I. Introduction
The use of wind power has been growing quickly around the globe and, particularly, in the last decade in Portugal. The strong investment in infrastructures was reflected in the fact that in the last year renewable energies accounted for 57% of the electricity supply in Portugal (REN - Rede Energética Nacional), ranging from hydroelectric power (28%) to wind (22%), biomass (5%) and photovoltaic (1.4%). Non-renewable production, however, accounted for 43% of consumption, divided between coal and natural gas [1].