Improved energy conversion for doubly-fed wind generators

There is currently significant interest in offshore wind turbines up to 5 MW. One of the preferred options is doubly-fed slip-ring generators. Turbine manufacturers have proposed an operating scheme for low speeds that is claimed to improve the overall energy extraction from the wind. The aim of this paper therefore is to examine the performance benefits for this new operational mode on a small experimental rig, confirmed by simulation, and to provide a fundamental understanding of the differences in the generator operation in both modes. The paper develops analytical steady-state models to provide this insight and correlates the operating performance with a dynamic real-time generator control scheme and experimental results obtained from a laboratory test machine. A theoretical study on a 2 MW commercial turbine is also undertaken.