CHAPTER 9: THEORY OF OPERATION SATURATION DETECTORB90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 9-79Figure 9-4: Directional principle operation during external faultsFigure 9-5: Directional principle operation during internal faultsThe B90 implementation calculates the maximum angle for the considered currents and compares it against a fixedthreshold of 90°. The flag indicating whether the directional protection principle is satisfied is available as the FlexLogicoperand BUS 1(4) DIR A/B/C.9.5 Saturation detector9.5.1 CT saturation detectionThe saturation detector of the B90 takes advantage of the fact that any CT operates correctly for a short period of time,even under very large primary currents that would subsequently cause a very deep saturation. As a result, in the case ofan external fault, the differential current stays very low during the initial period of linear operation of the CTs while the
