Torque controller of a doubly-fed induction generator impelled by a DC motor for wind system applications

The authors propose robust non-linear controllers for a wind system. The proposed controllers are based on a combination of the block control linearisation and super-twisting algorithm, as a second-order sliding mode technique. The main control loop is designed to regulate the electromagnetic torque and stator power factor of a doubly-fed induction generator which is connected to the grid. The generator is mechanically coupled with a DC motor, which can emulate the wind turbine operation. Therefore, an auxiliary control loop is designed to control the DC motor velocity. Additionally, a grid-side converter controller is proposed to regulate the DC-link voltage into AC/DC/AC converter, and displacement factor of the energy flow between the rotor and the grid. A robust stability analysis of the complete closed-loop system under external disturbances is presented. The robustness of the proposed control schemes is validated in real time using a workbench, which consists of a motor-generator group, AC/DC/AC electronic drive and dSPACE DS 1104 controller boards.