Effect of voltage sag on ageing of front end rectifier diodes of ASD

Adjustable-speed drives are the type of equipment most sensitive to voltage sags. This paper analyzes the behaviour of front end rectifier of three-phase ac adjustable-speed drives during balanced and unbalanced sags. Emphasis is placed on the i²t rating of diode assuming that the drive will not trip. By using a previously introduced classification of three-phase balanced and unbalanced sags, i²t curves for various capacitor values and different sag conditions are obtained for ac adjustable-speed drive. 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 up to 80% of voltage sag for type C & F sag. While higher value capacitor is selected for enhancing voltage sag ride through, at the same time the bridge rectifier diode I²t value should also be selected such that it should not cross the diode I²t limit during sag and sag recovery duration.