effects of such distributions on the effectiveness of the drying process were clearly evident during field tests with this paper probe arrangement.