GE Multilin B30 Bus Differential System 9-79 APPLICATION OF SETTINGS 9.4 SLOPES AND HIGH SET THRESHOLD99.4SLOPES AND HIGH SET THRESHOLD 9.4.1 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.9.4.2 EXTERNAL FAULTS ON C-1The following table 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 do 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 9.4)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 appears to the differential element protecting the North bus as a differentialsignal of 29.73 A, while the restraint signal is the maximum of the bus currents (112.8 A in this case). Consequently, thehigher slope of the characteristic should not be lower than 29.73 A / 112.8 A, or 26%, and the pickup of the high set differ-ential 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 can, however, saturate dueto the DC components. Saturation does not occur if , where ω is radian system frequency(2πf).If the above condition is violated, CT time-to-saturate for a full DC saturation can be estimated as follows. The CT satura-tion factor K s capability curve is defined as(EQ 9.5)whereT1 is a primary system time constantT2 is the secondary CT time constant, which can be estimated by the following equation:(EQ 9.6)whereN is the CT ratioVs is the CT voltage at 10 A exciting current obtained from the CT excitation curveCT limiting factor KS_LIM is defined by the following equation:(EQ 9.7)I relay I s2 I magnetizing2–=I relay R s× V magnetizing=V sat I s R s× 1 ω T dc×+( )×>K Sω T1 T2××T1 T2–------------------------- e t– T1⁄ e t– T2⁄– ωt( )sin–=T 2V s N×20 ω R s×( )×------------------------------=K S…LIMV s N×I p R s×--------------=
