A Current Transformer Saturation Detection Algorithm for Use in Current Differential Protection

A current transformer (CT) saturation detection algorithm for use in current differential protection is described. The algorithm evaluates the start and end of each CT saturation period using the "largest modulus series"; this is derived from the third order difference function applied to the secondary current. To reduce the possibility of false detection due to the effect of noise on the third order difference function, the ratios of the absolute values of the CT secondary induced voltage are also used as a criteria to determine when the CT is saturated. A blocking signal is activated when saturation is first detected and is maintained for the duration of the saturation period plus one cycle. Simulation results indicate that the algorithm can significantly enhance the stability of the difference method. The algorithm successfully detects saturation and prevents the maloperation of a current differential protection scheme during high current external faults; whilst ensuring high sensitivity on internal faults.