GE B90 Instruction Manual

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10-6 B90 Low Impedance Bus Differential System GE Multilin10.4 SLOPES AND HIGH SET THRESHOLD 10 APPLICATION OF SETTINGS1010.4SLOPES AND HIGH SET THRESHOLD 10.4.1 DESCRIPTIONTo set the higher slope and threshold of the high set (unbiased) differential operation, external faults must be analyzed.Consider an external fault for the North bus relay. It is justified to assume bus configurations that give maximum stress tothe maximum number of CTs. For this purpose we will assume the tie breaker, B-7 closed; all the circuitry capable of sup-plying the fault current to be in service; moreover, they are connected to the South bus in order to analyze the CT-7 and CT-8 carrying the fault current.10.4.2 EXTERNAL FAULTS ON C-1The table below presents the results of analysis of an external fault on circuit C-1 (C-1 is connected to the North bus; C-3,C-4, and C-5 are connected to the South bus).For security reasons, it has been assumed that the fault current being a sum of several contributors (C-3, C-4, and C-5 inthis case) has a time constant of the DC component of the maximum among the time constants of the contributors. Thefault current is supplied from circuits C-3, C-4, and C-5 connected to the South bus, thus through CT-3, CT-4, and CT-6.The current passes through the tie breaker threatening saturation of CT-7 and CT-8.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 earlier), it is concluded that CT-1 will saturate during this fault, producinga spurious differential signal for the North bus zone differential protection. All other CTs will not saturate due to the AC com-ponents. The amount of the spurious differential current (magnetizing current of CT-1) can be calculated using the burden,magnetizing characteristic and primary current of the noted CT by solving the following equations:(EQ 10.4)For I s = 116.67 A, Rs = 1.61 Ω and the characteristic shown earlier in the Approximate CT Magnetizing Characteristics fig-ure, the solution is I magnetizing = 29.73 A, I relay = 112.8 A.The magnetizing current of the saturated CT-1 will appear to the differential element protecting the North bus as a differen-tial signal of 29.73 A, while the restraint signal will be the maximum of the bus currents (112.8 A in this case). Conse-quently, the higher slope of the characteristic should not be lower than 29.73 A / 112.8 A, or 26%, and the pick up of thehigh set differential elements should not be lower than 29.73 A, or 2.97 pu.The CTs identified as operating in the linear mode as far as the AC components are considered may, however, saturate dueto the DC components. Saturation will not occur if , where ω is radian system frequency (2πf).If the above condition is violated, saturation will occur but not before:Columns 6 and 7 of the table below summarize the DC saturation threat for the fault on C-1. CT-4, CT-6, CT-7, and CT-8may 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.The transient saturation of the CTs due to the DC component may be neglected when setting the slopes of the characteris-tic as the saturation will be detected and the relay will use the current directional principle. It must however, be taken intoaccount when setting the high set (unbiased) differential element.Table 10–4: EXTERNAL FAULT CALCULATIONS ON C-1CT I FAULT (KA) IFAULT (A SEC) T DC (MS) ACSATURATION DCSATURATION TSAT (MS)CT-1 14.0 116.67 40 Yes Yes N/ACT-2 0 0.00 N/A No No 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 4.70I relay I s2 I magnetizing2–=I relay R s× V magnetizing=V sat I s R s× 1 ω T dc×+( )×>T sat T dc– 1 Vsat I s R s⁄( ) 1–ωT dc----------------------------------------–  ln×=