power distribution - Siemens

TechnicalGround Fault ProtectionSource Ground Current (or GroundReturn) MethodThis method of detecting the groundfault current Ig locates the groundsensor on the neutral connection toground at the service entrance. Thismeans that the ground sensor onlydetects ground fault current. This type ofdetection has some limitations becauseit is detecting the ground fault returncurrent. On multiple source systemswith multiple connections to ground, thisground fault current can return by morethan one path, therefore, somesensitivity in detecting these faultswould be lost.Residual Connection MethodCurrent sensors, one on each of thephase conductors and on the neutralconductors, are connected in common.This common (or residual connection)measures the vector summation of thephase currents and the neutral current.Under normal conditions, this vectorsummation will be zero, and no currentwill be applied to the ground relay.If a fault involving ground occurs, thecurrent summation is not equal to zero.Current flows into common connectionwhich is applied to the relay. Thismethod of detecting ground fault currentis used in circuit breakers with electronictrip device.Figure 11. Schematic for Zero SequenceFigure 12. Schematic for Source Ground CurrentGFS = Ground Fault SensorGFP = Ground Fault Protection(Relay or Trip Unit)SelectionResidual Ground CurrentSensing3-Wire SystemThis system is used with electronic tripunits, and always includes three currentsensors mounted on the circuit breaker.A trip element is connected in serieswith each sensor to provide phaseovercurrent protection. By adding aground trip element in the residual(neutral) circuit of the three currentsensors, it will sense ground fault currentonly, and not load current. This permitsmore sensitive settings to protectagainst low magnitude ground faults.This scheme is shown in Figure 14.Under normal conditions, the vector sumof the current in all of the phases equalszero. No current would flow in the GNDelement, which is also true under thecondition of a phase-to-phase fault.A phase-to-ground fault would cause acurrent to flow in the GND trip element.If the magnitude of this current exceedsthe pickup setting for the required time,the trip unit will operate to trip thebreaker.Figure 13. Schematic for Residual MethodFigure 14. Schematic for Ground Protection on 3-Wire Systems, Residual Sensing18 TECHNICALSiemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog 18-7