Solid State AC Variable Speed Drives have already

Voltage sags originating in ac supply systems can cause nuisance tripping of variable speed drives (VSDs) resulting in production loss and restarting delays. In ac VSDs having an uncontrolled rectifier front-end, the effects of voltage sags on the dc link causing dc under-voltage or ac over-current faults initiate the tripping. This paper suggests modifications in the control algorithm in order to improve the sag ride-through performance of ac VSDs. The proposed strategy recommends maintaining the dc link voltage constant at the nominal value during a sag by utilising two control modes, viz. (a) by recovering the kinetic energy available in the rotating mass at high motor speeds and (b) by recovering the magnetic field energy available in the motor winding inductances at low speeds. By combining these two modes, the VSD can be configured to have improved voltage sag ride-through perforamance at all speeds