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This paper is devoted to recovery voltage as a diagnostic non destructive method for an assessment of an insulating state of power transformers. The insulating pressboard used in power transformers is tested for different moisture levels. Experiments were carried out with samples from the same material intended to power transformers building. A charge and discharge of a capacitor having pressboard paper as dielectric is a suitable tool for an appreciation of the polarization and relaxation processes. The polar character of the water's molecules has an influence on the permittivity. Then the polarization of the paper will be affected by a moisture presence. Time evolution of the recovery voltage for different durations of the charges was examined. An increase of the recovery voltage with the moisture content was observed. The maximum of recovery voltage occurs much later starting from the moment of the opening of the circuit for higher moisture for all examined levels. Thus, the time separating the short circuit and the maximum's occurring can be considered like an important parameter allowing an appreciation of the moisture state of oil impregnated paper.

A Recovery Voltage as non-Destructive Tool for Moisture Appreciation of Oil Impregnated Pressboard: An Approach for Power Transformers Testing