Composite polar characteristics in multizone systems of phase-comparison distance protection

The static distance-protection schemes now in use are based on 2-input relays that respond either to the relative amplitudes of the two input relaying signals or to the phase displacement between them. Both kinds of response afford a range of relay polar characteristics, but this may be considerably increased by extending the basic modes of comparison from which the characteristics may be derived. Of the two different comparison techniques, the paper is devoted to that of phase comparison rather than amplitude comparison, and its scope is extended by using more than two relaying signals to contribute to the comparison, by varying the angular limits of comparison, and by using two or more comparators in combination. The purpose of the paper is to examine possible applications of these forms of relaying in complete 3-zone protection schemes incorporating directional-reactance and mho characteristics and also the more recently derived family of discontinuous characteristics. In a 3-zone scheme of static protection, three or more signals may be conveniently arranged to effect phase comparison by combining, in relays extended to receive more than two signals, the signals that are ordinarily present in pairs at the relay inputs in the different zones. It is shown how, in this way, a directional-reactance characteristic may be derived from a single multi-input relay simultaneously energised by the zone-1 and zone-3 relaying signals of a conventional 3-zone reactance scheme, thereby avoiding the use of reactance and mho starting relays in combination. In a similar way, it is shown that the long-reach zone-3 characteristic of a 3-zone mho scheme may be shaped by a 3-input comparison method to increase its discriminating margins against load encroachments on high-capacity transmission circuits. Other combinations of zone characteristics are given where alternative forms of control may be desirable in meeting different requirements.