7.1.3.8 Zone reach setting higher than minimum load impedanceGUID-78D0227F-2568-4C9A-8921-45812B4ABAF2 v5The impedance zones are enabled as soon as the (symmetrical) load impedancecrosses the vertical boundaries defined by RLdFw and RLdRv [4] or the linesdefined by ArgLd. So, it is necessary to consider some margin. It is recommendedto set RLdFw and RLdRv to 90% of the per-phase resistance that corresponds tomaximum load.min0.9 loadRLdFw R< ×IECEQUATION2419 V2 EN-US (Equation 73)min0.9 loadRLdRv R< ×IECEQUATION2420 V2 EN-US (Equation 74)The absolute value of the margin to the closest ArgLd line should be of the sameorder, that is, at least 0.1 • Zload min.The load encroachment settings are related to a per-phase load impedance in asymmetrical star-coupled representation. For symmetrical load or three-phase andphase-to-phase faults, this corresponds to the per-phase, or positive-sequence,impedance. For a phase-to-earth fault, it corresponds to the per-loop impedance,including the earth return impedance.RXRLdFwRLdRvArgLd90%10%10%ARGLdPossibleloadARGLdARGLdRXRLdFwRLdRvXLdXLdARGLdARGLd ARGLdARGLdIEC12000176-2-en.vsdIEC12000176 V2 EN-USFigure 63: Load impedance limitation with load encroachmentDuring the initial current change for phase-to-phase and for phase-to-earth faults,operation may be allowed also when the apparent impedance of the loadencroachment element is located in the load area. This improves the dependabilityfor fault at the remote end of the line during high load. Although it is not associatedto any standard event, there is one potentially hazardous situation that should beconsidered. Should one phase of a parallel circuit open a single pole, even thoughthere is no fault, and the load current of that phase increase, there is actually noway of distinguish this from a real fault with similar characteristics. Should this[4] RLdRv=RLdRvFactor*RLdFw1MRK 505 393-UEN B Section 7Impedance protectionLine differential protection RED650 2.2 IEC 129Application manual
