Impact of voltage sags on three-phase power transformers

This paper studies the behavior of three-phase power transformers under various voltage sag conditions. A nonlinear model of a three-phase three-leg transformer based on electric and magnetic circuit topologies is implemented and solved in time-domain using iterative numerical techniques. The corresponding transformer waveforms are computed and analyzed in both the time and frequency domains. Simulations are performed for different voltage sag magnitudes, starting and termination (recovery instant waveform) angles. It is shown that there can be significant distortions in transformer flux and magnetizing current, as well as three-phase output voltages and currents. Furthermore, high levels of voltage sag can severely deteriorate the power quality of input and output current and voltage waveforms.