**5. Conclusions**

Taiwan, due to its geographical location, has an excellent wind conditions and high potential for wind power production as well as strong typhoons which have damaged several wind turbines in recent years. The aerodynamic loads during typhoons were investigated by using the extreme wind conditions prescribed in IEC 61400-1 together with a measurement-calibrated gus<sup>t</sup> factor. The extreme wind conditions proposed in the present study were based on meteorological measurements from the Zhangbin area during typhoon invasions from 2007 to 2014. A statistical approach was employed to convert the raw data into a fitting formula to quantitatively describe the extreme wind conditions for typhoons. Our study shows that the 1-min average wind speed was 36 m/s, the maximum 3-s average wind speed was 45.36 m/s, and the maximum instantaneous wind speed was 64.85 m/s, where the maximum instantaneous wind speed in Taiwan was about 20 m/s higher than the values suggested in the IEC standards. Additionally, the gus<sup>t</sup> factor for typhoons was estimated at 1.26 with a gus<sup>t</sup> period of 6 s. This local gus<sup>t</sup> factor was used to calibrate the EOG model constant, such that a transient gus<sup>t</sup> time series was reconstructed for the unsteady simulation. The turbulence intensity during typhoons was found to be higher than those of turbulence types defined in the IEC standards at low wind speeds below 30 m/s, but it declines quickly with the growth of wind speed.

The aerodynamic loads acting on the NREL 5-MW wind turbine in a steady extreme wind condition as well as an unsteady extreme gus<sup>t</sup> were studied by RANS simulations. The simulation results show that the extreme aerodynamic loads acting on the wind turbine are obviously underestimated when the extreme wind condition only adopts a constant wind speed. The amplification factor of aerodynamic loads is predicted as follows: 1.54 for the drag force, 1.4 for the rotor torque, 1.3 for the overturning moment along the y direction,

1.1 for the blade pitch moment, and the yawing moment by 1.35 times. The amplification factor is governed by the nature of aerodynamic loading. It is close to the square of the gus<sup>t</sup> factor in a stagnation-pressure dominant case while it approximates to the gus<sup>t</sup> factor when viscous shear plays a major role. The quantitative amplification factors may not be universal for different wind turbines in different areas because the present study is limited to the Taiwan local wind conditions and this specific wind turbine. With a concern of lacking validation under extreme typhoons in this study, more load measurements of in-situ wind turbines are expected in the future. Lastly, it is suggested to conduct future studies on the dynamic amplification of this reference turbine by adopting the proposed extreme turbulence intensity in EWM, such that the full aerodynamics can be resolved and compared.

**Author Contributions:** Conceptualization, C.-Y.Y. and J.-S.K.; methodology, C.-Y.Y., T.-Y.L. and S.-W.C.; writing—original draft preparation, C.-Y.Y.; writing—review and editing, T.-Y.L. and S.- W.C.; visualization, C.-Y.Y. and T.-Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Ministry of Science and Technology (MOST), Taiwan [grant No. 109-3116-F-006-013-CC1] and [grant No. 109-2221-E-002-090-].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Bryan Nelson, a wind energy specialist at CR Classification Society, for English proofreading.

**Conflicts of Interest:** The authors declare no conflict of interest.
