The operational reliability of power transformers depends primarily on the oil-transformer-board 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 non-equilibrium 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.
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