Voltage sags on Quasi-Z-source adjustable-speed drives system

This paper presents a critical evaluation of the effect of voltage sags on Quasi-Z-source inverter based adjustable speed drives (qZSI-ASD), analyzes the behaviour of qZSI-ASD under the balanced and unbalanced sags. The voltage-tolerance curves are obtained to assess the sensitivity of the qZSI-ASD under the balanced voltage sags, which quite similar to the ZSI-ASD system. In particular the Quasi-Z-source network capacitor voltage variation under unbalanced voltage sag and its dependence on source impedance, Z-source network passive device values, and shoot-through duty cycle is derived. The larger Z-source network values, the smaller dc-link voltage variation under the voltage sag. The proposed representation can be established for any other three-phase equipment and used for sensitivity assessment. By controlling the shoot-through duty cycle, the qZSI-ASD system provides ride-through to voltage sags, both balanced and unbalanced sags. The simulation results are presented to verify the proposed concept and theoretical analysis.