IcIcIcILIRen05000216_ansi.vsdANSI05000216 V1 ENFigure 51: High impedance grounded networkThe operation of high impedance grounded networks is different compared to solidgrounded networks where all major faults have to be cleared very fast. In high impedancegrounded networks, some system operators do not clear single phase-to-ground faultsimmediately; they clear the line later when it is more convenient. In case of cross-countryfaults, many network operators want to selectively clear one of the two ground faults. Tohandle this type of phenomenon, a separate function called Phase preference logic(PPLPHIZ) is needed in medium and subtransmission network.In this type of network, it is mostly not possible to use distance protection for detection andclearance of ground faults. The low magnitude of the ground-fault current might not givepickup of the zero-sequence measurement elements or the sensitivity will be too low foracceptance. For this reason a separate high sensitive ground-fault protection is necessaryto carry out the fault clearance for single phase-to-ground fault.6.1.2.2 Fault infeed from remote endAll transmission and most all sub-transmission networks are operated meshed. Typical forthis type of network is that fault infeed from remote end will happen when fault occurs onthe protected line. The fault current infeed will enlarge the fault impedance seen by thedistance protection. This effect is very important to keep in mind when both planning theprotection system and making the settings.With reference to figure 52, the equation for the bus voltage VA at A side is:( )A A L A B fV I p Z I I R= × × + + ×EQUATION1273 V1 EN (Equation 41)If we divide VA by IA we get Z present to the IED at A side.1MRK 506 334-UUS A Section 6Impedance protection117Application manual
