I. Introduction
The wind power capacity installed in Portugal had a high rate of increase in recent years, becoming the second largest renewable energy source. By the end of 2013, Portugal had 4.4GW of installed wind power out of 17.8GW of the operating capacity in the Portuguese power system. During 2013, the wind generation reached 11.8TWh contributing with 24% of the total electrical supply [1], which constitutes the second highest wind energy penetration in the world, only surpassed by Denmark. The rapid growth of variable wind generation poses a serious challenge to the transmission system operators (TSOs) since, at any time 1)balance between power supply and consumption must exist to ensure stability of the power system and 2)the supply/demand balance must be achieved at a minimal economic cost.