Fault resistance GUID-83E3E475-3243-4308-9A91-B8DD9B47C276 v4The performance of distance protection for single phase-to-earth faults is veryimportant, because normally more than 70% of the faults on transmission lines aresingle phase-to-earth faults. At these faults, the fault resistance is composed ofthree parts: arc resistance, resistance of a tower construction, and tower-footingresistance.The resistance is also depending on the presence of earth shieldconductor at the top of the tower, connecting tower-footing resistance in parallel.The arc resistance can be calculated according to Warrington's formula:1.428707 LRarc I×=EQUATION1456 V1 EN-US (Equation 53)where:L represents the length of the arc (in meters). This equation applies for the distance protectionzone 1. Consider approximately three times arc foot spacing for the zone 2 and wind speedof approximately 50 km/hI is the actual fault current in A.In practice, the setting of fault resistance for both phase-to-earth RFPEZx andphase-to-phase RFPPZx should be as high as possible without interfering with theload impedance in order to obtain reliable fault detection.7.1.3 Setting guidelines IP14962-1 v17.1.3.1 General GUID-BA20D421-4435-44CA-A6E9-743E461F8C59 v2The settings for Distance measuring zones, quadrilateral characteristic (ZMFPDIS)are done in primary values. The instrument transformer ratio that has been set forthe analog input card is used to automatically convert the measured secondaryinput signals to primary values used in ZMFPDIS .The following basics must be considered, depending on application, when doingthe setting calculations:• Errors introduced by current and voltage instrument transformers, particularlyunder transient conditions.• Inaccuracies in the line zero-sequence impedance data, and their effect on thecalculated value of the earth-return compensation factor.• The effect of infeed between the IED and the fault location, including theinfluence of different Z0/Z1 ratios of the various sources.• The phase impedance of non transposed lines is not identical for all faultloops. The difference between the impedances for different phase-to-earthloops can be as large as 5-10% of the total line impedance.• The effect of a load transfer between the IEDs of the protected fault resistanceis considerable, the effect must be recognized.• Zero-sequence mutual coupling from parallel lines.Section 7 1MRK 505 393-UEN BImpedance protection122 Line differential protection RED650 2.2 IECApplication manual
