10-8 B30 Bus Differential System GE Multilin10.4 SLOPES AND HIGH SET THRESHOLD 10 APPLICATION OF SETTINGS10whereIp is the maximum CT primary fault currentThe figure illustrates the Ks CT saturation capability curve and K S_LIM limiting factor.Figure 10–5: DETERMINING CT TIME-TO-SATURATECT time-to-saturate (t SAT ) is determined as a projection of the intersection of the K s CT saturation capability curve andKS_LIM CT limiting factor, as shown in the figure.Practically, CT time-to-saturate can be obtained by either of the following methods:• Method 1 — Download the "CT Time-to-Saturate Estimator.xlsm" spreadsheet from the GE Multilin web site, locatedunder Support > Support Documents > B30 Bus Differential System. Enter the required system and CT parameters toobtain the CT time-to-saturate.• Method 2•Define several time instances t1, t2 ...t n•Use equation 10-5 to calculate the corresponding K S1, KS2 …KSn•Calculate K S_LIM using equation 10-7•Compare each K S value with K S_LIM. If KS at the given time instance t i is less than KS_LIM , it means that true tSAT isgreater than t i, otherwise it is less than tiColumns 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 5.02 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 is detected and the relay uses the current directional principle. It must, however, be taken into accountwhen 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) ACSATURATIONDCSATURATIONTSAT (MS)CT-1 14.0 116.67 40 Yes Yes 5.02CT-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 26.37
