Distance-type protective schemes for overhead lines and cables

Abstract

In Chapters 7–10 various schemes which are used to protect overload lines and cables have been considered. All of these schemes are of the unit type in that they should operate whenever faults above certain levels of current occur on the protected units, i.e. between the current transformers mounted at each of the ends of the units, and they should not operate when external faults are present or during any healthy conditions. In each case this desired performance is obtained by making use of information about the conditions at both ends of a protected unit at any time. As examples, the instantaneous input and output currents of lines are compared in current-comparison (Merz-Price) schemes, the phases of currents are compared in ‘phase-comparison’ schemes and the directions of currents are taken into account in interlock schemes. To effect the necessary comparisons, signals must be sent between the ends of protected units which may, of course, be quite long distances apart, and either pilot wires or optical fibres or other high frequency electronic communication equipment may be required for this purpose.