The Power Transformer Winding Input Admittance and Frequency Response

Abstract

The frequency response analysis is increasingly more widely used to estimate the electrical and mechanical condition of power transformer windings. The frequency response measurement procedure has been prescribed in the existing international and national standards. However, the interpretation of measurement results still remains a subject of discussion. Various approaches have been used to interpret the results, including an analysis of transformer windings’ complex impedances and admittances obtained by calculations from frequency responses. To improve the frequency response interpretation, the article presents an analysis of power transformer winding input and output currents and admittances based on simplified equivalent circuits. Qualitative and quantitative results are presented, which show that the admittances obtained from measured frequency responses differ essentially from the true input admittances of the windings. The main factors causing the differences between these winding admittances are considered. It is shown that despite the existing differences, the current through the neutral and the winding admittance determined from the measured frequency response can also be used in practice to determine the power transformer winding natural oscillation frequencies.