In order to meet the increasing demand of wind energy utilization, wind turbines (WTs) are developing toward the trend of large size and large capacity. In such a trend, various advanced yaw control strategies have been proposed to improve large WTs' comprehensive performance, but the analysis and summary of these strategies are still lacking. Therefore, it is necessary to have a review of yaw control, which not only enables readers to understand the current status of yaw control research but also promotes the development of wind energy technology. This paper presents a review of the current situation of yaw control for WTs, focusing on the mechanical/aerodynamic parts. The mechanical part is concerned with the WT yaw system and its effect on the fatigue load of the WT, and the aerodynamic part involves the wind energy capture and wake redirection to reduce the impact on adjacent WTs. In this review, the existing yaw control methods are classified in term of three control objectives: (1) increasing the wind energy capture of a single WT, (2) reducing the fatigue load of a single WT, and (3) maximizing the total power production of the whole wind farm and optimizing the wind farm fatigue load. On this basis, the control mechanism, the control algorithm, and the results are presented and analyzed in detail. Meanwhile, the advantages and disadvantages of the existing achievements are discussed. In addition, in a conclusion of the review, the future research direction has been identified.

With the wide application of distributed DC microgrids and the increasing of DC loads, the distributed control method has become one of the research hotspots. However, the distributed control method based on distributed communication network usually involves the problem of communication delay, which may lead to the failure of the controller in the practical application and even affect the stability of the system. This paper introduces a novel distributed secondary control algorithm without droop control, which can be used to realize voltage recovery and load sharing simultaneously. Based on this, two stability criterions under different conditions are derived for the DC microgrid system considering communication time delay, which are delay-dependent stability criterion under constant delay and time-varying delay-dependent stability criteria by using Linear Matrix Inequality (LMI). These stability criterions are helpful to guide the selection of control parameters. Simulation results verify the effectiveness and feasibility of the proposed control strategy, and show the correctness of the delay-independent stability criterion and delay-dependent stability criterion.