There is not a simple answer for that  question.

PMSG are expensive compared to other technologies. Brusless Doubly fed generatorsusually have lower power densities (than regular DFIGs) and this could increse the head mass.in the wind turbine. Cage induction generators have less efficiency and cannot operate at low speed.  

In addition to the very good answer of Roberto, it can also be said that the use of PMSG is an increasing trend in large wind turbines (from 2 or more MW) in the last years. It is true that PMSG are expensive but on the other hand heavy and expensive Gearboxes are not longer necessary. For example Enercon has a patented concept using only PMSG and other companies are developing very large (8 MW and more) PMSG.

The only one thing I would like to add, is that size and mass of the generator will not be an issue very soon. A new technologies based on the high temperature superconductors will allow to eliminate conductor joule losses and provide superior performance and compactness. Superconducting generators will be an advanced technology for wind power generation.

 Hi Muhammad,

I would suggest the following discouraging aspects of the BDFIG:

(Avoiding details here since the references below provide adequate treatment. You may check them out !)

Cheers,

-Sanjay

[1] F. Riincos, R. Carlson, N. Sadowski, P. Kuo-Peng, and H. Voltolini, “Performance and vibration analysis of a 75 kw brushless double-fed induction generator prototype,” in Proc. 41st IEEE IAS Annu. Meeting, vol. 5, Oct. 2006, pp. 2395–2402.

[2] P. Roberts, “A study of brushless doubly-fed (induction) machines,” Doctoral Thesis, University of Cambridge, Emmanuel College, Sep. 2004.

[3] B. Hopfensperger, D. J. Atkinson, and R. A. Lakin, “Steady state of the cascaded doubly-fed induction machine,” European Trans. on Electrical Power, vol. 12, no. 6, pp. 427–437, 2002. [Online]. Available: http://dx.doi.org/10.1002/etep.4450120607

http://www.site.uottawa.ca/~rhabash/ELG4126WindGenerators.pdf