8-6 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALBIASED CHARACTERISTIC BREAKPOINTS CHAPTER 8: APPLICATION OF SETTINGS8R 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 B30 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 B30 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. 8-3Table 8-3: Limits of linear operation of the CTSThe third and fourth columns of the table have the following significance.If an external fault occurs on circuit C-1, then CT-1 carries the fault current. As the fault current is higher than any of theother currents, the current supplied by CT-1 is 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,considering CT-1, the value of 8.96 pu is used as the higher breakpoint of the characteristic.Considering CTs that can be connected (depending on the positions of the switches) to the North bus, the HIGH BPNT for theNorth bus zone is selected as the minimum of (8.96, 9.13, 31.17, 22.88, 24.57, 31.17), or 8.96 pu.Considering CTs that can be connected (depending on the positions of the switches) to the South bus, the HIGH BPNT for theSouth bus zone is selected as the minimum of (9.13, 31.17, 22.88, 24.57, 31.17), or 9.13 pu.8.3.3 Low breakpointThe DC component in the primary current can 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-2operating principle upon detecting saturation.The residual magnetism (remanence) left in the core of a CT can limit the linear operation of the CT significantly. It isjustified to assume that the residual flux can be as high as 80% of the saturation level leaving only 20% to accommodatethe flux component created by the primary current. This phenomenon can be reflected by reducing the saturation voltagein the calculations by the factor of 100% / 20%, or 5. This, in turn, is equivalent to reducing the limit of linear operation bythe factor of 5, hence the last column in the Limits of Linear Operations of the CTs table.CT Rs (Ω) I max (A sec) I max (pu)(no remanence)I max (pu)(80% remanence)CT-1 1.61 89.55 8.96 1.79CT-2 1.58 91.25 9.13 1.83CT-3 1.85 155.84 31.17 6.23CT-4 1.75 137.30 22.88 4.58CT-5, CT-6 1.63 147.42 24.57 4.91CT-7, CT-8 1.85 155.84 31.17 6.23
