Interpretation of Transformer FRA Responses— Part II: Influence of Transformer Structure

In general, a transformer winding end-to-end frequency response analysis (FRA) response, usually measured between 10 Hz and 2 MHz, can be divided into three frequency regions of which from low to high frequencies, are dominated by the core, the windings, and the measurement setup correspondingly. The coupling interaction between windings may be originated either from nontested windings of the same phase with the winding-under-test or from nontested windings in the other phases. The latter is coupled to the winding-under-test through the three-phase delta winding connection. This paper addresses the issue of coupling between windings and how it affects the features of FRA responses. Using a 1000-MVA 400/275-kV autotransformer as an example, a double-peak feature is distinctively observed in the frequency band between 2 and 20 kHz on the series and common windings´ FRA responses. Through transformer modeling and simulation sensitivity studies, it is found that the coupling through the delta tertiary connection affects the first resonance of the double-peak feature while the coupling between windings of the same phase, especially between the series and the common winding, influences the anti-resonance and the second resonance of the double-peak feature.