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This paper presents an innovative implementation of the wavelet packet transform (WPT) using Butterworth passive (BP) filters for differential protection of power transformers. The proposed implementation is based on designing cascaded stages of high-pass third-order Butterworth filters with cutoff frequencies identical to the cutoff frequencies of WPT-associated digital quadrature mirror filters. These high-pass filters (HPFs) are designed to extract the second-level high-frequency components present in three-phase differential currents. Extraction of these frequency components is required in order to detect and classify transients in three-phase power transformers. The output of the designed Butterworth HPFs is utilized to initiate a trip signal in case of internal-fault currents. The third-order Butterworth HPFs are designed to simplify their practical implementation as well as their integration with the differential protective relay for the tested power transformer. Different magnetizing inrush, through-fault, and internal-fault currents are investigated for different loading conditions. Performances of the proposed BP-filter-based differential relay are compared with those of the digital WPT-based relay. Comparison results show that the Butterworth filter WPT-based differential relay is able to provide a low-cost, good diagnosis, and fast responses to internal-fault currents.