Beschreibung
The doubly-fed induction generators (DFIG) operating at synchronous speed offers some advantages. At this working mode, the power flowing in the rotor is not significant and thus the converter rating is reduced considerably. This research work develops control techniques which combine feedback and feedforward topologies to reduce oscillations in the electromagnetic torque, load angle, as well as the reactive power. According to the critical damping method, the damping torque is controlled to maintain the critical state of the entire system. This method is based on the dynamic analysis of torque components in the machine. As a consequence, an additional damping torque is fed to the control loop to eliminate the vibration of the variables during the alteration of operating parameters. The other method, namely coupling approach, is based on the analysis of the vibration components of the variables. Thus, the oscillation parts are provided simultaneously at their set values and the vibration is eliminated. On the other hand, as the share of wind energy in the power network increases, the generators are required to stay connected in cases of voltage depression. This work introduces control methods to ride-through the temporary low voltage for both symmetrical and asymmetrical voltage dip. For symmetrical dip, a rotor voltage control method is proposed with a determination of transient stator flux linkage. Under asymmetrical dip, the electromagnetic vibrations are deal with by controlling the positive and negative sequence separately. Optimal methods to determine the reference values of the negative sequence controller are proposed to eliminate the oscillations simultaneously. The control methods are presented with selected simulation and experimental results.
