8.13.2.4 Short line application GUID-7FA1B585-C979-4D51-A502-9C4070B99F0E v2In short line applications, the major concern is to get sufficient fault resistancecoverage. Load encroachment is not so common. The line length that can berecognized as a short line is not a fixed length; it depends on system parameterssuch as voltage and source impedance, see table 30.Table 30:Definition of short and very short lineLine category Un Un110 kV 500 kVVery short line 1.1-5.5 km 5-25 kmShort line 5.5-11 km 25-50 kmThe IED's ability to set resistive and reactive reach independent for positive andzero sequence fault loops and individual fault resistance settings for phase-to-phaseand phase-to-earth fault together with load encroachment algorithm improves thepossibility to detect high resistive faults without conflict with the load impedance,see figure 186.For very short line applications, the underreaching zone 1 cannot be used due to thevoltage drop distribution throughout the line will be too low causing risk foroverreaching.Load encroachment is normally no problem for short line applications.8.13.2.5 Long transmission line application GUID-AD1A1A5B-B4F5-4B48-9DBD-59D28D71BA9C v3For long transmission lines, the margin to the load impedance to avoid loadencroachment is a major concern. It is difficult to achieve high sensitivity forphase-to-earth fault at remote line end of long lines when the line is heavy loaded.What can be recognized as long lines with respect to the performance of distanceprotection can generally be described as in table 31, long lines have sourceimpedance ratio (SIR’s) less than 0.5.Table 31:Definition of long and very long linesLine category Un Un110 kV 500 kVLong lines 77 km - 99 km 350 km - 450 kmVery long lines > 99 km > 450 kmThe IED's ability to set resistive and reactive reach independent for positive andzero sequence fault loops and individual fault resistance settings for phase-to-phaseand phase-to-earth fault together with load encroachment algorithm improves thepossibility to detect high resistive faults at the same time as the security isimproved (risk for unwanted trip due to load encroachment is eliminated), seefigure 187.Section 8 1MRK 506 369-UEN BImpedance protection364 Line distance protection REL670 2.2 IECApplication manual
