Extended vector control of a rotating power electronic brushless doubly-fed induction generator under unsymmetrical voltage sags

This paper presents the implementation of the extended vector control on a new type of brushless doubly-fed generator when subjected to unsymmetrical voltage dips. This new type of generator has partially rated power electronic converter and exciter mounted on the shaft and rotating with it. The unbalanced voltages introduce positive and negative sequence components in the generator. The negative sequence components gives rise to torque oscillations and extra heating in the generator and power electronics which could affect the life time of the shaft, gearbox and electronic equipment. Two separate synchronous reference frames are defined and standard PI controllers are implemented in the reference frames in order to control the positive and negative sequence currents independently. Through these means the effect of the negative sequence currents is mitigated and it is shown that the oscillations in the behavior of this new type of generator are considerably reduced.