Frequency response analysis (FRA) is one of the most reliable and useful methods for mechanical fault detection in transformers. The winding axial displacement is one of the mechanical fault types that can occur in transformers. Axial displacements causes the detailed model parameters changing, in response to which FRA characteristics will also change. Numerous researchers have considered winding axial displacements as a symmetrical movement of disks. However, one of the most prevalent types of winding axial displacements is asymmetrical axial displacement which has not been considered by any prior studies. In this paper, physical asymmetrical and symmetrical axial displacements of the first disk of the LV winding were simulated and FRA characteristics were obtained for these cases. Finally, experimental tests were performed on an experimental transformer to validate the simulation results.
Condenser bushings are considered the Achilles' heel of power transformers because most of the explosions of transformers relate to their bushings' failure. Typical bushings monitoring methods measure the tan delta, power factor, and bushings' capacitance in line frequency. In contrast, the dielectric frequency response (DFR) or the frequency domain spectroscopy (FDS) measures these parameters over a wide frequency range. Some studies have revealed that the FDS curves have distinguished data that help detect bushing failures in their onset. However, there has not been a standard FDS interpretation method. Human experts' opinions play a considerable role in the interpretation process. In this paper, a finite element method model of a 96 kV oil-impregnated paper bushing is built with different OIP moisture content. The FDS test is simulated under these moisture levels. Seven twoarray and six one-array statistical indices are introduced to interpret the FDS results. The results have shown that all six one-array statistical indices, apart from average deviation (AVEDEV) and standard deviation (SD), can be perfect indicators of moisture levels of the OIP. Moreover, they can be used to assess the moisture content of the OIP bushings, and human errors are eliminated using this method.
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