8-8 B30 Bus Differential System GE Multilin8.5 SATURATION DETECTOR 8 THEORY OF OPERATION88.5SATURATION DETECTOR 8.5.1 CT SATURATION DETECTIONThe saturation detector of the B30 takes advantage of the fact that any CT operates correctly for a short period of time evenunder very large primary currents that would subsequently cause a very deep saturation. As a result of that, in the case ofan external fault, the differential current stays very low during the initial period of linear operation of the CTs while therestraining signal develops rapidly. Once one or more CTs saturate, the differential current will increase. The restraining sig-nal, however, yields by at least a few milliseconds. During internal faults, both the differential and restraining currentsdevelop simultaneously. This creates characteristic patterns for the differential - restraining trajectory as depicted below.Figure 8–7: CT SATURATION DETECTION: INTERNAL & EXTERNAL FAULT PATTERNSThe CT saturation condition is declared by the saturation detector when the magnitude of the restraining signal becomeslarger than the higher breakpoint (HIGH BPNT) and at the same time the differential current is below the first slope (LOWSLOPE). The said condition is of a transient nature and requires a seal-in. A special logic in the form of a “state machine” isused for this purpose as depicted in Figure 8–7: SATURATION DETECTOR state machine.As the phasor estimator introduces a delay into the measurement process, the aforementioned saturation test would fail todetect CT saturation occurring very fast. In order to cope with very fast CT saturation, another condition is checked thatuses relations between the signals at the waveform level. The basic principle is similar to that described above. Addition-ally, the sample-based stage of the saturation detector uses the time derivative of the restraining signal (di/dt) to bettertrace the saturation pattern shown in the above diagram.The saturation detector is capable of detecting saturation occurring in approximately 2 ms into a fault. It is worth emphasiz-ing that the saturation detector, although having no dedicated settings, uses the main differential characteristic for properoperation. This must be kept in mind when setting the characteristic as its parameters must retain their original meaning.The operation of the saturation detector is available as the FlexLogic operand BUS 1(2) SAT.Figure 8–8: SATURATION DETECTOR STATE MACHINE836728A1.CDRdifferentialrestrainingOPERATEBLOCKINTERNAL FAULT PATTERNEXTERNAL FAULT PATTERNEXTERNAL FAULT PATTERNNORMALSAT := 0EXTERNALFAULTSAT := 1EXTERNALFAULT and CTSATURATIONSAT := 1The differentialcharacteristicenteredThe differential-restraining trajectoryout of the differentialcharacteristic for acertain period of timesaturationconditionThe differentialcurrent below thefirst slope for acertain period oftime836729A1.CDR
