zero value. However, during heavy faults, CT saturation might cause the measuredphase angle to differ from 180 degrees for an external, and from 0 degrees for aninternal fault. See figure 36 for an example of a heavy internal fault with transient CTsaturation.en05000190.vsd0.5 kA302106024090270120300150330180 0HV s ide contribution to the tota l ne ga tive s e que nce diffe re ntia l curre nt in kADire ctiona l limit (within the re gion de limite d by ± 60 de gre e s is inte rna l fa ult)1.0 kA1.5 kAde finite lya n inte rna lfa ultInte rna l fa ultde cla re d 7 msa fte r inte rna lfa ult occure dtrip c o mmandin 12 mse xcurs ionfrom 0 de gre e sdue to CTs a tura tione xte rna lfa ultre gion35 msDire ctiona l Compa ris on Crite rion: Inte rna l fa ult a s s e e n from the HV s ideIEC05000190 V1 ENFigure 36: Operation of the internal/external fault discriminator for internal faultwith CT saturationIt shall be noted that additional security measures are implemented in the internal/external fault discriminator algorithm in order to guarantee proper operation withheavily saturated current transformers. The trustworthy information on whether afault is internal or external is typically obtained in about 10ms after the fault inception,depending on the setting IminNegSeq, and the magnitudes of the fault currents. Duringheavy faults, approximately 5ms time to full saturation of the main CT is sufficient inorder to produce a correct discrimination between internal and external faults.6.1.4.9 Unrestrained, and sensitive negative sequence protectionsTwo sub functions are based on the internal/external fault discriminator and have theability to trip a faulty power transformer, are parts to the traditional power transformerdifferential protection.The unrestrained negative sequence differential protectionThe unrestrained negative sequence protection is activated if one or more start signalshave been set by the traditional differential protection algorithm. This happens1MRK 504 135-UEN A Section 6Differential protection101Technical manual
