GE Multilin B90 Low Impedance Bus Differential System 10-710 APPLICATION OF SETTINGS 10.4 SLOPES AND HIGH SET THRESHOLD1010.4.3 EXTERNAL FAULTS ON C-2The following table presents the results of analysis of an external fault on circuit C-2 (C-2 is connected to the North bus; C-3, C-4 and C-5 are connected to the South bus).By comparing the secondary currents (column 3 in the following table) with the limits of linear operation for the CTs (column4 shown earlier in the Limits of Linear Operations of the CTs table) it is concluded that CT-2 will saturate during this faultproducing a spurious differential signal. All other CTs will not saturate due to the AC components. The amount of the spuri-ous differential current (magnetizing current of CT-2) can be calculated using the burden, magnetizing characteristic andthe primary current of the said CT.For I s = 116.67 A, Rs = 1.23 and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics fig-ure, the solution is I magnetizing = 27.69 A, I relay = 113.3 A.The magnetizing current of the saturated CT-2 will appear to the differential element protecting the North bus as a differen-tial signal of 27.69 A, while the restraint signal will be the maximum of the bus currents (113.3 A). Consequently, the higherslope of the characteristic should not be lower than 27.69 A / 113.3 A, or 24% and the pick up of the high-set differentialelements should not be lower than 27.69 A, or 2.77 pu.Columns 6 and 7 of the following table summarize the DC saturation threat for the fault on C-2. CT-4, CT-6, CT-7, and CT-8 may saturate due to the DC components and may generate spurious differential signal for both the North and South busrelays depending on the bus configuration. The saturation will not occur before 4.7 ms and will be detected by the Satura-tion Detector.10.4.4 EXTERNAL FAULTS ON C-3The following table presents the results of analysis of an external fault on circuit C-3 (C-3 is connected to the North bus; C-4 and C-5 are connected to the South bus).By comparing the secondary currents (column 3 in the table below) with the limits of linear operation for the CTs (column 4in the Limits of Linear Operations of the CTs table shown earlier), it is concluded that none of the CTs will saturate due tothe AC currents during this fault.Columns 6 and 7 of the table below summarize the DC saturation threat for the fault on C-3. CT-3, CT-4, CT-6, CT-7, andCT-8 may saturate due to the DC components and may generate a spurious differential signal for both the North and Southbus relays depending on the bus configuration. The saturation will not occur before 11.18 ms and will be detected by theSaturation Detector.Table 10–5: EXTERNAL FAULT CALCULATIONS ON C-2CT I FAULT (KA) IFAULT (A SEC) TDC (MS) ACSATURATIONDCSATURATIONTSAT (MS)CT-1 0 0.00 N/A No No N/ACT-2 14.0 116.67 40 Yes Yes N/ACT-3 6.0 25.00 5 No No N/ACT-4 5.0 25.00 30 No Yes 15.19CT-6 3.0 15.00 40 No Yes 35.25CT-7, CT-8 14.0 58.33 40 No Yes 4.70Table 10–6: CALCULATIONS FOR THE EXTERNAL FAULTS ON C-3CT I FAULT (KA) IFAULT (A SEC) TDC (MS) ACSATURATION DCSATURATION TSAT (MS)CT-1 0 0.00 N/A No No N/ACT-2 0 0.00 N/A No No N/ACT-3 8.0 33.33 40 No Yes 11.18CT-4 5.0 25.00 30 No Yes 15.19CT-6 3.0 15.00 40 No Yes 35.25CT-7, CT-8 8.0 33.33 40 No Yes 11.18
