CHAPTER 9: THEORY OF OPERATION DIFFERENTIAL PRINCIPLEB90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 9-59• 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 carries 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 becomes 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) needs to be set no higher than 5000 A,where 1000 A = 5 pu (1000 A is the base unit as outlined in the previous the example).The same approach applies to the setting of the lower breakpoint, LOW BPNT.9.3.3 Enhanced securityTo enhance the performance of the B90, the differential characteristic is divided into two regions having diverse operatingmodes, as shown in following figure.The first region applies to comparatively low differential currents and has been introduced to deal with CT saturation onlow-current external faults. Certain distant external faults can cause CT saturation due to extremely long time constants ofthe DC component or multiple autoreclosure shots. The saturation, however, is difficult to detect in such cases. Additionalsecurity via the “directional check” is permanently applied to this region without regard to the saturation detector.Figure 9-3: Two regions of differential characterThe second region includes the remaining portion of the differential characteristic and applies to comparatively highdifferential currents. If, during an external fault, the spurious differential current is high enough so that the differential-restraining current trajectory enters the second region, then saturation is guaranteed to be detected by the saturationdetector.The B90 operates in the 2-out-of-2 mode in the first region of the differential characteristic. Both differential anddirectional principles (Differential Principle and Directional Principle) must confirm an internal fault in order for the biaseddifferential element to operate.
