## 1 Introduction
There are many different forms for storing the wind energy: gravitational potential energy, compressed air, electrochemical energy, chemical energy, and kinetic energy [13]. [PERSON] et al. improved the quality of frequency regulation in isolated power systems by combining a hybrid wind-pumped storage hydropower plant (PSHP) with variable speed wind turbines (VSWT) [14; 15]. Battery energy storage systems are regarded as one of the most promising technologies, which can help to overcome the issue mentioned above [16; 17]. For a hydraulic wind turbine, compressed air energy storage has a great number of advantages of all of the energy storage forms, which can make full use of hydraulic energy without an energy conversion process and are more suitable for very large scales [18]. [PERSON] et al. presented a novel offshore wind turbines with an open compressed air energy storage system. A nonlinear controller was designed to capture maximal wind energy and satisfy power demand. Case studies showed that the storage system not only can downsize the electrical component to 1/5 of the turbine's capacity, but also can output a constant mean electrical power [19]. [PERSON] et al. investigated the open hybrid wind-tidal turbine with fluid power transmission and energy storage system. The open hydraulic system of these turbines are mainly comprised of a hydraulic pump and a pelton turbine. The simulation results demonstrated that the energy storage system can damp out the power fluctuations and deliver the desired generator power [20; 21]. [PERSON] et al. designed a compressed air energy storage system for the wind turbine with hydrostatic powertrain. The design parameters of the energy storage system, as the compression ratios, the expander ratios, and the air tank size, were chosen based on the simulations to realize the best stable performance [22]. The authors in [23] added an energy storage system into the closed-loop hydraulic system of hydraulic wind turbine to eliminate the randomness and fluctuation of wind power. A proportional valve was adopted to keep the motor speed constant and produce stable frequency electrical energy. The rotor speed has an important influence on the wind energy absorbed by wind turbine. In order to achieve maximum wind power coefficient, the rotor speed varies with the wind speed, which is determined by the optimal tip speed ratio. Therefore, a novel closed-loop hydraulic system with hydraulic accumulator is presented and applied to the wind energy generation system to realize simultaneously variable speed constant frequency and wind energy storage, as shown in Figure 1.  
Given existing research achievement, many researchers have made great contributions to the development of hydraulic wind turbine. However, the study on the dynamic responses of the rotor and motor to the step changes of the wind and rotor speed is seldom found for the closed hydraulic wind turbine with an energy storage system. Meanwhile, the compensation ability of the energy storage system to load power change also needs to be analyzed and validated. The rest of this paper is constructed as follows. Section 2 details the system configuration and energy flow diagram of the hydraulic wind turbine with an energy storage system. Section 3 depicts the mathematical models of all components in this hydraulic wind turbine. Section 4 presents first a speed control diagram for the rotor and motor speed controls. The speed control strategy is described later. Section 5 shows the main parameters of this hydraulic wind turbine aiming at a Micron 600 KW. Treating the step changes of wind speed, rotor speed input and load power as an input to evaluate the dynamic response performance of the control system. Section 6 concludes this work.