Impact of transformer winding dry-out on frequency response analysis

Frequency Response Analysis (FRA) has been in use since the last decade as a sensitive method in transformer internal defect recognition. This study has concentrated on the influence of transformer winding dry-out process on FRA spectrum. A single phase glassy model transformer involving concentric LV and HV windings is fabricated and taken as a test object. FRA traces for HV and LV windings of the test object are recorded for wet and dry condition of the paper insulation. KarlFischer Titration (KFT) is employed to determine the moisture content of the oil, and paper insulation humidity is derived using equilibrium curves. FRA trace deviations for HV and LV windings before and after dry-out process are elaborated and total capacitance variations due to the moisture diffusion through transformer windings are calculated and discussed.