Multi-Level Inverter Integration for Low Voltages Ride Through for Controlling Renewable Wind Energy Conversion Systems
In this study, a new control of grid-connected doubly-fed induction generators (DFIGs) is introduced to attain low-voltage ride-through capability. The multi-level converter (MLC) is designed and integrated into the grid-connected DFIG. The grid-side converter is proposed to be a MLC controlled by the conventional DC-link voltage controller, while the rotor-side converter is a two-level conventional converter controlled by the proposed virtual voltage strategy. The proposed virtual controller is designed to use busbar voltage during normal operation, while it utilizes the designed virtual voltage during wide range conditions of low grid voltage. Based on the proposed design of the virtual voltage, the generator currents are limited to the rated values, in which it protects the generator against low grid voltages that may be due to close or remote grid faults. The simulation results prove the validity of the proposed control scheme during extreme low voltages.