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This paper proposes an algorithm for detecting current transformer (CT) saturation by analyzing a secondary current. The current has points of inflection where saturation begins and ends. Then, discontinuity in the first difference of the current arises at the next instants of points of inflection, where the second and third differences of the current may have large values enough to detect saturation. In this paper, the third difference is used because it is more effective than the second difference in terms of saturation detection. However, an anti-aliasing low-pass filter softens the current and, thus, reduces the values of the third difference at those instants. This paper includes the effect of a low-pass filter on the proposed algorithm. The sampling rate of 64 samples/cycle (s/c) is studied by varying a remanent flux and the cutoff frequency of the filter. The test results clearly indicate that the proposed algorithm successfully detects the interval of saturation even with a remanent flux and the softening effect. However, special attention should be paid to the decision of the threshold value when a cutoff frequency is much lower than half the sampling frequency. This paper concludes by implementing the algorithm into a TMS320C6701 digital signal processor and the results are satisfactory.
Date of Publication: Jan. 2004