protection zone, due to long duration of transformer inrush current the parameter should beset to 60 s. Otherwise a setting of 1 s is sufficient.IdUnre M12541-180 v5IdUnre is set as a multiple ofIBase. Values of differential currents above the unrestrained limitgenerate a trip disregarding all other criteria, that is, irrespective of the internal or externalfault discriminator and any presence of harmonics. It is intended for fast tripping of internalfaults with high fault currents. The recommended setting is 120% of the highest through faultcurrent that can appear on the protected line. Consequently, to set this value properly, thefault current must be calculated in each specific case.For a short line or a situation with a 1½ breaker bay, the through fault current might bepractically the same as the differential current at an internal fault. Extreme unequal CTsaturation at external faults could then be a risk of unwanted operation if the unrestrainedoperation is used. Consequently, if the through fault currents can be of the same order as themaximum differential currents at internal faults, for example, when there is a source of poweronly on one side (in the branch with the first CT), and only load on the other side (in the branchwith the other CT), it is recommended to refrain from using the unrestrained operation, bysetting the max valueIdUnre = 50 ×IBase.On long lines, the through fault current is often considerably less than the maximumdifferential current at internal faults, and a suitable setting of the unrestrained level is theneasy to calculate.When a transformer is included in the protected zone, the maximum inrush current must beconsidered when the unrestrained level is calculated. The inrush current appears from one sideof the transformer, while the maximum differential current at internal faults is limited by thesource impedances on all sides of the transformer. Observe that if the power transformer isenergized via a power line, even a short one, the inrush phenomenon is much less pronounced.The fundamental frequency of the inrush current will not be as high as when the powertransformer is connected directly to a source with very low internal impedance.EndSection1 M12541-186 v3EndSection1 is set as a multiple ofIBase. The default value 1.25 is generally recommended. Ifthe conditions are known more in detail, other values can be chosen in order to increase ordecrease the sensitivity.EndSection2 M12541-189 v3EndSection2 is set as a multiple ofIBase. The default value 3.00 is generally recommended. Ifthe conditions are known more in detail, other values can be chosen in order to increase ordecrease the sensitivity.SlopeSection2 M12541-192 v4SlopeSection2 is set as a percentage value: [delta operate current/delta restrain current] ×100%. The default value 40.0 is generally recommended. If the conditions are known more indetail, other values can be chosen in order to increase or decrease the sensitivity.SlopeSection3 M12541-195 v4SlopeSection3 is set as a percentage value: [delta operate current/delta restrain current] ×100%. The default value 80.0 is generally recommended. If the conditions are known more indetail, other values can be chosen in order to increase or decrease the sensitivity.6.2.3.3 The 2nd and 5th harmonic analysis M12541-198 v5The 2nd and 5th harmonic block scheme is permanently active only if an in-line powertransformer (A or B, alternatively both A and B) is part of the protected circuit. This is a mustdue to phenomena specific for power transformers, such as inrush and over-excitation. If thereis no power transformer included in the protected multi-terminal line circuit, then the 2nd and5th harmonic block scheme is only active under the following conditions:1MRK 505 363-UEN A Section 6Differential protection85Application manual
