Abstract:
The work presented in this thesis includes control system design, analysis and grid synchronization of a DFIG (doubly-fed induction generator) driven by a wind turbine using stator-voltage and stator-flux oriented frames. The DFIG is a special type of induction machine which is comprised of two back-to-back converters. One converter connects the DFIG stator to the grid, and the second converter is connected to the rotor of the machine through a DC-link capacitor. In this work, DFIG steady-state and transient models have been created in the d-q reference frame. The steady-state model is used to obtain the relationship between the rotor d-q currents and stator real/reactive power references in a particular orientation frame. The transient model is used to develop the DFIG power control mechanisms. The wind turbine driving torque is modeled by considering typical wind turbine aerodynamic characteristics under variable wind and pitch angle conditions. Comparisons are made to evaluate the differences between DFIG current/power controller in stator-voltage and stator-flux oriented frames. A speed control system has been designed to analyze maximum energy extraction from a DFIG for a particular wind speed. Lastly, the grid synchronization control technique and synchronization method have been proposed as this system requires some care during the machine start-up and integration with the grid. The main aim of the synchronization control process is to avoid heavy start-up currents and mechanical stresses on the turbine shaft and other integrated components. This is achieved by properly matching the phase angle, frequency, and magnitude of the grid voltage and the stator induced voltage irrespective of whether it is a stator-voltage or stator-flux oriented frame used for modeling the generator. Instead of a traditional control scheme using a PLL (phase-locked loop), the rotor d-q reference current is generated with grid voltage as the reference so as to induce identical voltage in the stator as that of the grid. The machine is started by a driving torque and the switch between stator and the grid can be closed for synchronization. However, appropriate timing of switch closure plays a critical role in satisfying the magnitude condition of synchronization. Simulation models have been developed using Matlab®/Simulink® for a GE 1.5 MW generator.
