8.5.3 Setting guidelines SEMOD171961-4 v9The sensitive earth fault protection is intended to be used in high impedance earthed systems,or in systems with resistive earthing where the neutral point resistor gives an earth faultcurrent larger than what normal high impedance gives but smaller than the phase-to-phaseshort circuit current.In a high impedance system the fault current is assumed to be limited by the system zerosequence shunt impedance to earth and the fault resistance only. All the series impedances inthe system are assumed to be zero.In the setting of earth fault protection, in a high impedance earthed system, the neutral pointvoltage (zero sequence voltage) and the earth fault current will be calculated at the desiredsensitivity (fault resistance). The complex neutral point voltage (zero sequence) can becalculated as:phase0f0UU 3 R1 Z= ×+EQUATION1943 V1 EN-US (Equation 102)WhereUphase is the phase voltage in the fault point before the fault,Rf is the resistance to earth in the fault point andZ0 is the system zero sequence impedance to earthThe fault current, in the fault point, can be calculated as:phasej 00 f3 UI 3I Z 3 R×= = + ×EQUATION1944 V1 EN-US (Equation 103)The impedance Z0 is dependent on the system earthing. In an isolated system (without neutralpoint apparatus) the impedance is equal to the capacitive coupling between the phaseconductors and earth:phase0 cj3 UZ jX j I×= - = -EQUATION1945 V1 EN-US (Equation 104)WhereIj is the capacitive earth fault current at a non-resistive phase-to-earth faultXc is the capacitive reactance to earthIn a system with a neutral point resistor (resistance earthed system) the impedance Z0 can becalculated as:Section 8 1MRK 505 363-UEN ACurrent protection156Application manual
