CHAPTER 10: THEORY OF OPERATION SATURATION DETECTORG60 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 10-710See the Application of Settings chapter for information on setting calculations.10.2 Saturation detector10.2.1 CT saturation detectionThe saturation detector of the G60 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 therestraining signal develops rapidly. Once one or more CTs saturate, the differential current increases. The restraining signal,however, yields by at least a few milliseconds. During internal faults, both the differential and restraining currents developsimultaneously. This creates characteristic patterns for the differential - restraining trajectory, as shown.Figure 10-3: CT saturation detection internal and 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 the following figure on saturation detector state machine.As the phasor estimator introduces a delay into the measurement process, the aforementioned saturation test fails 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. Additionally, thesample-based stage of the saturation detector uses the time derivative of the restraining signal (di/dt) to better trace thesaturation pattern shown in the following figure.The saturation detector is capable of detecting saturation occurring in approximately 3 ms from fault inception. Thesaturation detector, although having no dedicated settings, uses the main differential characteristic for proper operation.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 STATOR DIFF SAT A/B/C.
