10-6 B30 Bus Differential System GE Multilin10.3 BIASED CHARACTERISTIC BREAKPOINTS 10 APPLICATION OF SETTINGS10If an external fault occurs on circuit C-1, CT-1 will carry the fault current. As the fault current is higher than any of the othercurrents, the current supplied by CT-1 will be used as the restraint signal. CT-1 is guaranteed to saturate if the currentexceeds 89.55 A secondary, or 17.9 times its rated current, or 8.96 pu of the bus differential zone. Consequently, consider-ing CT-1, the value of 8.96 pu should be used as the higher breakpoint of the characteristic.Considering CTs that could be connected (depending on the positions of the switches) to the North bus, the HIGH BPNT forthe North bus zone should be selected as the minimum of (8.96, 9.13, 31.17, 22.88, 24.57, 31.17), or 8.96 pu.Considering CTs that could be connected (depending on the positions of the switches) to the South bus, the HIGH BPNT forthe South bus zone should be selected as the minimum of (9.13, 31.17, 22.88, 24.57, 31.17), or 9.13 pu.10.3.3 LOW BREAKPOINTThe DC component in the primary current may saturate a given CT even with the AC current below the suggested value ofthe higher breakpoint. The relay copes with this threat by using the Saturation Detector and applying a 2-out-of-2 operatingprinciple upon detecting saturation.The residual magnetism (remanence) left in the core of a CT can limit the linear operation of the CT significantly. It is justi-fied to assume that the residual flux could be as high as 80% of the saturation level leaving only 20% to accommodate theflux component created by the primary current. This phenomenon may be reflected by reducing the saturation voltage inthe calculations by the factor of 100% / 20%, or 5. This, in turn, is equivalent to reducing the limit of linear operation by thefactor of 5, hence the last column in the Limits of Linear Operations of the CTs table.For example, if the residual flux left in the core of the CT-1 is as high as 80% of its saturation level, the CT will saturate at17.92 A secondary, or 3.58 times its rated current, or at 1.79 pu of the bus differential zone.The reduced limit of linear operation should be used as the lower breakpoint of the biased differential characteristic (theLOW BPNT setting). In this way the interval spanning from the lower to higher breakpoints covers the indistinct area of possi-ble saturation due to the random factor of residual magnetism.The LOW BPNT should be set at 1.79 pu for the North bus zone, and at 1.83 pu for the South bus zone.A combination of very high residual magnetism and a DC component with a long time constant may saturate a given CTeven with the AC current below the suggested value of the lower breakpoint. The relay copes with this threat by using a 2-out-of-2 operating mode for low differential currents.
