Contents I. Introduction
The utilization of Renewable Energy Resources (RESs) is increasing rapidly throughout the world due to environmental concerns to come up with cleaner energy sources under the constraints of lower prices and maximum reliability. Although a high-level of RESs penetration mitigate negative environmental impact compared to traditional power plants, control issues are burdensome in the converters-based power system. The traditional power system is synchronous based machines and they can release their kinetic energy stored in their rotating mass when needed. Unfortunately, RESs cannot do the same since their rotating mass is decoupled from the grid through the power electronics converters. By its nature, wind energy is a variable source. Consequently, active power generated by a wind turbine is variable as well. To produce electrical energy, the kinetic energy present in the wind is converted to electrical energy through rotor, gearbox, and generator. The larger the blades of a wind turbine, the higher is the power produced. When a wind turbine is connected to a weak transmission grid (grid with low Short Circuit Ratio), power quality issues such as voltage fluctuation, flicker, harmonics occur. To significantly contribute to the grid, several standard sizes of wind turbines are clustered at a site where there is a vast enough wind field, flat or in a valley In the literature of power system engineering, such an arrangement is called a wind farm. The operation of wind turbines causes air turbulence from its back as well as from its sides. The region of turbulence created is called the wake of the turbine. Wind turbines are placed in such a way that no turbines are in the wake of the forward and side turbines. Failure to do so leads to the reduction of the output power of any turbines that lies in the wake of another. To be connected to the transmission grid, it is required that each wind turbine meet certain requirements. Voltage sags, faults, and unbalances are very frequent in wind power systems since wind farms are located far away from the load center where the electrical energy is needed. Wind turbines, during their operations, are subjected to a variety of harsh conditions in terms of loads and weather. The modeling of the behavior of these machines are not an easy task for the designers and system planners. Generally, TSOs use simplified models to perform stability studies like loss of generation, power lines switching just to name a few. Thus, the modeling of the wind turbines is of the utmost importance in the impact studies of the large scale wind farms on the Bulk Electric System (BES). This paper attempts to address the issues of modeling of wind turbines which has huge importance during the design stage, operation and maintenance.
