Fundamental concepts of using water activity probes to assess transformer insulation water content

Abstract: The water concentration in transformer paper insulation must be monitored because, if it becomes too high, the paper insulation will age rapidly [1]-[4] and the transformer may fail on overload [5]-[9]. A failure can occur on overload if the temperature of the paper rises sufficiently to cause evolution of water bubbles. If these bubbles cause dielectric failure of the oil gap, the transformer may fail catastrophically. Paper insulation becomes brittle as it ages, and eventually becomes too brittle to act as an effective insulator. Oxygen increases the aging rate because the acids created by chemical reactions occurring in the oil, paper, and oxygen react further with water [10]-[12]. A study of the thermal aging of Kraft paper found that wet oxygenated paper degrades 40 times faster than the same paper heated in a dry oxygen-free environment [4]. The paper within a transformer is likely to degrade fastest at the top of the winding, where it is hottest [13]. Since the water concentration in the paper significantly influences its aging, it is important that the water concentration be measured accurately at the top of the winding