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 analyses the behavior of three-phase AC adjustable-speed drives during balanced and unbalanced sags. Emphasis is placed on the DC 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 DC bus capacitance