GE B90 Instruction Manual

Also see for B90: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

9-4 B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALDIFFERENTIAL PRINCIPLE CHAPTER 9: THEORY OF OPERATION9Figure 9-2: Biased operating characteristicThe higher slope used by the B90 acts as an actual percentage bias regardless of the value of the restraining signal. This isso because the boundary of the operating characteristic in the higher slope region is a straight line intersecting the originof the ‘differential - restraining’ plane. The advantage of having a constant bias specified by the HIGH SLOPE setting createsan obstacle of a discontinuity between the first and second slopes. This is overcome by using a smooth approximation(cubic spline) of the characteristic between the lower and higher breakpoints. Consequently, the characteristic ensures• A constant percentage bias of LOW SLOPE for restraining currents below the lower breakpoint of LOW BPNT,• A constant percentage bias of HIGH SLOPE for restraining currents above the higher breakpoint of HIGH BPNT, and• A smooth transition from the bias of LOW SLOPE to HIGH SLOPE between the breakpoints9.3.2 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, for example applying the dynamic bus replica of the protected zone. Thedifferential current is scaled to the maximum rated primary current. The scaling must be taken into account when settingthe PICKUP value of the biased differential characteristic and the HIGH SET operating point of the unbiased differentialfunction.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, for example applying the dynamic bus replica of the protected bus zone. Therestraining current is scaled to the maximum rated primary current. The scaling must be taken into account when settingthe breakpoints of the biased differential characteristic.The “maximum of” definition of the restraining signal biases the relay toward dependability without jeopardizing securityas the relay uses additional means to cope with CT saturation on external faults. An additional benefit of this approach isthat the restraining signal always represents a physical — compared to an “average” or “sum of” — current flowing throughthe CT that is most likely to saturate during given external fault. This brings more meaning to the breakpoint settings of theoperating characteristic.The following example is provided with respect to the breakpoint settings.9.3.2.1 Example 2Proceed with the previous example and assume that taking into account the relevant factors such as properties of the CTsthemselves, resistance of the leads, and burden of the CTs, the following primary currents are guaranteed to betransformed without significant saturation: