Study of moisture equilibrium in oil-paper system with temperature variation

Transformer owners make every effort to keep the water at a low level inside a transformer. Moisture is an important parameter for the safe operation, service life and reliability of a transformer. A code of practice is set and used by utilities for the maintenance of transformers. In accordance with these maintenance guides an accurate measurement of the moisture level in a transformer becomes an important factor in transformer maintenance. When oil in a transformer is working at a constant relatively elevated temperature for a long period, the thermodynamic equilibrium between water absorbed by cellulose and water dissolved in oil is closely approached. This equilibrium is temperature dependent such that at elevated temperature more water is dissolved by oil. In a transformer the bulk of water is in the paper and any small changes in temperature significantly modify the dissolved water content of the oil but only slightly modify the water content of the paper. To ensure the proper interpretation of the moisture content of a transformer the analytical results need to be corrected. This paper will report on an investigation into several methods devised for the calculation of moisture content of the paper. This will be achieved by using analytical results for moisture content of the oil at different temperatures and make a comparison of them.