Sensitivity of Connection Schemes for Detection of Axial Displacement of Transformer Winding by Frequency Response Analysis

Axial displacement of winding is one of the most severe mechanical faults of power-transformer windings because it may result in an increase of mechanical force generated when high short-circuit current flows into the winding. Frequency response analysis (FRA) has a possibility to detect them with high sensitivity. However, the sensitivities of FRA in various connection schemes have not been fully investigated yet. In this paper, transfer functions of a transformer whose winding is axially displaced are measured in several connection schemes. The sensitivities of the measured transfer functions in these connection schemes to the axial displacement are evaluated objectively by employing some numerical indices. From the comparison of the calculated numerical indices, it is found that the inductive inter-winding measurement is the most sensitive to the axial displacement. Furthermore, the measured transfer function is reproduced by a simple circuit model. Based on discussion using the circuit model, it is found that some resonances in the TF of IIW measurement are sensitive to the variation of the mutual inductance, which would be varied due to the AD.