Diagnostic Application of Moisture Equilibrium for Power Transformers

This paper aims at providing reliable methods of assessing moisture in oil-paper-insulated power transformers. The traditional method of moisture evaluation, oil sampling with the subsequent application of an equilibrium diagram, suffers from severe errors resulting in an overestimation of water concentration. To improve this, moisture sorption in cellulose (paper, pressboard) and moisture solubility in insulation oils were theoretically described and represented as sorption isotherms and equilibrium diagrams under the special consideration of aging. Several steps lead to the key recommendation of this paper, which is to use water saturation in oil and in cellulose to describe water in power transformers. The practical application and the use of vegetable oils are considered as well.