7.1.2.4 Short line application GUID-331451C9-EA93-481A-BA2E-D729EDB98828 v4In short line applications, the major concern is to get sufficient fault resistancecoverage. Load encroachment is not such a common problem. The line length thatcan be recognized as a short line is not a fixed length; it depends on systemparameters such as voltage and source impedance, see table 13.Table 13: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.For very short line applications, the underreaching zone 1 can not be used due tothe fact that the voltage drop distribution throughout the line will be too lowcausing risk for overreaching.7.1.2.5 Long transmission line application GUID-0C99A197-06CD-4668-A017-3CA9E5E01ED2 v4For long transmission lines, the margin to the load impedance, that is, to avoid loadencroachment, will normally be a major concern. It is well known that it is difficultto achieve high sensitivity for phase-to-earth fault at remote line end of long lineswhen 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 14. Long lines have Sourceimpedance ratio (SIR’s) less than 0.5.Table 14: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 54.1MRK 505 393-UEN B Section 7Impedance protectionLine differential protection RED650 2.2 IEC 113Application manual
