# On the Contribution of Wind Farms in Automatic Generation Control: Review and New Control Approach
## 1 Introduction
Since frequency control is directly related to active power control, and active output power of wind farms has intermittent characteristic, due to wind uncertainty; this aspect of grid ancillary capability (frequency control) for wind farms has more significance from the viewpoint of the power system. Frequency control in wind farms is analyzed in three levels: Inertial control, primary control, and secondary control or automatic generation control. The inertial and primary control systems are well addressed in the literature [17,18]. Imbalance between load and generated power by generators in the system are exhibited through frequency deviations. An indelible off-normal frequency deviation directly affects power system operation, security, reliability, and efficiency by damaging equipment, degrading load performance, overloading of the transmission lines, and triggering the protection devices. Frequency performance of the power system is regulated in three levels; the first two levels by generation units, and in the third level, by loads through load shedding in severe situations. These three levels are known as primary control or inertial response [19], secondary or supplementary control [20] and tertiary or emergency control or load shedding [21]. Automatic generation control (AGC) is the manner of action of participating generation units in frequency control. Supplementary frequency control, which is known as load-frequency control (LFC), is a major function of AGC systems as they operate online to control system frequency and power generation [22]. Frequency control can be directly dependent on the speed control of the turbines in the generation units, due to the fact that the frequency generated in the electric network is proportional to the rotational speed and mechanical power of the generator. In conventional generators (non-wind turbine), the issue is initially sensed by the governor of the generators, which can adjust the valve position to change the fuel amount and subsequently change the mechanical power for the electrical part to track the load change and to restore the frequency to be near the nominal value. AGC responsibilities can be classified as a significant control process that operates constantly to balance the generation and load in the power system at a minimum cost, adjust the generation to minimize frequency deviation and regulate tie-line power flows. Briefly, the AGC system is responsible for frequency control and power interchange, as well as optimal economic dispatch. The AGC system realizes generation changes by sending signals to the under-control generation units. Due to newly advanced technologies in wind turbines and their controllability, the wind farms can be mentioned as an acceptable choice to contribute to the AGC. The AGC performance is highly dependent on how those generating units respond to the commands. The generation unit response characteristics is mainly affected by the control strategy that the unit utilizes, like robust control methods [23; 24], intelligent algorithms [25; 26] and optimization approaches [27; 28]. This part can be solved by integrating appropriate and efficient controller in the wind turbine structure, which is demonstrated in this study. The control strategy that is utilized for wind farm contribution in the AGC is the main topic for this paper. This paper firstly reviews the contribution of wind farms in load frequency control as a major function of AGC. Different aspects of such contribution, like different control strategies (optimization, model predictive control and intelligent methods) and coordination with other devices, such as conventional generation units, flexible AC (alternative current) transmission systems (FACTS) and energy storage systems are reviewed. As the second stage, a new control methodology by implementing fractional order proportional-integral-differential (FOPID) controller for supplementary loop of a variable speed wind generator-based wind farms is proposed.