**7. Conclusions**

In this study, a new LQR-PI control algorithm was designed and proposed to improve the performance of conventional PI control. For this, numerical modeling of a target 100 kW horizontal-axis PMSG-type wind turbine was performed, and an LQR-PI control algorithm using an LQR controller as a feedforward controller to the conventional PI control was introduced. To verify the proposed control algorithm by simulation, a conventional PI and an LQR controller were also designed for the target wind turbine, and comparisons of the simulation results for the three di fferent controllers were carried out. The simulations were performed with and without noise.

The results showed that the LQR control improved the performance only in the rated power region where the noise was not considered, but the proposed LQR-PI control was able to maintain the stability by reducing the standard deviation of the generator speed in all cases, with and without considering noise in the generator speed signal. With the proposed LQR-PI, the standard deviation of the generator speed was reduced by 38.85% in the transition region and by 26.86% in the rated power region when the noise was not considered. Also, it was reduced by 35.29% in the transition region and by 21.48% in the rated power region when the noise was considered. Therefore, it can be concluded that the LQR-PI control was e ffective in improving the stability of the wind turbine with a minimal change to the existing PI control. In particular, the proposed LQR-PI control is expected to improve the annual energy production of the target 100 kW wind turbine because it can significantly reduce the standard deviation of the generator speed and, finally, the frequency of shutdowns due to overspeed in the generator.

**Author Contributions:** All authors equally contributed to the data analysis and the simulation, the results analysis, the writing, and review.

**Funding:** This work was partly supported by a Korea Institute of Energy Technology Evaluation and Planning (KETEP) gran<sup>t</sup> funded by the Korean governmen<sup>t</sup> (MOTIE) (20173010025010, Advancing of micrositing technology for wind farm development, and 20184030201940, Graduate Track for Core Technologies of Wind Power System Engineering).

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