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Nowadays, using renewable energy sources (RESs) such as wind, solar and tidal power for generation of energy is increasing. Among all RESs, the wind power has the maximum contribution to the renewable power generation. China, USA, Germany, Spain and India at the end of 2010 had maximum installed capacities in the world with 44.7 GW, 40.18 GW, 27.21 GW, 20.67 GW and 13.06 GW, respectively; it is expected that, at the end of 2020, total capacity in the world will reach to 1500 GW (World Wind Energy Association WWEA, 2011). With growth of wind energy, the power system is faced with new problems such as new challenges in frequency control (Bevrani, Ghosh, & Ledwich, 2010) and power reserve estimation. With increasing wind power in a power system, the initial frequency drop may increase following a load disturbance (Erlich, Rensch, & Shewarega, 2006).
Currently, using of Doubly Fed Induction Generators (DFIGs) is more usual than the other wind turbine technologies, because they can easily provide ancillary services and active/reactive power control. Naturally, the DFIG wind turbines cannot contribute to the inertial response because the rotational speed of them is not directly connected to the network (Lalor, Ritchie, Rourke, Flynn, & O'Malley, 2004; Ekanayake & Jenkins, 2004; Lalor, Mullane, & O'Malley, 2005). But, adding a supplementary loop control to the DFIGs, they can release their kinetic energy and contribute to inertial response and primary control (Lalor, Ritchie, Rourke, Flynn, & O'Malley, 2004; Lalor, Mullane, & O'Malley, 2005; de Almeida & Lopes, 2005; Morren, de Haan, Kling, & Ferreira, 2006; Anaya-Lara, Hughes, Jenkins, & Strbac, 2006; Conroy & Watson, 2008; Chowdhury & Ma, 2008; El Mokadem, Courtecuisse, Saudemont, Robyns, & Deuse, 2009; Tarnowski, Kjar, Sorensen, & Ostergaard, 2009; Ping-Kwan, Pei, Banaka, & Boon Teck 2009; Erlich & Wilch, 2010).
Wang, Sun, Li, and Ooi (2010) have proposed a voltage and frequency control method for DFIG. This method makes DFIG equivalent to a synchronous generator and controls active and reactive power of stator by controlling the voltage magnitude and frequency of the rotor. Kaneko, Uehara, Senjyu, Yona, and Urasaki (2011) used an integral control method for wind farms to reduce frequency deviation in a small power system. In this method, wind farm achieves the frequency regulation objective based on two control schemes: load estimation and short-term ahead wind speed prediction. The proposed methods have some drawbacks such as reduction of wind farm output power, as well as increased pitch action. Bhatt, Ghoshal, and Roy (2010) have introduced an extra frequency control support function which affects the rotational speed of rotor in the case of frequency disturbance and reduces the frequency dip by releasing short term transient active power. For optimal dynamic frequency performance, Thyristor controlled phase shifter (TCPS) and Superconducting Magnetic Energy Storage (SMES) are also used in coordination with DFIG frequency control loop. Mokadem, Courtecuisse, Saudemont, Robynsand, and Deuse (2009) have introduced a primary frequency control based on fuzzy logic for variable-speed wind generators. This method controls the torque generator and pitch angle to keep primary reserve. Senjyu, Kaneko, Uehara, Yona, Sekine, and Kim (2009) have presented an output power control of wind turbine using pitch angle control. In this approach, wind turbines achieve flexible output power control and therefore frequency deviation reduces. Xiangyu, Heming, and Yi (2010) have added a PD controller to the DFIG, and they have introduced a new control loop for participation of DFIG in the frequency control, but this loop only provides the primary frequency control.