**6. Conclusions**

WTs are exposed to harsh conditions year-round, and the variability of ambient conditions can affect WT monitoring directly. The main bearing temperature can be different in the same working state but different under ambient temperature and wind speed changes. Conversely, a data-driven model may fall into local minima due to the randomness of the initial coefficients, which can also affect the monitoring accuracy.

To solve these problems, a novel WT monitoring method is proposed. The changes in ambient conditions are used as inputs of the WT model, and the GA-ELM is applied to construct the WT model. Model testing shows that, compared to ELM and BPNN, GA-ELM can avoid local minima and achieve smaller MSE, MAE, and MAPE. With cases in different ambient conditions and real failure, it proves that, compared to the method without considering the change in ambient conditions, the proposed method could reduce false alarms when WT is in a normal working state under extreme ambient conditions, and generate an earlier alarm when a failure is about to occur. This means that the proposed method can reduce operation and maintenance costs, thereby improving the economic efficiency of wind power generation.

It should be noted that this study focuses on ambient temperature and wind speed to describe ambient conditions. However, ambient conditions also include other factors, such as air humidity and air pressure. In future research, these factors should also be considered in order to improve the accuracy of WT monitoring.

**Author Contributions:** Methodology, Z.H. and X.L.; formal analysis, Z.H.; writing—original draft preparation, Z.H.; writing—review and editing, X.L.; project administration, S.Z.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key R&D Program of China, gran<sup>t</sup> number 2017YFB1200703.

**Data Availability Statement:** Not applicable.

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