With the development of large-scale wind power generation and offshore wind energy, reducing the nacelle weight and the gear failure rate is increasingly important.Hydraulic transmission is characterized by its flexible layout and transmits large energy with small volume and weight, which suits the demands of wind power generation. In this paper, a thorough review of hydraulic technology application in wind energy is carried out, in the aspect of pitch, brake, yaw, transmission, new applications, and the potential problems. K E Y W O R D S hydraulic brake, hydraulic pitch, hydraulic transmission, hydraulic yaw, new applications, wind energy 1 | INTRODUCTIONHydraulic transmission applied to wind energy is not a new concept, and early works by JERICO 1 showed that a lack of component availability is the main factor hindering its implementation. Some commercial wind turbines are equipped with hydraulic pitch or yaw mechanism, but after several years, oil leakages affected the turbine exterior and the control performance forcing the reuse of electrical systems. With the development of hydraulic components and the growing size of wind power generation, hydraulic technology has gradually been applied in wind energy, such as the hydraulic pitch system 2 listed in Table 1, the hydraulic braking system, 3 and hydraulic transmission system 4,5 depicted in Table 2. With the development of hydraulic components and offshore wind energy (which has been estimated by the MAKE to be a 3.9GW grid capacity in 2018), hydraulic wind turbine will find its new development.In this paper, an overall review of the hydraulic technology applied in wind energy, including the hydraulic structure and the corresponding control strategy, is carried out. The article is structured as follows: In the first section, hydraulic technology and wind energy basic situation are mainly introduced, especially their potential binding points. In the second section, the hydraulic application in the pitch, brake, yaw, transmission between the rotor and the generator, and other novel application are mainly described. In the third section, a review about the hydraulic components studied specially for wind energy is made. In the fourth section, the deficiency and advantage of hydraulic energy applied in wind energy and the application prospect are discussed.

A wind turbine is the most typical machine used to capture energy from the wind. The system design goal of a wind turbine is to obtain as much energy as possible from the wind while transmitting as much of that energy to the grid as possible, all under the highest possible level of stability. Currently, many inherent weaknesses result in a high failure rate in common commercial wind turbines with gear drive systems. To improve the performance, in this study, a typical 1.5 MW gear transmission was redesigned into a novel hydromechanical transmission system (HMTS). Parameters related to the HMTS efficiency were analysed to ensure that the efficiency was relatively high. The variable speed ratio (VSR) and torque triple absorption (TTA) principle were thoroughly deduced. Moreover, a MATLAB/Simulink-AMESim cosimulation model was proposed, and a 30 kW proportional prototype was established. Both simulation and experimental results show that the structural design and proposed control strategies meet the design goals, including a relatively high transmission efficiency around the rated wind speed, speed control below the rated wind speed, and torque control above the rated wind speed. These achievements should guarantee the future application of the novel transmission system in wind turbines.

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