to-ground fault together with load encroachment algorithm improves the possibility todetect high resistive faults without conflict with the load impedance, see figure 138.For very short line applications the underreaching zone 1 can not be used due to that thevoltage drop distribution through out the line will be too low causing risk foroverreaching.Load encroachment is normally no problems for short line applications so the loadencroachment function could be switched off (OperationLdCmp = Off). This will increasethe possibility to detect resistive close-in faults.8.5.2.6 Long transmission line applicationFor long transmission lines the margin to the load impedance that is, to avoid loadencroachment, will normally be a major concern. It is difficult to achieve high sensitivityfor phase-to-ground fault at remote end of a long lines when the line is heavy loaded.The definition of long lines with respect to the performance of distance protection is notedin table 25.Table 25:Definition of long linesLine category Vn Vn110 kV 500 kVLong lines 45-60 miles 200–250 milesVery long lines >60 miles >250 milesAs mentioned in the previous chapter, the possibility in IED to set resistive and reactivereach independent for positive and zero sequence fault loops and individual faultresistance settings for phase-to-phase and phase-to-ground fault together with loadencroachment algorithm improves the possibility to detect high resistive faults at the sametime as the security is improved (risk for unwanted trip due to load encroachment iseliminated).8.5.2.7 Parallel line application with mutual couplingGeneralIntroduction of parallel lines in the network is increasing due to difficulties to getnecessary area for new lines.Parallel lines introduce an error in the measurement due to the mutual coupling betweenthe parallel lines. The lines need not to be of the same voltage in order to experiencemutual coupling, and some coupling exists even for lines that are separated by 100 meters1MRK 506 369-UUS - Section 8Impedance protectionLine distance protection REL670 2.2 ANSI 285Application manual
