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

In this paper, a critical evaluation of the effect of voltage sags on adjustable speed drives (ASDs) is presented. In particular the DC-link voltage variation under voltage sag and its dependence on source impedance, DC-link inductance and output load is computed. It is shown that for larger source impedance, the DC-link voltage variation under a voltage sag is also large 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. In order to improve the performance of ASDs, this paper proposes an integrated boost converter approach. This approach provides ridethrough to critical ASD load during voltage sags without any additional energy storage device. Upon detection of a voltage sag the boost converter operates with suitable duty ratio and maintains the DC-link voltage within acceptable limits. This prevents nuisance tripping and facilitates continuous operation of critical ASD load at rated torque. The proposed integrated boost converter does not introduce any additional semiconductors in the series path of the power flow and is low in cost. A commercially available 480 V, 22 kVA ASD is modified with the integrated boost converter approach and details are discussed. Analysis, simulation and experimental performance of the ridethrough approach are presented.