1.428707 LRarc I×=EQUATION1456 V1 EN (Equation 129)where:L represents the length of the arc (in meters). This equation applies for the distance protection zone 1.Consider approximately three times arc foot spacing for the zone 2 and to give extra margin to theinfluence of wind speed and temperature.I is the actual fault current in A.In practice, the setting of fault resistance for both phase-to-ground RFPE and phase-to-phase RFPP should be as high as possible without interfering with the load impedance inorder to obtain reliable fault detection. However for zone1 it is necessary to limit the reachaccording to setting instructions in order to avoid overreach.8.3.3 Setting guidelines8.3.3.1 GeneralThe settings for Distance measuring zones, quadrilateral characteristic (ZMQPDIS, 21)are done in primary values. The instrument transformer ratio that has been set for theanalog input card is used to automatically convert the measured secondary input signalsto primary values used in ZMQPDIS (21).The following basics must be considered, depending on application, when doing thesetting calculations:• Errors introduced by current and voltage instrument transformers, particularly undertransient conditions.• Inaccuracies in the line zero-sequence impedance data, and their effect on thecalculated value of the ground-return compensation factor.• The effect of infeed between the IED and the fault location, including the influenceof different Z0/Z1 ratios of the various sources.• The phase impedance of non transposed lines is not identical for all fault loops. Thedifference between the impedances for different phase-to-ground loops can be aslarge as 5-10% of the total line impedance.• The effect of a load transfer between the IEDs of the protected fault resistance isconsiderable, the effect must be recognized.The load transfer on the protected lineshould be considered for resistive phase to ground faults• Zero-sequence mutual coupling from parallel lines.1MRK 506 369-UUS - Section 8Impedance protectionLine distance protection REL670 2.2 ANSI 241Application manual
