A moisture-in-oil model for power transformer monitoring - Part I: Theoretical foundation

Nowadays, there is an increasing interest in using transformer monitoring systems. Some monitoring systems use models to predict the value of certain variables. One of the key variables to be monitored is moisture content in oil, as moisture has a high influence on transformer exploitation. In this paper, the theoretical foundation of a model for monitoring moisture in transformer oil is reported. The moisture model estimates moisture content in transformer oil under certain working conditions. Comparing the value of estimated moisture with the measured one, the model is able to detect failures producing an abnormal water content in the transformer. The model considers the equilibrium relations between paper and oil, the transients that take place until the steady-state equilibrium is attained, and the water increase caused by the insulating paper ageing. In addition, the model can alert to the possibility of oil saturation and the consequent free water formation in the transformer. In this paper, the theoretical foundation of the moisture model is described; in a companion paper, its experimental verification is reported.