Effect of voltage sags on adjustable-speed drives: A critical evaluation and an approach to improve performance

In this paper, a critical evaluation of the effect of voltage sags on adjus table-speed drives (ASD's) is presented, In particular, the dc-link voltage variation under voltage sag and its dependence on source impedance, dc-lin k inductance, and output load is computed, It is shown that, for larger sou rce impedance, the dc-link voltage variation under a voltage sag is also la rge and increases the susceptibility of an ASD and may result in a nuisance trip, The results from the analysis are plotted in per-unit quantities and serve as a design guide to assess ASD performance for a variety of sags. I n order to improve the performance of ASD's, this paper proposes an integra ted boost converter approach, This approach provides ride-through to critic al ASD load during voltage sags without any additional energy storage devic e, Upon detection of a voltage sag, the boost converter operates with suita ble duty ratio and maintains the dc-link voltage within acceptable limits, This prevents nuisance tripping and facilitates continuous operation of cri tical ASD load at rated torque, The proposed integrated boost converter doe s not introduce any additional semiconductors in the series path of the pow er flow and is low in cost, A commercially available 480-V 22-kVA ASD is mo dified with the integrated boost converter approach, and details are discus sed, Analysis, simulation, and experimental performance of the ride-through approach are presented.