IEC05000216 V2 ENFigure 81: High impedance earthing networkThe operation of high impedance earthed networks is different compared to solidearthed networks, where all major faults have to be cleared very fast. In highimpedance earthed networks, some system operators do not clear single phase-to-earth faults immediately; they clear the line later when it is more convenient. In caseof cross-country faults, many network operators want to selectively clear one of thetwo earth faults.In this type of network, it is mostly not possible to use distance protection for detectionand clearance of earth faults. The low magnitude of the earth-fault current might notgive start of the zero-sequence measurement elements or the sensitivity will be too lowfor acceptance. For this reason a separate high sensitive earth-fault protection isnecessary to carry out the fault clearance for single phase-to-earth fault. For cross-country faults and when using phase preference, it is necessary to make sure that thedistance protection is operating in the phase-to-earth loops independently, wheneverpossible. See guidelines for setting INReleasePE.8.2.2.2 Fault infeed from remote endAll transmission and most all sub-transmission networks are operated meshed.Typical for this type of network is that fault infeed from remote end will happen whenfault occurs on the protected line. The fault current infeed will enlarge the faultimpedance seen by the distance protection. This effect is very important to keep inmind when both planning the protection system and making the settings.With reference to figure 82, 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 (Equation 39)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 (Equation 40)1MRK 502 071-UEN - Section 8Impedance protectionGenerator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 191Application manual
