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This paper presents a novel formulation for differential protection of three-phase transformers. The discrete wavelet transform (DWT) is employed to extract transitory features of transformer three-phase differential currents to detect internal faulty conditions. The performance of the proposed algorithm is evaluated through simulation of faulty and non-faulty test cases on a power transformer using ATP/EMTP software. The optimal mother wavelet selection includes performance analysis of different mother wavelets and resolution number of levels. In order to test the formulations performance, the proposed method was implemented on MatLabreg environment. Simulated comparative test results with a percentage differential protection with harmonic restraint formulation shows that the proposed technique improves the discrimination performance. Simulated test cases of magnetizing inrush and close by external faults are also presented in order to test the performance of the proposed method in extreme conditions.