5.3.6.3 Supplementary criteriaTo relieve the burden of constructing an exact optimal operate-restraincharacteristic, two special features supplement the basic stabilized differentialprotection function, making Generator differential protection GENPDIF a veryreliable 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.It discriminates 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(that is, 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 quicklywithout any 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,are free of harmonic pollution, that is, if no harmonic block signal has been set,then a (minor) internal fault, simultaneous with a predominant external fault, canbe suspected. 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 indicationof a 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 fundamentalfrequency components (50/60 Hz) of the decaying DC offset of the fault currents,the system is not fully symmetrical immediately after the fault. Due to the transientexistence of the negative sequence system, faults can be distinguished as internal orexternal, even for three-phase faults.The internal or external fault discriminator responds to the relative phase angles ofthe negative sequence fault currents at both ends of the stator winding. Observethat the source of the negative sequence currents at unsymmetrical faults is at thefault point.Section 5 1MRK 502 034-UEN -Differential protection108Technical Manual
