| Wind energy is often installed in rural, remote areas characterized by weak, unbalanced power transmission grids. In induction wind generators, unbalanced three-phase stator voltages cause a number of problems, such as overheating, over-current, and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of unbalance, induction wind generators are switched out of the network. This can further weaken the grid. In doubly-fed induction generators, control of rotor currents allows for adjustable speed operation and reactive power control. In addition, it is possible to control the rotor currents to correct for the problems caused by unbalanced stator voltages, including torque pulsations, stator reactive power pulsations, and unbalanced stator currents. This thesis presents a novel voltage mode controller design for a doubly-fed induction generator that provides variable speed, reactive power control, and, under stator voltage unbalance conditions, the presented control eliminates torque pulsations, stator reactive power pulsations, and draws more balanced currents from the utility. The net effect is to make wind generators more robust and valuable to the grid. |
