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The basic relaying philosophy of the American Electric Power 765-kV system is covered. The use of duplicate primary protection, each based on a different operating principle, is explained. Of significance is the unique use of single sideband power-line carrier and transfer tripping. The effect of line oscillations and asymmetry of transmission line impedances is discussed. Included also is a discussion of relay characteristics, ac and dc control power, shielding and overvoltage, and equipment protection.