The protection of electrical power systems: a critical review of present-day practice and recent progress

The paper deals with the protection of generators, power transformers, feeders and busbars and with the characteristics of protective transformers. The subject-matter has been selected for the following purposes: (a) To raise some controversial issues relating to protective-gear practice in order to promote discussion upon them in the hope that this will lead to more general agreement. Among these issues are over-current and negative-phase-sequence protection of generators, earthing of the neutral point, the use of Post Office pilot wires for protection, the application ofcarrier-distance protection, the duplication and supervision of busbar-zone protection, and the overlapping of current-transformers. (b) To outline some new problems which have arisen. (c) To describe some new developments of importance, such as instantaneous differential protective system for power transformers, high-resistance pilot-wire scheme, a starting scheme for carrier protection and a biased differential system of busbar-zone protection. (d) To examine briefly the performance requirements of protective transformers.