9-2 T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
SATURATION DETECTOR CHAPTER 9: THEORY OF OPERATION
9
Figure 9-1: Directional principle operation during external faults
Figure 9-2: Directional principle operation during internal faults
The T35 implementation calculates the maximum angle for the considered currents and compares it against a fixed
threshold of 90°. The flag indicating whether the directional protection principle is satisfied is available as the FlexLogic
operand XFMR PCNT DIFF DIR A/B/C.
9.2 Saturation detector
9.2.1 CT saturation detection
The saturation detector of the T35 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 of
an external fault, the differential current stays very low during the initial period of linear operation of the CTs while the
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