6.2.7 Operation principle6.2.7.1 Fundamental principles of the restricted earth-fault protectionRestricted earth-fault protection, low impedance function (REFPDIF) detects earthfaults on earthed power transformer windings, most often an earthed star winding.REFPDIF is a winding protection of the differential type. Since REFPDIF is based onthe zero sequence current, which theoretically only exists in case of an earth fault,REFPDIF can be made very sensitive regardless of normal load currents. It is thefastest protection a power transformer winding can have. Remember that the highsensitivity and the high speed tend to make such a protection unstable. Specialmeasures must be taken to make it insensitive to conditions for which it should notoperate, for example, heavy through faults of phase-to-phase type or heavy externalearth faults.REFPDIF is of the low impedance type. All three-phase currents, and the neutral pointcurrent, must be fed separately to REFPDIF. The fundamental frequency componentsof all currents are extracted from all input currents, while other eventual zero sequencecomponents, such as the 3rd harmonic currents, are fully suppressed. Then the residualcurrent phasor is calculated from the three line current phasors. This zero sequencecurrent phasor is then added to the neutral current vectorially, in order to obtaindifferential current.The following facts may be observed from figure 45 and figure 46, where the three lineCTs are shown as connected together in order to measure the residual 3Io current, forthe sake of simplicity.Return path viapower systemL2L3L1L2L3IN = -3Izs1Uzs Current in the neutral(measured as IN ) servesas a directional referencebecause it has the samedirection for both internaland external faults.zone of protectionIzs1Izs1Izs1IfaultROAblockINExternalfaultregionL1Zero-sequence differentialcurrent for external faultIdiff = abs(3I0 + IN )Idiff = 3Izs1 - 3Izs1 = 03I0blockblockoperateReturn path throughtransformerIL1+ IL2+IL3 = 3I03I0 = 3Izs1(Summation in the IED)PowersystemExternalfaultregionInternalfaultregion ROA = Relay Operate Angle3Izs1IEC09000107-3-en.vsdThese threezero-sequencecurrents are not measuredIEC09000107-3 V1 ENFigure 45: Zero sequence currents at an external earth fault1MRK 504 135-UEN A Section 6Differential protection113Technical manual
