Case studies of factors influencing frequency response analysis measurements and power transformer diagnostics

In this article, changes in FRA signatures due to changes in temperature of transformers and in the moisture content of their insulation have been presented and discussed. In the authors´ experience, changes in FRA signatures as a result of changes in transformer temperature and moisture content follow a common pattern, independent of the type of winding. However, the magnitude of the changes depends on the transformer type and the complexity of its windings and insulation. The change in FRA signature as a result of mechanical displacement of the windings would follow a different pattern, depending on the nature of the displacement and the winding arrangement. Further study is needed to quantify the dependence of FRA signatures and the insulation properties ε(ω), Z(ω), and Y(ω) on insulation condition (temperature, WCP, aging) in the high frequency range of up to 10 MHz. This may result in the development of a new approach to characterization and assessment of transformer insulation.