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Seminar: ECE Faculty Candidate Thursday May 4th 11:00 – 11:50am KEC 1007 Ted Brekken A Novel Control Scheme for a Doubly-fed Induction
Wind Generator Under Unbalanced Grid Voltage Conditions 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 saturation, over-current, and stress on the
mechanical components from torque pulsations. Therefore, for high levels of
unbalance, induction wind generators are switched out of the network. This can
further weaken the grid. A special type of induction generator, called a
doubly-fed induction generator, has found wide use in high-power wind
applications. The wound rotor construction of a doubly-fed induction machine
allows for rotor current control via a rotor-connected power converter. Control
of the rotor currents in turn allows for variable speed operation and reactive
power control. The goal of the research is to further extend the control
strategy beyond the standard variable speed and reactive power control to also
include compensatory features that can correct for the problems caused by an
unbalanced stator voltage. This 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. Unbalanced
stator voltages appear as a positive and negative sequence 3-phase voltage at
the stator terminals. The negative sequence causes a flux field in the machine
to rotate counter to the main, positive sequence direction. This causes a
second harmonic pulsation in the machine torque (active power) and reactive
power. Also, the machine stator currents become unbalanced. When using
synchronous frame vector control, the second harmonic disturbance caused by the
unbalance will be propagated through all synchronous frame variables. The
developed control compensates for the second harmonic, and hence the unbalance,
by injecting a supplementary rotor voltage to cancel the second harmonic in
both the torque and stator reactive power synchronous frame control loops. This
also has the effect of reducing the stator current unbalance. This novel
control was implemented and tested on a 15 kW doubly-fed induction machine.
Theory and results will be presented. Biography: Ted Brekken is postdoctoral researcher at
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