Computer-aided setting calculation for distance zone 2 and zone 3 protection

Distance protection has been widely used in the protection of EHV and UHV transmission lines. One of the advantages of using distance protection is that its built-in zone 2 and zone 3 elements provide local relay back-up as well as remote system back-up protection. Typically the reach settings of zone 2 and zone 3 are taken respectively as the protected line plus 25% of the shortest second line and the protected line plus the longest second line plus 25% of the longest third line. Distance reach setting determination can be a relatively complicated procedure in view of the large volume of data that has to be processed. With the help of computer, the impedance seen by a distance relay at a particular fault location under various generation and system configuration can be calculated. The use of a computer allows the protection engineer to concentrate on judgment and evaluation of the performance of the distance relay at the chosen reach setting, such as the zone 2 or 3 coverage, the load encroachment and the power swing problem. With computerization, the time for distance setting review is shortened. The systematic approach allows regular reviews to be conducted as required, such as after the introduction of new generation plants, new circuits or transformers in the system