# On the Contribution of Wind Farms in Automatic Generation Control: Review and New Control Approach
## 1 Introduction  
In the 21 st century, electrical energy is needed more than ever, and the harmful effect of using fossil fuels to generate electrical energy, such as carbon dioxide emission, has become more serious. Accordingly, the demand on renewable energy sources to produce electricity from clean energies, such as wind, solar, hydro, biomass and geothermal, have globally increased. Renewable energies are salient choice to solve the air pollution problem, however the intermittent output power can create new challenges in the operation of power systems. Impacts of renewable energies on power systems operation cannot be ignored, and should be analyzed, along with developing effective mitigation strategies and technologies--especially for higher level of renewable penetrations. In the territory of renewable energy sources, development of wind turbine as a source to produce electrical energy from wind is going on swiftly around the world. In 2017, the installed wind energy worldwide was more than 539 GW [1]. Such a high generation needs more and more attention, in order to address the intermittency issues produced by wind turbine itself. Wind turbines are divided into two groups: Fixed speed and variable speed. The first group, fixed speed, generally use an inductiongenerator (IG) that is connected directly to the grid, and are known as fixed speed wind turbine (FSWT). The second group, variable speed, typically use permanent magnet synchronous generator (PMSG), or doubly-fed induction generator (DFIG) in their structure, and are known as variable speed wind turbines (VSWTs). Taking advantage of the power electronic converters, the PMSG is fully decoupled from the grid. It means that, the stator of PMSG is connected to the back-to-back fully rated power electronic converters in order to inject the power into the grid. In the other case, DFIGs have both direct connection and converter-based connection to the grid. The stator of DFIG is connected directly to the grid while the rotor is connected through partially rated back-to-back converters. The power electronic converter used in a variable speed wind turbine enables the wind turbine to regulate the output power over a wide range of wind speeds [2,3]. The variable type is the dominant and promising type of wind turbines for application in large-scale wind farms. In the domain of wind turbines, VSWT technology has attracted a lot of attention for integration in power networks, because of its salient features. The primary advantage of VSWT driven wind generators is that, they allow the amplitude and frequency of their output voltages to be maintained at a constant value, no matter the speed of the wind blowing on the wind turbine rotor. Therefore, it can be inferred that they can be directly connected to the ac power network and remain synchronized at all times with the ac power network. Other advantages include the ability to control the voltage at point of common coupling and power factor control (e.g., to maintain the power factor at unity). Furthermore, the VSWT-based wind generators can produce the maximum power at variable speeds of wind [4]. Penetration level of large scale wind farm is increased in power systems [5]. Since that the large-scale wind farms have high capacity, they should be investigated like conventional power plants as they are connected to the electrical power network. In Reference [6], static planning of wind farms in power networks is reported by optimal load flow. Contribution of wind farms in ancillary services of power system are investigated in Reference [7]. In this regard, wind farms can contribute to voltage and reactive power control [8,9], frequency stability and control [10,11,12], power system stability enhancement [13,14], harmonic mitigation [15], oscillation damping [16] etc.