## 1 Introduction
So even if the expected power generation was calculated by selecting the hub height wind speed as the representative wind speed within the rotor disk, the error compared with the actual power generation was not large [21; 22]. However, in recent years, as the diameter of the rotor area of the wind turbine has exceeded 200 m, the hub height wind speed cannot be the representative value in complex terrain, such as mountains, or even in flat terrain, where wind speed fluctuations by height occur in short time, due to the meteorological characteristics within the local atmospheric boundary layer. In regions showing these characteristics, to analyze the effect of the difference in wind speed by height on power generation, it is necessary to consider several wind speeds corresponding to multiple heights at the same time [24; 25; 26]. Moreover, a detailed cause analysis is required that correlates atmospheric stability with the time when large wind speed difference by height occurs. The International Electrotechnical Commission (IEC) proposes a method to calculate the rotor equivalent wind speed by using the wind speed measured at (at least) three random heights [27]. It is maintained that the effect of the kinetic energy flux of the wind speed passing through the rotor disk area can be reflected in the power generation calculation considering the vertical wind shear [27]. In foreign countries, the concept has been introduced, and empirical studies of the rotor equivalent wind speed have already been actively conducted. The IEC Technical Committee 88 (TC88) recognizes that if the wind shear coefficient is kept constant, the difference between the rotor equivalent wind speed and the hub height wind speed is generally small [24]. However, in special cases, the vertical wind speed profile cannot be sufficiently explained by a constant wind shear coefficient, and such cases may be influenced by local meteorological characteristics or atmospheric stability distribution [12; 21; 22]. In this case, it is necessary to recognize that the difference between the rotor equivalent wind speed and the hub height wind speed may be considerable. In particular, it was revealed that the vertical wind speed shear of the rotor disk shows a difference in speed gradient according to atmospheric stability, and that this difference in vertical wind speed causes a change in the average wind speed [28]. In addition, the scattering of the data collected by the output curve of the wind turbine was analyzed less when using the rotor equivalent wind speed [23; 29]. It was recommended to use the rotor equivalent wind speed in the area outside the wind shear coefficient of (\(-\)0.05-0.4) [24], and claimed that REWS helps to improve the accuracy in the area where non-linear strong wind shear occurs [30]. As a result of predicting wind power generation output according to physical patterns by correlating meteorological characteristics and rotor equivalent wind speed through machine learning, it was confirmed that the prediction accuracy was improved by 22% [31]. In addition, since both the vertical wind shear and the horizontal wind shear (wind direction shear) can affect the amount of power generation, a new formula applied to the rotor equivalent wind speed has also been proposed [32; 33; 34]. In addition, the wind power generation based on the rotor equivalent wind speed can quantify the sensitivity to wind speed shear and direction change, as well as atmospheric stability [35; 36; 37]. On the other hand, it has been revealed that there is a statistically significant difference between the applied output curve and the conventional output curve in a form that considers atmospheric stability variables, such as the strength of wind turbulence passing through the rotor disk, the Richardson number, and the Monin-Obukhov length [12; 38; 39; 40; 41]. Since such atmospheric stability can have a significant effect on both the amount of power generation of a wind turbine, and stability and durability, a thorough preliminary investigation is absolutely required [12].