Distance protection: optimum dynamic design of static relay comparators

The ease with which transistor comparators for distance relays can be designed for high-speed operation must be tempered with consideration of overall performance and integrity of operation. Operating speed must be defined over the whole of the working range of the relay, and neither the speed nor the measuring accuracy should be unduly affected by the severe transients generated by modern e.h.v. networks. The comparator, as well as being proof against damaging voltage surges, must operate accurately in the presence of long-duration offset transients accentuated by low-loss modern-plant parameters and the random point-on-wave inception of faults due to natural hazards and closure or reclosure of modern pressure-head circuit breakers. The attainment of high-speed operation under these practical conditions precludes the adoption of many, apparently practical transistor-comparator circuits and favours the adoption of circuits with well defined dynamic performances. Extensive laboratory investigation has shown that the block-average comparison principle is amenable to precise design in all respects, and practical fast-operating relays can be designed with good transient-free characteristics. The results obtained on such a practical relay are presented in the paper for a phase comparator with a polarised-mho characteristic. It is shown that a minimum inherent operating time of one halfcycle of the power frequency can be defined for this comparator arrangement and that both the static and dynamic operating characteristics are predictable over the whole working range. Equivalent performance for the amplitude-comparator counterpart is justified in an appendix, and underlines earlier work. Relays using the block-average comparison principle have been used successfully in field trials since 1957, and this principle now forms the basis for various production designs. Sufficient field experience is now available to justify the theoretical analysis and laboratory test results gi- ven in the paper.