Effect of CT transient saturation on transfer of sympathetic inrush

Second harmonic proportion and dead-angle of sympathetic inrush transferred by transiently saturated CT(Current Transformer) are investigated. The formula for calculating the second harmonic proportion of sympathetic inrush is derived, the transfer characteristics of saturated CT for each harmonic are analyzed, and the second harmonic proportion of sympathetic inrush transferred by transiently saturated CT is calculated. The influence of CT saturation on dead-angle is studied. The results indicate that, as the second harmonic proportion of sympathetic inrush is similar to that of magnetizing inrush, the sympathetic inrush itself is not the essential cause of differential relay mal-operation; as higher harmonic is transferred more easily by the saturated CT than lower harmonic, the second harmonic proportion of sympathetic inrush at CT secondary side is higher than that at CT primary side; the dead-angle of sympathetic inrush transferred by saturated CT is decreased, and the more saturated CT is, the smaller dead-angle is.