7.1.2.2 Fault infeed from remote end GUID-E28CCC52-497F-4B79-9430-8A2155FED936 v4All transmission and most all sub-transmission networks are operated meshed.Typical for this type of network is that fault infeed from remote end will happenwhen fault occurs on the protected line. The fault current infeed will enlarge thefault impedance seen by the distance protection. This effect is very important tokeep in mind when both planning the protection system and making the settings.With reference to figure 53, the equation for the bus voltage UA at A side is:U I p Z I I RfA A L A B= ⋅ ⋅ + +( )⋅EQUATION1273-IEC-650 V2 EN-US (Equation 34)If we divide UA by IA we get Z present to the IED at A side.IA + IBUAIA IAZA = = p ·ZL + ·RfEQUATION1274-IEC-650 V1 EN-US (Equation 35)The infeed factor (IA+IB)/IA can be very high, 10-20 depending on the differencesin source impedances at local and remote end.Z <ZLZ <ESAUA UBA B ESBIA IBRfp*ZL (1-p)*ZLZSA ZSBIEC09000247-1-en.vsdIEC09000247 V1 EN-USFigure 53: Influence of fault current infeed from remote line endThe effect of fault current infeed from the remote line end is one of the mostdriving factors to justify complementary protection for distance protection.When the line is heavily loaded, the distance protection at the exporting end willhave a tendency to overreach. To handle this phenomenon, the IED has an adaptivebuilt-in algorithm, which compensates the overreach tendency of zone 1 at theexporting end. No settings are required for this feature.1MRK 505 393-UEN B Section 7Impedance protectionLine differential protection RED650 2.2 IEC 111Application manual
