Start-up and Smooth Grid Connection of the Doubly Fed Induction Generator

This paper deals with the start-up and smooth grid connexion of the doubly-fed induction generator (DFIG) system driven by the wind turbine. Stator flux-oriented control for back-to-back converters in the DFIG rotor circuit is presented. The start-up procedure of DFIG begins with the encoder calibration and ends with the grid connection. First, a new algorithm for estimating the initial rotor position is investigated. This algorithm is provided to determine the initial angle, whatever the rotor state (blocked or in rotation). Then, the synchronization of open stator and grid voltages is discussed in depth. By controlling d-axis and q-axis the magnitude and phase of the induced stator voltage are adjusted respectively to be equal to the grid voltage. The proposed method ensures a smooth connection and zero exchange power at the moment of connection. Furthermore, the steps for safe disconnection are presented. Finally, the overall methodology is validated on a 3 kW DFIG laboratory setup.