• The impedance corresponding to the protected line, plus the first zone reach of theshortest adjacent line.• The impedance corresponding to the protected line, plus the impedance of themaximum number of transformers operating in parallel on the bus at the remote endof the protected line.Larger overreach than the mentioned 80% can often be acceptable due to fault currentinfeed from other lines. This requires however analysis by means of fault calculations.If the chosen zone 2 reach gives such a value that it will interfere with zone 2 on adjacentlines, the time delay of zone 2 must be increased by approximately 200 ms to avoidunwanted operation in cases when the telecommunication for the short adjacent line at theremote end is down during faults. The zone 2 must not be reduced below 120% of theprotected line section. The whole line must be covered under all conditions.The requirement that the zone 2 shall not reach more than 80% of the shortest adjacent lineat remote end is highlighted in the example below.If a fault occurs at point F, see figure 121, the IED at point A senses the impedance:Z VI Z I II Z I I II R Z II ZAFAAACA CACFA C BAF ACCAC= = + + ⋅ + + + ⋅ = + + ⋅1 FFC BAFI II R+ + + ⋅1EQUATION302 V5 EN (Equation 380)A B21CI AIBZ AC Z CBZ CFI A+ IBANSI05000457-2-en.vsdFANSI05000457 V2 ENFigure 220: Setting of overreaching zone8.13.4.4 Setting of reverse zoneThe reverse zone (zone RV) is applicable for purposes of scheme communication logic,current reversal logic, weak-end infeed logic, and so on. The same applies to the back-upprotection of the bus bar or power transformers. It is necessary to secure, that it alwaysSection 8 1MRK 506 369-UUS -Impedance protection422 Line distance protection REL670 2.2 ANSIApplication manual
