As the magnitude of the residual current is independent of the fault location, the selectivity of theground fault protection is achieved by time selectivity.When should the sensitive directional residual overcurrent protection be used and when shouldthe sensitive directional residual power protection be used? Consider the following:• Sensitive directional residual overcurrent protection gives possibility for better sensitivity.The setting possibilities of this function are down to 0.25 % of IBase, 1 A or 5 A. Thissensitivity is in most cases sufficient in high impedance network applications, if themeasuring CT ratio is not too high.• Sensitive directional residual power protection gives possibility to use inverse timecharacteristics. This is applicable in large high impedance grounded networks, with largecapacitive ground fault currents. In such networks, the active fault current would be small andby using sensitive directional residual power protection, the operating quantity is elevated.Therefore, better possibility to detect ground faults. In addition, in low impedance groundednetworks, the inverse time characteristic gives better time-selectivity in case of high zero-resistive fault currents.PhasecurrentsPhasegroundvoltagesINVNANSI13000013-1-en.vsdANSI13000013 V1 EN-USFigure 56: Connection of SDEPSDE to analog preprocessing function blockOvercurrent functionality uses true 3I0, i.e. sum of GRPxA, GRPxB and GRPxC. For 3I0 to becalculated, connection is needed to all three phase inputs.Directional and power functionality uses IN and VN. If a connection is made to GRPxN this signal isused, else if connection is made to all inputs GRPxA, GRPxB and GRPxC the internally calculatedsum of these inputs (3I0 and 3V0) will be used.8.4.3 Setting guidelines SEMOD171961-4 v8The sensitive ground-fault protection is intended to be used in high impedance groundedsystems, or in systems with resistive grounding where the neutral point resistor gives an ground-fault current larger than what normal high impedance gives but smaller than the phase to phaseshort circuit current.1MRK 511 364-UUS A Section 8Current protection127Application manual
