## 1 Introduction  
The energy crisis and environmental pollution have been a threat to human survival and world economic growth because of excessive exploitation and use of fossil fuel. Renewable energy as the best replacement of fossil fuel is widely used in almost all countries in the world to provide energy during the past decades. Among all kinds of renewable energies, wind energy is experiencing rapid development for its clean, abundant, free, and environmentally friendly characteristics [1; 2]. The global total wind power industry increased by 12.6% (54.6 GW) in capacity in 2016. The global total growing capacity and China's cumulative installations were 486.8 GW and 168,732 MW at the end of 2016. The Five-Year Plan for Energy (2016-2020) calls for 210 GW of wind by 2020 [3]. The mechanical gearbox is a troublesome component within the traditional wind turbine, which has high failure rates and causes high maintenance costs [4]. Compared to the wind power with gearbox, the wind turbine with the direct-drive permanent-magnet generator is more attractive for the highest energy yield. However, this is more expensive and heavier [5]. In recent years, the gearless wind turbine with fluid power transmissions has attracted the attention of researchers around the world because of the unique advantages of variable transmission ratio, high power density,and reliability. The use of hydraulic drives not only can remove an AC frequency converter and a voltage transformer, but also make it possible to significantly reduce the weight of the nacelle and tower [6; 7]. [PERSON] et al. established the simulation model of a gearless hydraulic wind energy harvesting and transfer system using the SimHydraulic toolbox in the MATLAB. A prototype of the hydraulic system was made and the experimental results verified the availability of the proposed model [8]. In [9], a simplified model of the NREL 5 MW turbine with a hydrostatic drivetrain was derived and the control scheme and controller were designed to operate over the wind speed range of the wind turbine. The simulation results showed the controller has a satisfactory performance in its entire operating range. The authors in [10] presented a complete mathematical model of the hydraulic transmission used in wind turbines and proposed a valve control system to decrease the pressure and power fluctuations. Simulations under both below and above the rated wind speed displayed that the wind turbine with the hydraulic transmission has the same variable characteristics as the conventional variable speed wind turbines with gear. The paper [11] showed a hydraulic wind power transfer system which is used as a substitution of the traditional mechanical drive. The mathematical modeling of this gearless wind power transfer system was established and its accuracy was confirmed by comparing with test results. The authors in [12] proposed a secondary control hydrostatic transmission (SC-HST), which was used for the wind energy conversion system. A PID controller was designed for motor speed control. The simulation results demonstrated that the relative error of the motor speed was less than 2% and the efficiency of the novel system was 70.4%. Electric power produced by wind turbines is highly erratic because of the stochastic and intermittent nature of wind. The output instability of electric power will reduce the power quality and affect the planning of power systems. Therefore, an energy storage system will be indispensable for a wind energy conversion system. The investment cost of an energy storage system accounts for a large proportion of the total price of the wind turbine with an energy storage system. The larger the capacity of the energy storage system is, the bigger the total cost becomes. An energy storage system can control wind power plant output and provide additional energy to the power system via storing the excess energy and releasing the stored energy.