IEC05000215 V2 EN-USFigure 51: Solidly earthed networkThe earth-fault current is as high or even higher than the short-circuit current. Theseries impedances determine the magnitude of the fault current. The shuntadmittance has very limited influence on the earth-fault current. The shuntadmittance may, however, have some marginal influence on the earth-fault currentin networks with long transmission lines.The earth-fault current at single phase-to-earth in phase L1 can be calculated asequation 28:L1 L101 2 0 f 1 N f3 U U3I Z Z Z 3Z Z ZZ×= =+ + + + +EQUATION1267 V3 EN-US (Equation 28)Where:UL1 is the phase-to-earth voltage (kV) in the faulty phase before faultZ1 is the positive sequence impedance (Ω/phase)Z2 is the negative sequence impedance (Ω/phase)Z0 is the zero sequence impedance (Ω/phase)Zf is the fault impedance (Ω), often resistiveZN is the earth-return impedance defined as (Z0-Z1)/3The high zero-sequence current in solidly earthed networks makes it possible to useimpedance measuring techniques to detect earth faults. However, distanceprotection has limited possibilities to detect high resistance faults and shouldtherefore always be complemented with other protection function(s) that can carryout the fault clearance in those cases.Effectively earthed networks GUID-39CAF169-315E-4E3E-9EE6-28CBF624B90E v5A network is defined as effectively earthed if the earth-fault factor fe is less than1.4. The earth-fault factor is defined according to equation 29.f UUepn= maxEQUATION1268 V4 EN-US (Equation 29)Section 7 1MRK 505 393-UEN BImpedance protection108 Line differential protection RED650 2.2 IECApplication manual
