Wind energy as one of the renewable energies is serving as an indispensable role in generating new electric power. The worldwide installation of wind farms has considerably increased recently. To extract more wind resources, multi-megawatt wind turbines are usually designed and constructed with large rotors and slender tower. These flexible structures are susceptible to external dynamic excitations such as wind, wave and seismic loads. The excessive vibrations can compromise the wind energy conversion, lead to the structural fatigue damage and even result in the catastrophic failure of wind turbines in harsh environmental conditions. Various control devices have been proposed and used to mitigate the unwanted vibrations of wind turbines to enhance their safety and serviceability. This paper aims to provide a state-of-the-art review of the current vibration control techniques and their applications to wind turbines. Firstly, the widely used control strategies in engineering structures are briefly introduced. Their applications to suppress the adverse vibrations of the structural components of wind turbines, mainly the tower and blades, are then reviewed and discussed in detail. It can be concluded that the vibration mitigation of wind turbines is very challenging due to the fact that the dynamic behaviours of wind turbines are very complicated, which are associated with the aerodynamics, rotation of the blades, interaction between the tower and rotating blades, and soil-structure interaction, etc. Moreover, it is a challenge to straightforwardly use many of the conventional control devices because of the limited spaces in the tower and blades.

Introduction

Wind energy as one of the renewable energies plays an attractive means to generate new electric power. Wind farms have experienced rapid growth and expansion recently especially in the last decade. As reported by the Global Wind Energy Council, the worldwide installations of wind turbines reached about 539 GW at the end of 2017, with an increase of 2155% compared to that in 2011 [1]. Moreover, numbers of wind turbines are erected far away from coastlines to more efficiently extract the huge wind resources. The offshore wind power reached a historical record in 2017 with 4334 MW new installations, and the cumulative capacity was 18,814 MW [1]. Fig. 1 shows the global annual and cumulative installations of wind turbines from 2001 to 2017, and the total installations of offshore wind farms in 2011–۲۰۱۷ are shown in Fig. 2. It can be seen that though the offshore wind capacity in 2017 was only about 3.5% of the total wind capacity, it is growing very quickly and with great prospects. Wind turbines can be broadly grouped into the horizontal and vertical axis categories depending on the orientation of the rotation axis of the blades. Fig. 3 shows the typical geometrical configurations of the horizontal and vertical axis wind turbines (HAWTs and VAWTs) [2]. As implied by the name, the blades of HAWTs rotate about the horizontal axis, and they are perpendicular to the direction of wind flow, while the blades of VAWTs rotate vertically and they are not required to face the wind. As the HAWTs dominate the utility-scale wind turbine market currently, only the research works on the vibration control of HAWTs are reviewed and discussed herein.