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This paper presents a study of the impact of two important parameters, moisture and aging of the oil/paper dielectric used as insulation in power transformers.The way in which these two parameters influence different parameters of the Frequency Domain Spectroscopy (FDS) measurements, is emphasized.Different FDS parameters were measured by varying the moisturecontent and the aging degree of the oil impregnated paper.The use of two types of neural networks for analysis of the results was necessary in order to help discriminating the impact of moisture and aging on the FDS measurements and, in some cases, to estimate the aging duration of the paper impregnated with oil.Index Terms -Power transformer, neural networks, FDS, power factor, dielectric dissipation factor, paper oil insulation.

Abstract:It is now well-established that moisture in the oil paper insulation used in power and instrument transformers significantly reduces the transformers' lifetimes, and can eventually lead to premature failure. This moisture should, therefore, always be removed, not only during production but also after repairs. At the final stage of manufacturing, the drying process should be carried out to remove water and air vacuoles contained in the cellulose-based paper before impregnation. Successful drying helps increase the residual life of transformers, because the presence of moisture and air vacuoles accelerates the aging/degradation process of the oil paper insulation. Proper estimation of residual moisture before impregnation and the determination of the time required for drying play key roles in the time-consuming process of drying. In this paper, the disadvantages of inadequate drying are addressed, followed by a mathematical approach to model the paper drying process. A mathematical model describing the kinetics of drying according to temperature, initial moisture, paper weight, final moisture, and extraction rate is proposed. This model also estimated the amount of moisture removed at the end of the drying process.

In this contribution, dielectric measurements in the frequency domain spectroscopy (FDS) were undertaken on oil impregnated paper samples in order to assess, with accuracy, the state of the solid insulation used in power transformers. The dielectric dissipation factor (tand d d d) and the angle (phase shift) of the impedance of new and aged oil impregnated paper samples were measured; the moisture content served as parameter. These investigations aim at improving our understanding onto the effects of aging and moisture on the quality of the solid insulation (paper and paperboard) used in power transformers. Such investigations might be helpful in the separation of these two effects when doing FDS measurements.I.