Applicability of the FRA method for characterization of insulation of power transformers

Frequency response analysis (FRA) is usually used for diagnostics of mechanical deformations and displacements of transformer windings caused by electromagnetic forces during short circuits. In general, a high frequency response of a transformer is affected by the properties of all transformer components, and, thus, the FRA carries information about the state of the whole transformer including its main electrical insulation. From this point of view, the FRA might be considered as an alternative to the existing methods for assessment of the insulation. In the present paper, the applicability of the FRA is analysed using a high frequency model of a transformer, which takes into account the complex structure of the oil-paper insulation and frequency dependencies of the dielectric permittivities of the insulation components as well as winding resistances. A sensitivity analysis of the computed frequency responses to the variations in geometrical structure and complex permittivities of the components is presented.