6.4.6.3 Supplementary criteriaTo relieve the burden of constructing an exact optimal operate-restrain characteristic,two special features supplement the basic stabilized differential protection function,making Generator differential protection GENPDIF a very reliable one.The supplementary criteria are:• Internal/external fault discriminator (enhances, or blocks, the trip command)• Harmonic restrain (blocks only)The internal/external fault discriminator is a very reliable supplementary criterion. Itdiscriminates with a high speed between internal and external faults. Thediscriminator is the main part of what is here called the negative-sequence-current-based differential protections. It is recommended that this feature is always used (thatis, enabled, OpNegSeqDiff = On).If a fault is classified as internal, then any eventual block signals by the harmoniccriterion are ignored, and the differential protection can operate very quickly withoutany further delay.If a fault (disturbance) is classified as external, then generally, but notunconditionally, a trip command is prevented. If a fault is classified as external,harmonic analysis of the fault conditions is initiated.If all the differential currents which caused their respective start signals to be set, arefree of harmonic pollution, that is, if no harmonic block signal has been set, then a(minor) internal fault, simultaneous with a predominant external fault, can besuspected. This conclusion can be drawn because at external faults, major falsedifferential currents can only exist when one or more current transformers saturatetransiently. In this case, the false instantaneous differential currents are highlypolluted by higher harmonic components, the 2nd, and the 5th.The existence of relatively high negative-sequence currents is in itself an indication ofa disturbance, as the negative-sequence currents are superimposed, pure-faultquantities. The negative-sequence currents are measurable indications of abnormalconditions. The negative sequence currents are particularly suitable for directionaltests. The negative sequence internal or external fault discriminator workssatisfactorily even in case of three-phase faults. Because of the fundamental frequencycomponents (50/60 Hz) of the decaying DC offset of the fault currents, the system isnot fully symmetrical immediately after the fault. Due to the transient existence of thenegative sequence system, faults can be distinguished as internal or external, even forthree-phase faults.The internal or external fault discriminator responds to the relative phase angles of thenegative sequence fault currents at both ends of the stator winding. Observe that thesource of the negative sequence currents at unsymmetrical faults is at the fault point.• If the two negative sequence currents, as seen by the differential relay, flow in thesame direction (that is with the CTs oriented as in figure 50), the fault is internal.1MRK 502 048-UEN A Section 6Differential protection131Technical manual
