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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. [ABSTRACT FROM AUTHOR]

This paper studies the effects of symmetrical and unsymmetrical voltage sags on three-phase three-legged transformers. The transformer model includes saturation and the parameters have been obtained from experimental measurements. The study shows that sags can produce transformer saturation when voltage recovers. This saturation produces an inrush current that is similar to the inrush current produced during the transformer energizing. The study takes into account that the voltage recovery instant can take only discrete values, since the fault-clearing is produced in the natural current zeroes. The paper also studies the influence of the sag type, duration, depth and fault current angle on the inrush current peak value. The current peak has a periodical dependence on sag duration, and a linear dependence on depth. Unsymmetrical sags can result in current peaks as high as those of symmetrical sags. [ABSTRACT FROM AUTHOR]

This paper proposes a new saturated single-phase transformer model and a three-phase three-legged transformer model for their implementation in a harmonic load flow. The nonlinear magnetizing behavior of the transformer is represented in both models by a simple saturated reluctance function. The procedure for the nonlinear magnetizing current calculation is described. The model incorporation into the harmonic load flow is also analyzed. Simulations have been developed in order to show the performance of the proposed models, and have been compared with real measurements. [ABSTRACT FROM AUTHOR]

This paper describes the effects of symmetrical voltage sags on three-phase three-legged transformers. The transformer model includes saturation and is based on experimental parameters. It has been implemented in PSpice. Voltage recovery after a voltage sag can produce transformer saturation. This saturation produces an inrush current similar to that of the transformer energizing. The variables that influence the peak value of the inrush current, namely depth, duration, and initial point-on-wave are studied. A result that must be emphasized is the periodical influence of the sag duration. [ABSTRACT FROM AUTHOR]