The electrical strength of transformer oil in a transformerboard-oil system during moisture non-equilibrium

The operational reliability of power transformers depends primarily on the oil-transformerboard condition. It´s well known, that the presence of moisture has significant influence on ageing of transformerboard and also on the dielectric strength of transformer oil. At constant temperature a moisture equilibrium between oil and transformerboard exists. The change of load conditions causes a change of temperature and as a result a migration of moisture between oil and transformerboard. These interactions are very important for the breakdown phenomena in oil-cellulose insulation. The disturbance of moisture equilibrium results in a significant reduction of the electrical strength of the oil, especially if the transformerboard has a water content more than 2.5%. This value is normally in an ageing transformer. The experimental investigations of the electrical strength of transformer oil during moisture nonequilibrium are presented. The transition conditions are simulated in an indirectly heated test setup, which represents the insulation system of a power transformer. This model has the same oil-transformerboard ratio (volume and surface area) as a real 200 MVA transformer. The parameters, having the main influence of the breakdown-voltage during the non-equilibrium state, are discussed.