8.12.2.3 Load encroachment GUID-6785BF05-2775-4422-8077-A663D01C6C07 v6In some cases the measured load impedance might enter the set zone characteristicwithout any fault on the protected line. This phenomenon is called loadencroachment and it might occur when an external fault is cleared and highemergency load is transferred onto the protected line. The effect of loadencroachment is illustrated on the left in figure 172. A load impedance within thecharacteristic would cause an unwanted trip. The traditional way of avoiding thissituation is to set the distance zone resistive reach with a security margin to theminimum load impedance. The drawback with this approach is that the sensitivityof the protection to detect resistive faults is reduced.The IED has a built in feature which shapes the characteristic according to thecharacteristic shown in figure 172. The load encroachment algorithm will increasethe possibility to detect high fault resistances, especially for phase-to-earth faults atthe remote line end. For example, for a given setting of the load angle ArgLd, theresistive blinder for the zone measurement can be set according to figure 172affording higher fault resistance coverage without risk for unwanted operation dueto load encroachment. Separate resistive blinder settings are available in forwardand reverse direction.The use of the load encroachment feature is essential for long heavily loaded lines,where there might be a conflict between the necessary emergency load transfer andnecessary sensitivity of the distance protection. The function can also preferably beused on heavy loaded, medium long lines. For short lines, the major concern is toget sufficient fault resistance coverage. Load encroachment is not a majorproblem. .RZ1ArgLdRLdFwIEC09000248-3-en.vsdxXRLdRv[1]IEC09000248 V3 EN-USFigure 172: Load encroachment phenomena and shaped load encroachmentcharacteristic[1][1] RLdRv=RLdRvFactor*RLdFw1MRK 506 369-UEN B Section 8Impedance protectionLine distance protection REL670 2.2 IEC 337Application manual
