A novel control of dual-excited synchronous machines for variable-speed wind turbines

Compliancy to grid codes has been a driving force behind the development of variable-speed technologies for wind generation. This resulted in the doubly-fed induction generator (DFIG) to become a common technology. The operational benefits of the DFIG derive from the possibility of controlling the operating speed of the generator from the rotor windings. Even if constructively different, the Dual-Excited Synchronous Generator (DESG) is functionally identical to the DFIG and offers an equivalent controllability. However, the mechanical design of the DESG can be exploited to be comparatively better than a DFIG for low speed operations. Thus, DESG can be a valid candidate as non conventional variable-speed constant-frequency generators for wind power generation. This paper presents a novel control technique for wind turbines that allows the power flowing into the rotor to be actively regulated and even zeroed. The algorithm is exemplified on a DESG but can be equally implemented also on a DFIG. The control of the rotor power can be translated into a simplified power electronics configuration, since the grid-side inverter in the back-to-back converter can be replaced by an unregulated rectifier, leading perhaps to a cost reduction and to an increase of reliability.