Theoretical Calculation of Inrush Currents in Three- and Five-Legged Core Transformers

In this paper, a theoretical study of the three-phase transformer behavior in the presence of a sag is presented by considering that the fault clearing is produced simultaneously in all phases. Analytical expressions of the magnetic flux and the inrush current after voltage recovery are obtained, and the results presented in the literature are analytically justified. The influence of depth, duration, and the initial point-on-wave on the peak value of the inrush current are studied for all sag types. Simple expressions to obtain the value of the current peaks are presented