Forced oscillations in wind energy generation systems

Use of the doubly fed induction generator (DFIG) in wind energy generation systems allows variable speed operation by using partially rated back-to-back quadruple active and reactive power PWM converters. The control of the system is very complex. Despite numerous new control schemes reported in the literature, these control techniques, which are designed to solve some particular control problems, have rarely been tested for general stability problems systematically. Specifically, nonlinear stability analysis of the system has not been reported before. In this paper, we use a commonly used control scheme to study the system stability in terms of the oscillatory magnitude of the voltage link capacitor of the system under unbalanced grid voltage. An important variation parameter, frequency of the unbalanced grid voltage, is used in this paper. Stability diagrams is collected using the voltage of the link capacitor of the back-to-back PWM power converters, exposing the possibility of chaotic behavior of the wind generation system. This paper thus plays the role of characterizing wind energy generation systems using nonlinear stability analysis.