Effects of balanced and unbalanced voltage sags on VSI-fed adjustable-speed drives

This paper analyzes the effects of voltage sags on the three-phase rectifier of a voltage-source-inverter (VSI)-fed adjustable-speed drive. These effects are dc voltage drop and ac current peaks. As rectifier behavior is univocally characterized by two parameters (which depend on the ac reactance and the dc capacitor), their influence on the dc voltage drop and the ac current peaks is studied. Diagrams in function of these two parameters are presented in order to easily predetermine the worst effects of voltage sags originated by single-phase faults. The results of the study suggest that if the trip levels are correctly adjusted, the drive can ride through the voltage sags produced by single-phase faults (assuming the drive has typical values of L and C).