# On the Contribution of Wind Farms in Automatic Generation Control: Review and New Control Approach
## 2 Wind Farm Contribution in Frequency Regulation  
Granted by new technologies and control methods of wind turbines, wind farms are worthy choice to contribute to the frequency regulation of the power system. In this regard, operation under maximum power point tracking, connecting to power system with AC/DC (direct current)/AC converters and coordination with other conventional power plants are the main issues that to be investigated for wind farms in order to able to contribute to frequency regulation. Figure 1 shows the frequency regulation process and related strategies for wind farms to contribute in AGC. This section generally explains the wind farms contribution in primary frequency control, and their inertial response to support frequency regulation. Wind farms contribution in AGC and its details are provided in the next section. Normally, wind turbines operate in maximum power point of tracking (MPPT) mode to extract maximum possible power from the wind and convert it into electrical power. It is obvious that such models with MPPT operation mode is not appropriate in AGC studies. In Reference [29], the performance of wind farms by VSWTs were investigated when they are modeled under the set-point tracking control strategies for intermittent wind. The superiority of set-point tracking mode over MPPT mode was proven by the contribution of the wind farms in AGC. To reach high quality frequency performance, a new method is introduced in Reference [30] to preserve a certain amount of wind power reserve to contribute in frequency regulation of power system. Inertial response of wind turbines is the other important issue for frequency regulation. The FSWTs can provide an inertial response to the frequency deviation, because of its direct coupling; however, this inertia is small compared to the synchronous generator. On the other hand, the VSWTs are connected to the grid by back-to-back voltage or current source converters, which decouples the complete wind turbine in the case of PMSG and the DFIG partially from the grid to support frequency regulation. In this situation, the wind turbine cannot contribute to the frequency regulation through inertial response with its original controller, since it is not connected directly to the grid. Therefore, it is needed to add supplementary controllers to converter part of VSWTs to prepare them for frequency change tracking and then do efficient control. In this regard, the VSWTs are equipped with efficient supplementary frequency control loops to support inertial response, primary frequency control and secondary frequency control [31; 32; 33]. A combination of inertial response and a droop active power support in VSWTs was shown in Reference [34], in which the wind turbine reached efficient frequency control for different levels of penetration. A new control strategy to coordinate the inertial control,  
Figure 1: (**a**) Frequency regulation process; (**b**) control strategies for wind farms to contribute to the AGC. AGC, automatic generation control; FACTS, flexible AC transmission systems; MPC, model predictive control. rotor speed and pitch angle control in a DFIG are presented in Reference [35]. In Reference [36], a deloading strategy was proposed alongside with inertial control loop for the PMSGs to contribute to the frequency control, by increasing their active power production. Efficient damping of frequency oscillations and improving the frequency regulation capability were the main advantages for the strategy. In sharp contrast to MPPT, deloaded strategy is a method to limit the generation of wind turbine through power curve of turbine to make it feasible to contribute to frequency control [37]. In Reference [38] a novel strategy was proposed to strengthen the primary control capability of VSWTs for frequency control with fast response, which can improve the VSWTs contribution in the AGC.