Polyphase distance protection characteristics

The paper considers the steady-state performance of existing schemes of polyphase distance protection for high-voltage systems and introduces new polyphase schemes for phase and earth-fault protection. Differences in the performance of various schemes are highlighted and a comparison is given for close-up faults and zone 1 boundary faults. A new approach is suggested to assess the high-speed tripping characteristic of a polyphase unit and a method is given to establish whether or not high-speed protection is possible during close-up faults. Results of computer studies indicate that prefault load and power swings have a less marked effect on the reach of the new scheme during resistive faults, than is the case with a conventional 6-element scheme. Comparisons are also made with five switched-distance schemes supplied by relay manufacturers in Western Europe and the advantages and disadvantages of the polyphase approach are listed