Control of a Doubly-Fed Induction Wind Generator Under Unbalanced Grid Voltage Conditions Ted Brekken, Ned Mohan, Tore Undeland UNIVERSITY OF MINNESOTA 200 Union St. S.E. Minneapolis, MN 55455, USA Tel.: +47 – 976 69 048 Fax: +47 – 735 94 279 E-Mail: tedbrekken04@fulbrightweb.org URL: http://www.ece.umn.edu Acknowledgements I would like to thank my advisor, Ned Mohan. I would also like to thank Tore Undeland and the Energiomforming department at the Norwegian University of Science and Technology for their support. Keywords «Doubly fed induction motor», «Vector control», «Windgenerator systems», «Active damping», «Power quality» Abstract 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 and unbalanced stator currents. This paper presents a novel voltage mode controller design for a doubly-fed induction generator that provides variable speed, reactive power control. Also, under stator voltage unbalance conditions, the proposed control eliminates torque pulsations and draws more balanced currents from the utility. Introduction Rural grids are prone to three-phase voltage unbalances that can cause many problems for induction machines. The goal of the research is to develop a control method for doubly-fed induction wind generators that addresses issues associated with weak rural grids. Commonly used control methods for doubly-fed machines pay no special attention to these problems. The presented control method improves the robustness and all-around performance of doubly-fed induction wind generators, allowing them to operate in conditions in which they would normally be removed from the grid. A controller is designed and tested in simulation and also tested on a 15 kW DSP-based hardware setup.