Study of voltage sag effects on three-phase transformers

This paper studies the effects of symmetrical and unsymmetrical voltage sags on a three-phase three-legged transformer and a three-phase transformer bank by considering that fault clearing is produced instantaneously in all the phases. The transformer model includes saturation and the parameters have been obtained from experimental measurements. The influence of the sag type, depth, duration and initial point-on-wave on the inrush current peak value is studied. The current peak has periodical dependence on sag duration and the initial point-on-wave, and a linear dependence on depth. Unsymmetrical sags can result in current peaks as high as those of symmetrical sags.