CT saturation causes 2nd harmonics of considerable value on the CT secondaryside, which contributes to the stabilization of the relay at through fault conditions.It is strongly recommended to maintain a sensitive setting of the I2/I1Ratio alsowhen a power transformer is not included in the protected zone.I5/I1Ratio M12541-206 v4The set value is the ratio of the 5th harmonic of the differential current to thefundamental frequency of the differential current. To obtain this information, theinstantaneous differential current must be analyzed.A 20...30% over-excitation of a transformer can cause an increase in the excitationcurrent of 10...100 times the normal value. This excitation current is a truedifferential current if the transformer is inside the protected zone. It has a highdegree of the 5th harmonic, and the default setting of 25% will be suitable in mostcases to detect the phenomenon. As CT saturation also causes 5th harmonics on thesecondary side, it is recommended to maintain this setting at 25% or lower even ifno power transformer is included in the protected zone.6.1.3.4 Internal/external fault discriminator M12541-210 v2NegSeqDiffEn M12541-213 v4The negative sequence fault discriminator can be set On/Off. It is an importantcomplement to the percentage restrained differential function. As it is directional, itcan distinguish between external and internal faults also in difficult conditions,such as CT saturation, and so on. It is strongly recommended that it is alwaysactivated (On).NegSeqROA M12541-218 v4This is the setting of the relay operate angle of the negative sequence current basedinternal/external fault discriminator. The directional test is made so that the endwith the highest negative sequence current is found. Then, the sum of the negativesequence currents at all other circuit ends is calculated. Finally, the relative phaseangle between these two negative sequence currents is determined. See figure 40.Ideally the angle is zero degrees for internal faults and 180 degrees for externalfaults. However, measuring errors caused by, for example, CT saturation as well asdifferent phase angles of the negative sequence impedances, require a safetymargin, expressed as the ROA (Relay Operate Angle). The default value 60degrees is recommended in most cases. The setting NegSeqROA is a compromisebetween the security and dependability of the differential protection. The valueNegSeqROA = 60 degrees emphasizes security against dependability. Tests haveproven that 60 degrees is a good choice.Section 6 1MRK 505 393-UEN BDifferential protection92 Line differential protection RED650 2.2 IECApplication manual
