GE Multilin B90 Low Impedance Bus Differential System 9-39 THEORY OF OPERATION 9.3 DIFFERENTIAL PRINCIPLE99.3DIFFERENTIAL PRINCIPLE 9.3.1 BIASED DIFFERENTIAL CHARACTERISTICThe B90 uses a dual-slope dual-breakpoint operating characteristic as shown in the figure below.The PICKUP setting is provided to cope with spurious differential signals when the bus carries a light load and there is noeffective restraining signal.The first breakpoint (LOW BPNT) is provided to specify the limit of guaranteed linear operation of the CTs in the most unfa-vorable conditions such as high residual magnetism left in the magnetic cores or multiple autoreclosure shots. This pointdefines the upper limit for the application of the first slope (LOW SLOPE).The second breakpoint (HIGH BPNT) is provided to specify the limits of operation of the CTs without any substantial satura-tion. This point defines the lower limit for the application of the second slope (HIGH SLOPE).Figure 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 origin ofthe ‘differential - restraining’ plane. The advantage of having a constant bias specified by the HIGH SLOPE setting creates anobstacle of a discontinuity between the first and second slopes. This is overcome by using a smooth approximation (cubicspline) 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 breakpoints.differentialrestrainingLOWSLOPEOPERATEBLOCKIr|Id|HIGHSLOPELOW BPNTHIGH BPNTPICKUP836720A1.CDR
