Limitations of distance-type protective equipment when applied to long extremely-high-voltage power lines

On power lines the currents which flow solely in the phase conductors travel near the velocity of light whereas those which return through the ground and overhead earth wires travel more slowly, the speed depending on the line spacings, conductor sizes and nature of the ground. The full steady-state equations for a 3-phase line are developed and a particular line is examined to show the magnitude of this effect. The effect of transposing a line is also indicated. The steady-state performance of basic distance protection fitted with compensating equipment to allow for the effects of the currents in the sound phases during earth faults is studied, using the line equations. It is shown that large errors of measurement occur under earth-fault conditions on long lines fed from small sources. The performance for faults which do not involve earth is satisfactory. It is shown that non-transposition of a line makes the apparent impedance of a fault on a line depend on the source impedance. This causes further errors in the assessment of the positions of faults of all types. The high transient time-constants on long lines increase the chance of maloperation during the transient period. It is concluded that distance protection may not be satisfactory on very long lines and that a study must be made of the complete power system before a decision can be made about its suitability for any particular application.