An algorithm for detecting CT saturation using the secondary current third-difference function

When a current transformer (CT) saturates, the wave-shape of the secondary current distorts and contains points of inflection at the start and end of each saturation period. These can be observed as discontinuities in the first-difference function. The second- and third-difference functions convert the points of inflection into pulses that can be easily used to detect saturation. An algorithm for detecting CT saturation based on the third-difference function is described in this paper. An antialiasing low-pass filter is required to prevent aliasing and reduce the effect of noise. Inadvertently, the filter softens or smooths the current signal and reduces the peaks in the third-difference function. The performance of the algorithm was validated by changing the remanent flux in the core, considering the effect of a low-pass filter and also experimentally using a test system with a real CT. Test results indicate that the detector can correctly detect each saturation period when the secondary current is severely distorted.