To maintain the safe operation of power transformers the water content of their insulation must be monitored, and if it becomes too high, corrective action taken. While measuring the water activity of the oil is one method to determine the water content of paper, there is no commonly accepted method used by the utilities in Australia. Consequently, a field study was performed, using four operating power transformers, to compare the determination of the water content of paper with measurements made using dielectric response. The transformers were chosen to have a wide range of insulation water content. The measurements compared very well, even with transformers with degraded insulation. However, as insulation paper degrades its adsorption isotherms change, which will form the basis of future work
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