**1. Introduction**

There are several sources of wind power variability, including short-term (seconds to mi‐ nutes) and long-term (hours of the day, or seasonal). The type of wind turbine, the control algorithm, and wind speed fluctuations all affect the short-term power fluctuation at each turbine. The blade of a Type 1 induction generator wind turbine experiences the tower shad‐ ow effect every time a blade passes the tower; the output of the Type 1 turbine commonly fluctuates because of this. Wind turbines Type 3 and Type 4 are equipped with power elec‐ tronics and have a reasonable range of speed variation; thus, the impact of the tower shad‐ ow effect is masked by the power converter control. Because of wind speed fluctuations and wind turbulence, the power output also influences the output of the wind turbine generator.

However, in the big picture, the power fluctuation at a single turbine is not as important as the total power output of a wind power plant. The interface between the wind power plant and the power system grid is called the point of interconnection (POI). All the meters to cal‐ culate revenue, measure voltage and frequency, and measure other power quality attributes are installed at the POI. At the POI, all the output power from individual turbines is injected into the power grid. A wind power plant covers a very large area; thus, there are various diversities within each plant (e.g., wind speed, line impedance, and instantaneous terminal voltage at each turbine). The power measurement from a single wind turbine usually shows a large fluctuation of output power; however, because many turbines are connected in a wind power plant, the power fluctuation from one turbine may cancel that of another, which effectively rectifies the power fluctuation of the overall plant.

© 2012 Martin-Martínez et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Martin-Martínez et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Additionally, many wind power plants are co-located in the same region where wind resour‐ ces are excellent; thus, the spatial diversity among wind power plants contributes to a smooth‐ er output power of the region than the output power of an individual wind power plant.

**Figure 1.** Real and reactive power output of a wind power plant; (a) Single turbine representation (b) Sixteen turbines representation

Figure 1(a) shows the fluctuation in the output power when the wind power plant is repre‐ sented by a single turbine. Figure 1(b) illustrates the real and reactive power output when the wind power plant is represented by sixteen turbines. With greater wind diversity, as shown in Figure 1(b), the power fluctuation is smoother than that with less wind turbine representation, as shown in Figure 1(a) [1].
