CHAPTER 8: APPLICATION OF SETTINGS BUS DIFFERENTIAL SETTINGSB30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 8-118Table 8-8: External fault calculations on C-58.5 Bus differential settings8.5.1 DescriptionTaking the previous analysis from this chapter into account, the settings have been calculated as follows.Table 8-9: North bus differential protection settingsTable 8-10: South bus differential protection settingsCT I FAULT (kA) IFAULT (A sec) TDC (ms) AC saturation DC saturation t SAT (ms)CT-1 0 0.00 N/A No No 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 26.37CT-5 11.0 55.00 30 No Yes 9.45CT-7, CT-8 11.0 45.83 30 No Yes 11.54Setting Value CommentsPICKUP 0.1 pu Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The pu value is for the base of 1200A. This means the actual pickup is 120 Aprimary.LOW SLOPE 25% Default value. Lower or higher values can be entered upon security/dependabilityrequirements.LOW BPNT 1.79 pu None of the CTs saturate for AC currents below 1.79 pu even with 80% remanence. The DCcomponent, however, combined with the remanence can saturate some CTs even for currentsbelow 1.79 pu. The B30 copes with saturation using the current directional principle.HIGH SLOPE 60% Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The value of 60% ensures that the differential characteristic alone (without thedirectional principle) work correctly under AC saturation of the CTs (26% of spuriousdifferential during the fault on C-1 saturating CT-1).HIGH BPNT 8.96 None of the CTs saturate for AC currents below 8.96 pu. The DC component, however, cansaturate some CTs even for currents below 8.96 pu. The B30 copes with saturation using thecurrent directional principle.HIGH SET 5.94 The maximum spurious differential current is 2.97 pu. Due to limited accuracy of analysis andthe effect of DC saturation a security factor of 2 has been adopted. The highest internal faultcurrent is 14kA, or 11.67 pu giving a good chance to clear a number of faults by the unbiaseddifferential operation.Setting Value CommentsPICKUP 0.1 pu Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The pu value is for the base of 1200A. This means the actual pickup is 120 Aprimary.LOW SLOPE 25% Default value. Lower or higher values can be entered upon security/dependabilityrequirements.LOW BPNT 1.83 pu None of the CTs saturate for AC currents below 1.83 pu even with 80% remanence. The DCcomponent, however, combined with the remanence can saturate some CTs even forcurrents below 1.83 pu. The B30 copes with saturation using the current directional principle.HIGH SLOPE 60% Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The value of 60% ensures that the differential characteristic alone (without thedirectional principle) work correctly under AC saturation of the CTs (24% of spuriousdifferential during the fault on C-2 saturating CT-2).HIGH BPNT 9.13 pu None of the CTs saturate for AC currents below 9.13 pu. The DC component, however, cansaturate some CTs even for currents below 9.13 pu. The B30 copes with saturation using thecurrent directional principle.
