7.6.2 Application SEMOD171959-4 v11In networks with high impedance grounding, the phase-to-ground fault current is significantlysmaller than the short circuit currents. Another difficulty for ground fault protection is that themagnitude of the phase-to-ground fault current is almost independent of the fault location in thenetwork.Directional residual current can be used to detect and give selective trip of phase-to-ground faultsin high impedance grounded networks. The protection uses the residual current component 3I0 ·cos φ, where φ is the angle between the residual current and the residual voltage (-3V0),compensated with a characteristic angle. Alternatively, the function can be set to strict 3I0 levelwith a check of angle φ.Directional residual power can also be used to detect and give selective trip of phase-to-groundfaults in high impedance grounded networks. The protection uses the residual power component3I0 · 3V0 · cos φ, where φ is the angle between the residual current and the reference residualvoltage, compensated with a characteristic angle.A normal non-directional residual current function can also be used with definite or inverse timedelay.A backup neutral point voltage function is also available for non-directional residual overvoltageprotection.In an isolated network, that is, the network is only coupled to ground via the capacitancesbetween the phase conductors and ground, the residual current always has -90º phase shiftcompared to the residual voltage (3V0). The characteristic angle is chosen to -90º in such anetwork.In resistance grounded networks or in Petersen coil grounded, with a parallel resistor, the activeresidual current component (in phase with the residual voltage) should be used for the groundfault detection. In such networks, the characteristic angle is chosen to 0º.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.1MRK 511 358-UUS A Section 7Current protection155Application manual
