When the magnetic circuit of a conventional protection current transformer (CT) enters saturation, the wave-shape of the secondary current distorts due to the increase in the excitation current. The distortion can be detected by analysing the secondary current using an algorithm that evaluates the first, second and third difference functions. The first difference contains points of inflection, which correspond to the start and end of each saturation period. The second and third differences convert the discontinuities at the points of inflection into pulses that can be used to detect saturation. The design and evaluation of an algorithm for detecting CT saturation using the third difference function is described. A low-pass filter, required for anti-aliasing and noise rejection, softens the discontinuities and reduces the magnitude of the pulses seen in the second and third difference signals. The softening effect is pronounced when the cut-off frequency of the filter is reduced. The sampling rate of 64 samples/cycle is used and the currents are passed through a first-order low-pass RC anti-aliasing filter with a cut-off frequency of from 1920 to 480 Hz. Experimental test results clearly demonstrate that the algorithm successfully detects the start and end of each saturation period. Results are obtained when a prototype detector based on a digital signal processor was used on a high current test-rig. Results indicate that the detector can correctly detect when the secondary current signal is distorted and in all the test cases the detector operated correctly.
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