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A transformer is one of the most important units in power networks; thus, fault diagnosis of transformers is quite significant. In this paper, the frequency-response analysis, deemed as a suitable diagnostic method for electrical and/or mechanical faults of a transformer, is employed to make a decision over a defective phase. To deal with wideband frequency responses of each phase, a synthetic spectral analysis is proposed, which augments low- and medium-frequency components, and equalizes the frequency intervals of a resulting combined curve by a log-frequency interpolation. Furthermore, for discriminating a defective phase through computing overall amounts of deviation with other phases, the two well-known criteria and three proposed criteria are examined with experiment data. The overall diagnosis results show that the proposed criterion discriminates a defective phase with the highest average hit ratio among all of the provided criteria for selected faults.