Frequency Response Analysis (FRA) is a powerful technique to detect internal abnormalities within power transformers. However, diagnosis criteria of power transformers by FRA have not been fully established yet. In this paper, a new evaluation method of degree of identification between two transfer functions is proposed. In the proposed method, a frequency window is employed and Cross Correlation Coefficient (CCF) is calculated with moving the window. Furthermore, the proposed method is applied to transfer functions of real transformers with or without some abnormalities. Based on this discussion, objective criteria for diagnosis of abnormalities of transformer windings by FRA is proposed. The proposed criteria are based on a few data of transformers with some abnormalities. Therefore the proposed criteria are tentative and they would be improved by accumulation of further data.
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