Investigation of Low-Frequency Oscillations in VSD Driven Induction Motors in Microgrids

In recent years, direct torque-control (DTC) based variable-speed drives (VSDs) have been widely used in broad-range of industrial and commercial applications. This is due to the fact that DTC based VSDs offer high-speed and torque precision compared to scalar control and vector control techniques. The DTC combines flux oriented control, i.e., both the flux and torque are directly controlled for high accuracy. This paper investigates the effect of low-frequency oscillations (LFOs) on DTC based VSD driven induction motors (IMs) under various disturbances in a hybrid AC/DC microgrid. The study has shown that when an electro-mechanical oscillation induced by a step change occurs at the VSD, it could give rise to speed and torque oscillations at the VSD driven IM; however, the dynamic response is far better than the direct-on-line (DOL) based IM. Furthermore, when a significant voltage dip occurs in the microgrid, it significantly affects the response of the VSD driven IM. The DC-link capacitor of the VSD reduces the effect of voltage variations at the PCC on the VSD driven IM. Thus, with a high DC-link capacitor, the VSD has a high capability to reduce the impact of voltage disruption. Furthermore, voltage and frequency oscillations created by load disturbances in microgrids could lead to power oscillations at the VSD driven IM.