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A circuit model of a real transformer is established to reproduce the transfer functions of the transformer. This model is based on the configuration and dimensions of the real transformer. Using this circuit model, the sensitivities of the circuit elements to the resonant frequencies are analyzed. In frequency response analysis (FRA), this sensitivity analysis is helpful in the diagnosis of abnormalities by extracting the circuit elements having relatively high sensitivity to the shift of the resonant frequency. Furthermore, the ability to detect abnormalities in a transformer winding is discussed quantitatively by relating the shift of the resonant frequency and the degree of abnormality via variations of circuit elements on the basis of electromagnetic theory. The accumulation of such data for transformers with different classes of rated voltage and capacity, connections of phases, winding configurations, and so will contribute construction of diagnosis criteria for transformer windings by FRA.