This DC desensitization is not active, if a disturbance has been detectedand characterized as internal fault.6.3.6.3 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 (87G) 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. The discriminatoris the main part of what is here called the negative-sequence-current-based differentialprotections. 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 quickly withoutany further delay.If a fault (disturbance) is classified as external, then generally, but not unconditionally,a trip command is prevented. If a fault is classified as external, harmonic analysis ofthe fault conditions is initiated.If all the differential currents which caused their respective pickup 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 highly pollutedby 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 works satisfactorilyeven in case of three-phase faults. Because of the fundamental frequency components(50/60 Hz) of the decaying DC offset of the fault currents, the system is not fullysymmetrical immediately after the fault. Due to the transient existence of the negativeSection 6 1MRK 502 043-UUS BDifferential protection124Technical Manual
