Performance requirements for relays on unusually long transmission lines

Analysis of fundamental bases of relaying made as part of the engineering for the 270-mile 154-kv interconnection between systems of Nebraska Power Company and Kansas Gas and Electric Company reveals that, of current, phase angle, impedance, and impedance angle, none alone offers satisfactory discrimination among open-circuit, short-circuit, and load conditions. Combination of input-impedance magnitude Z and impedance angle ψ = tan⁻¹ X/R is studied by means of “impedance circle diagrams” of the interconnecting circuit. On these diagrams is charted the dynamic behavior of the interconnected systems during faults and surges of synchronizing power. The desirable areas of trip and no trip are mapped, and general specifications for the performance of relay systems are presented in terms of impedance magnitude and angle, Z ∠ ψ. Thereby full advantage can be taken of all possible synchronizing power inherently available over the interconnection and at the same time faults and approach of out-of-step condition can be recognized properly.