9-4 B90 Low Impedance Bus Differential System GE Multilin9.3 DIFFERENTIAL PRINCIPLE 9 THEORY OF OPERATION99.3.2 DIFFERENTIAL AND RESTRAINING CURRENTSThe differential current is produced as a sum of the phasors of the input currents of a differential bus zone taking intoaccount the status signals of the currents, i.e. applying the dynamic bus replica of the protected zone. The differential cur-rent is scaled to the maximum rated primary current as explained in Section 8.1 Introduction. The scaling must be takeninto account when setting the PICKUP value of the biased differential characteristic and the HIGH SET operating point of theunbiased differential function.The restraining current is produced as a maximum of the magnitudes of the phasors of the zone input currents taking intoaccount the status signals of the currents, i.e. applying the dynamic bus replica of the protected bus zone. The restrainingcurrent is scaled to the maximum rated primary current as explained in Section 8.1 Introduction. The scaling must be takeninto account when setting the breakpoints of the biased differential characteristic.The “maximum of” definition of the restraining signal biases the relay toward dependability without jeopardizing security asthe relay uses additional means to cope with CT saturation on external faults. An additional benefit of this approach is thatthe restraining signal always represents a physical – compared to an “average” or “sum of” – current flowing through the CTthat is most likely to saturate during given external fault. This brings more meaning to the breakpoint settings of the operat-ing characteristic.The following example is provided with respect to the breakpoint settings.EXAMPLE 3:Proceed with the previous example (see page 8–2) and assume that taking into account the relevant factors such as prop-erties of the CTs themselves, resistance of the leads and burden of the CTs, the following primary currents are guaranteedto be transformed without significant saturation:• 1A CT: 6.0 kA• 1B CT: 7.5 kA• 1C CT: 5.0 kA• 1D CT: 13.0 kA• 1E CT: 8.0 kA• 1F CT: 9.0 kAAs having the lowest primary current guaranteeing operation without saturation, the CT associated with the 1C input ismost exposed to saturation. During an external fault on the 1C circuit, the 1C CT will carry the fault current contributed bypotentially all the remaining circuits. The fault current is higher than any contributing current, and therefore, the current ofthe 1C CT will become the restraining signal for the biased differential characteristic for external faults on the 1C circuit.Consequently, the higher breakpoint of the differential characteristic (HIGH BPNT) should be set not higher than5000A : 1000A = 5 pu (1000A is the base unit; see page 8–2 for the example).The same approach applies to the setting of the lower breakpoint, LOW BPNT.
