since it increases the magnitude of fault current in healthy circuit after the openingof first circuit breaker. The so called current reversal phenomenon may causeunwanted operation of protection on healthy circuit and this way endangers evenmore the complete system stability.To avoid the unwanted tripping, some manufacturers provide a feature in theirdistance protection which detects that the fault current has changed in direction andtemporarily blocks distance protection. Another method employed is to temporarilyblock the signals received at the healthy line as soon as the parallel faulty lineprotection initiates tripping. The second mentioned method has an advantage inthat not the whole protection is blocked for the short period. The disadvantage isthat a local communication is needed between two protection devices in theneighboring bays of the same substation.Distance protection used on series compensated lines must have a high overreachto cover the whole transmission line also when the capacitors are bypassed or outof service. When the capacitors are in service, the overreach will increasetremendously and the whole system will be very sensitive for false teleprotectionsignals. Current reversal difficulties will be accentuated because the ratio of mutualimpedance against self-impedance will be much higher than for a non-compensatedline.If non-unit protection is to be used in a directional comparison mode, schemesbased on negative sequence quantities offer the advantage that they are insensitiveto mutual coupling. However, they can only be used for phase-to-earth and phase-to-phase faults. For three-phase faults an additional protection must be provided.8.13.4 Setting guidelines IP14962-1 v18.13.4.1 General GUID-B9958CEF-90ED-4644-B169-C6B4A018193B v1The settings for Distance measuring zones, quadrilateral characteristic(ZMFCPDIS) are done in primary values. The instrument transformer ratio that hasbeen set for the analog input card is used to automatically convert the measuredsecondary input signals to primary values used in ZMFCPDIS.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.Section 8 1MRK 506 369-UEN BImpedance protection392 Line distance protection REL670 2.2 IECApplication manual
