A novel CT saturation detecting algorithm unaffected by a remanent flux

This paper proposes a novel current transformer (CT) saturation detecting algorithm unaffected by a remanent flux. At the instants of the beginning/end of saturation, the magnetizing inductance of the core changes significantly, resulting in severe wave distortion of the secondary current. In other words, although the secondary current is continuous, the shape of the secondary current during saturation is quite different from that during unsaturation after a fault. In addition to that, at the instants, the increment of the secondary current is reduced significantly, depending on a remanent flux and saturation time. The third difference of the secondary current is used to detect CT saturation in this paper. That is, if the third difference is less than a predetermined threshold value, the CT saturation is at the instant of beginning or end. The proposed method does not depend on the amount of a remanent flux and any frequency components contained in the current. The results of various tests with a remanent flux from -80% to +80% indicate satisfactory performance of the method.