GE Multilin B90 Low Impedance Bus Differential System 9-59 THEORY OF OPERATION 9.3 DIFFERENTIAL PRINCIPLE99.3.3 ENHANCED SECURITYIn order to enhance the performance of the B90, the differential characteristic is divided into two regions having diverseoperating modes as shown in following diagram.The first region applies to comparatively low differential currents and has been introduced to deal with CT saturation on low-current external faults. Certain distant external faults may cause CT saturation due to extremely long time constants of theDC component or multiple autoreclosure shots. The saturation, however, is difficult to detect in such cases. Additional secu-rity via the “directional check” is permanently applied to this region without regard to the saturation detector.Figure 9–3: TWO REGIONS OF DIFFERENTIAL CHARACTERISTICThe second region includes the remaining portion of the differential characteristic and applies to comparatively high differ-ential currents. If, during an external fault, the spurious differential current is high enough so that the differential-restrainingcurrent trajectory enters the second region, then saturation is guaranteed to be detected by the saturation detector.The B90 operates in the 2-out-of-2 mode in the first region of the differential characteristic. Both differential and directionalprinciples (see Sections 9.3 Differential Principle and 9.4 Directional Principle) must confirm an internal fault in order for thebiased differential element to operate.The relay operates in the dynamic 1-out-of-2 / 2-out-of-2 mode in the second region of the differential characteristic. If thesaturation detector (see Section 9.5 Saturation Detector) does not detect CT saturation, the differential protection principlealone is capable of operating the biased differential element. If CT saturation is detected, both differential and directionalprinciples must confirm an internal fault in order for the biased differential element to operate.Because of diverse operating modes in the first and second regions of the differential characteristic, the user gains doublecontrol over the dependability and security issues. The first level includes slopes and breakpoints of the characteristic withregard to the amount of the bias. The second level includes control over the split between the first and second regions ofthe characteristic.differentialrestrainingRegion 1(low differentialcurrents)Region 2(high differentialcurrents)836725A1.CDR
