The application of nonunit protection to six-phase transmission lines

Six-phase transmission offers increased power carrying capacity over existing right-of-ways and hence is an attractive option for future system development. As with other transmission systems, these lines will normally be protected with a combination of unit and nonunit protection. Although few problems are expected with unit protection schemes, nonunit protection, typically distance relays, does present some difficulties due to limitations with phase selection. The suitability of using nonunit distance protection has been examined by studying the impedances seen by the array of comparators required to protect a six-phase circuit for a variety of fault conditions.