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Established harmonic restraint differential relay concepts have been enlarged upon to provide a new static multiple-winding transformer differential relay. The resulting design includes simplification of the frequency selective circuits and new techniques for rapidly determining the harmonic content of the applied currents. The relay is shown to have improved sensitivity, to be responsive to internal transformer winding faults in less than two cycles, and to the more severe type of fault in less than one cycle. Full security against inrush currents and overexcitation as well as to external faults is demonstrated. Poor performance of current transformers is shown not to adversely affect either the security or dependability of the relay. Heavy current test results are presented which confirm the relay principles.