## 3 NTF Derivation and Application
### NTF Application  
NTF can be applied to other wind turbines if they are the same type as the turbine tested, and further, the requirements for terrain class are satisfied [8]. The two test wind turbines met the requirements; they are the same type, and their terrain classes are the same because they are operating at the same wind farm. The nacelle anemometer wind speeds of wind turbines no. 1 and 15 were corrected to free-stream wind speeds by applying the derived NTFs. The corrected wind speeds were corrected again to the normalized wind speed at the reference air density to exclude the effect of wind variation with air density. The power curves were finally drawn using the bin method with an interval of 0.5 m/s.  
Figure 4a,b show \(\mathrm{PC_{NTF,\ NL}}\), \(\mathrm{PC_{NTF,\ Cup}}\), \(\mathrm{PC_{Cup}}\) and \(\mathrm{PC_{NL}}\) with respective power coefficients for the test wind turbines. In the figures, the power output ratio represents the measured power output, \(\mathrm{P_{m}}\), divided by the rated power output, \(\mathrm{P_{r}}\), and the wind speed ratio denotes the measured wind speed, \(\mathrm{V_{m}}\), divided by the rated wind speed, \(\mathrm{V_{r}}\). Each bin of all power curves had a minimum of three sample data points.  
In Figure 4a, the power curves and the power coefficients for wind turbine no. 1 were compared. Here, \(\mathrm{PC_{NL}}\) is a nacelle LIDAR power curve that was drawn according to the procedure proposed by [PERSON] et al. [23; 24; 25]. \(\mathrm{PC_{NTF,\ NL}}\) and \(\mathrm{C_{P,\ NTF,\ NL}}\) were very similar to the references \(\mathrm{PC_{NTF,\ Cup}}\) and \(\mathrm{C_{P,\ NTF,\ Cup}}\), respectively. On the other hand, \(\mathrm{PC_{NL}}\) and \(\mathrm{C_{P,\ NL}}\) were lower than the others. The power  
Figure 3: Relationship between the nacelle LIDAR and the nacelle anemometer wind speeds.  
curves and the power coefficients for wind turbine no. 15 were compared in Figure 3(b). Here, \(\text{PC}_{\text{Cup}}\) was drawn in accordance with IEC 61400-12-1 1 st edition, which was a reference. \(\text{PC}_{\text{NTF, NL}}\) was higher and \(\text{PC}_{\text{NTF, Cup}}\) was lower than \(\text{PC}_{\text{Cup}}\). \(\text{PC}_{\text{NTF, NL}}\), was similar to that for wind turbine no. 1. A higher \(\text{C}_{\text{P, Cup}}\) than the others was observed under low wind speeds, which was caused by very low wind speeds compared to the high power output in the early stage of the wind speeds.  
Table 6 lists the results of the two-parametric linear regression analysis between the power curves when \(\text{PC}_{\text{NTF, Cup}}\) of wind turbine no. 1 and \(\text{PC}_{\text{Cup}}\) of wind turbine no. 15 were references. The correlations of \(\text{PC}_{\text{NTF, NL}}\) of wind turbines no. 1 and 15 were very high, with slopes of 1.002 and 0.997 and a coefficient of determination, \(\text{R}^{2}\), of 0.999 and 0.997, respectively. Because \(\text{PC}_{\text{NTF, NL}}\) were very similar to the reference power curves, \(\text{PC}_{\text{NTF, Cup}}\) and \(\text{PC}_{\text{Cup}}\), which were derived from current IEC standards, the NTF from nacelle LIDAR measurements could be applied to other wind turbines for power performance testing. The slopes and \(\text{R}^{2}\) values of the other power curves had a high correlation.  
Figure 4: Comparison of power curves and power coefficients.