Analysis of voltage tolerance of AC adjustable-speed drives for three-phase balanced and unbalanced sags

Adjustable-speed drives are the type of equipment most sensitive to voltage sags. This paper analyzes the behavior of three-phase ac adjustable-speed drives during balanced and unbalanced sags. Emphasis is placed on the de-bus voltage and on the drop in speed, both assuming that the drive will not trip. By using a previously introduced classification of three-phase unbalanced sags, voltage-tolerance curves are obtained for ac adjustable-speed drives. The conclusion from the analysis is that voltage sags due to three-phase faults are a serious problem for adjustable-speed drives. However, single-phase and phase-to-phase faults, causing the majority of voltage sags, can be tolerated by adding a relatively small amount of de-bus capacitance.