8-12 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALENHANCING RELAY PERFORMANCE CHAPTER 8: APPLICATION OF SETTINGS88.6 Enhancing relay performance8.6.1 Using setting groupsIn the example of the South bus, CT-2 is the weakest (most prone to saturation) CT dictating values of some settings.However, CT-2 may not be a part of the South bus protection zone if the S-2 switch is opened. As the position of the switchmust be provided for the dynamic bus replica, the status of the switch can be re-used to control the setting groups andapply more sensitive settings if the weakest CT is not part of the bus zone at a given time. For example, if the S-2 switch isopened while the S-6 switch is closed, the CT-4 becomes the weakest CT connected to the South bus. The higherbreakpoint (HIGH BPNT) could be increased to 22.88 pu (fourth column of the Limits of Linear Operations of the CTs table).The lower breakpoint (LOW BPNT) could be increased to 4.58 pu (fifth column of the Limits of Linear Operations of the CTstable). The higher slope (HIGH SLOPE) could be decreased as no AC saturation is possible for the South bus CTs (see theexternal fault calculation tables for each circuit).The concept could be implemented by using:• FlexLogic to process the status signals in order to identify the weakest CT• Setting Groups to switch dynamically from one setting group to another (adaptive settings)This approach can be extended for buses that do not require the dynamic bus replica mechanism. This can includeapproximation of the total bus fault current using positions of all switches and breakers and optimizing the settingsdepending on the amount of stress imposed on the CTs in any particular bus configuration.HIGH SET 5.54 The maximum spurious differential current is 2.77 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 Comments
