Sub-Harmonics in Wind Driven SM-DFIG in the Super-Synchronous Range of Operation

In this paper the sub-harmonics associated with a specially designed single machine brushless doubly fed induction generator (SM-BDFIG) are analyzed at supersynchronous speeds. The SM-BDFIG is coupled to a variable speed wind turbine, and its rotor circuit is connected to a 3- phase rectifier feeding Li-ion batteries. A mathematical model, using Fourier expansion, is developed to define the subharmonics created by the time harmonics of the rotor currents irrespective of any other space or time harmonics due to winding distribution, unbalanced grid phases, or presence of harmonics in the grid voltages. The effect of the positive and negative sequence harmonics on the performance of the generator is then presented, showing the speed ranges within which these sub-harmonics are most effective, as well as the speed ranges within which these sub-harmonics are least effective. The obtained results help to design a SM-BDFIG with less harmonic distortion, hence improving its performance in wind energy conversion systems.