CHAPTER 8: APPLICATION OF SETTINGS BIASED CHARACTERISTIC BREAKPOINTSB90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 8-588.3 Biased characteristic breakpoints8.3.1 DescriptionThe limits of linear operation of the CTs need to be found in order to set the breakpoints of the biased differentialcharacteristic. The settings for the North and South bus relays are analyzed simultaneously from this point on as the twodifferential zones share some CTs and the results of computations apply to both the relays.For microprocessor-based relays it is justified to assume the burden of the CTs to be resistive. The limits of the linearoperation of a CT, neglecting the effects of the DC component and residual magnetism, can be approximated as follows:Eq. 8-1whereImax is the maximum secondary current transformed without saturation (AC component only, no residual magnetism),R s is the total burden resistanceV sat is the saturation voltage of the CTThe total burden resistance depends on both the fault type and connection of the CTs. For single-line-to-ground faults andCTs connected in Wye, the burden resistance is calculated as:Eq. 8-2whereR lead is the lead resistance (one way, hence the factor of 2)R CTsec is the secondary CT resistanceR relay is the relay input resistanceAssuming 0.003 Ω/m lead resistance and approximating the B90 input resistance for the 5A input CTs as 0.2 VA / (5 A)2 or0.008 Ω, the limits of the linear operation of the CTs have been calculated and presented in the Limits of Linear Operationsof the CTs table.8.3.2 High breakpointAs an external fault can happen on any of the connected circuits, threatening saturation of any of the CTs, take theminimum value of the linear operation limit as the HIGH BPNT setting. The limit of linear operation that neglects both theresidual magnetism and the effect of the DC component should be the base for setting the higher breakpoint of the biaseddifferential characteristic.The B90 requires the breakpoints to be entered as ‘pu’ values. The relay uses the largest primary current of the CTsbounding the bus differential zone as a base for the pu settings. Both the North and South buses have the largest primarycurrent of the CTs of 1200 A (CT-7 and CT-8), thus upon configuration of the relays, 1200 A is automatically selected asbase for the pu quantities. With a given Ibase current, the limits of linear operation have been recalculated to pu values asfollows:Eq. 8-3
