7.1.3.2 Setting of zone 1 GUID-ABD9A3B1-E420-471E-A798-4A56371E715E v1The different errors mentioned earlier usually require a limitation of theunderreaching zone (normally zone 1) to 75 - 90% of the protected line.In case of parallel lines, consider the influence of the mutual coupling according tosection "Parallel line application with mutual coupling" and select the case(s) thatare valid in the particular application. By proper setting it is possible to compensatefor the cases when the parallel line is in operation, out of service and not earthedand out of service and earthed in both ends. The setting of earth-fault reach shouldbe selected to be <95% also when parallel line is out of service and earthed at bothends (worst case).7.1.3.3 Setting of overreaching zone GUID-8D7D4C9D-468B-465F-85D9-3D64500474DB v2The first overreaching zone (normally zone 2) must detect faults on the wholeprotected line. Considering the different errors that might influence themeasurement in the same way as for zone 1, it is necessary to increase the reach ofthe overreaching zone to at least 120% of the protected line. The zone 2 reach canbe even higher if the fault infeed from adjacent lines at remote end is considerablehigher than the fault current at the IED location.The setting shall generally not exceed 80% of the following impedances:• The impedance corresponding to the protected line, plus the first zone reach ofthe shortest adjacent line.• The impedance corresponding to the protected line, plus the impedance of themaximum number of transformers operating in parallel on the bus at theremote end of the protected line.Larger overreach than the mentioned 80% can often be acceptable due to faultcurrent infeed from other lines. This requires however analysis by means of faultcalculations.If any of the above gives a zone 2 reach less than 120%, the time delay of zone 2must be increased by approximately 200ms to avoid unwanted operation in caseswhen the telecommunication for the short adjacent line at remote end is downduring faults. The zone 2 must not be reduced below 120% of the protected linesection. The whole line must be covered under all conditions.The requirement that the zone 2 shall not reach more than 80% of the shortestadjacent line at remote end is highlighted in the example below.If a fault occurs at point F see figure 62, 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-US (Equation 54)1MRK 505 393-UEN B Section 7Impedance protectionLine differential protection RED650 2.2 IEC 123Application manual
