Variable speed and constant frequency control of hydraulic wind turbine with energy storage system

Abstract

To eliminate the adverse effect of the fluctuation and intermittence of wind power on the quality and stability of electrical power system, an energy storage system is introduced into the closed-loop hydraulic system of hydraulic wind turbine for the first time. The whole hydraulic system consists of a fixed displacement pump, a variable displacement motor, two proportional control valves and an energy storage system. The energy storage system absorbs or releases oil as the wind fluctuates. When the wind suddenly disappears, the generator can continue to produce electrical energy by means of the discharge of the hydraulic energy stored in the energy storage system. On the basis of modelling all hydraulic components, the simulation model of the hydraulic system is established. A new control method is presented within this article, which keeps the motor speed constant to generate constant frequency electrical power when the rotational speed of the wind wheel changes. Ultimately, simulations under the two conditions of step and sine wind wheel speeds are done. The simulation results demonstrate how the motor, the proportional valves and the energy storage system work together when the wind wheel speed varies and also prove validity of the control method we designed.

Combined constant speed control method for a wind generator equipped with hydraulic energy storage

“…The following assumptions are considered before establishing the mathematical model of the system: The leakage flow of the hydraulic motor is laminar. The gas state in the accumulator changes to an isothermal process, and the gas is an ideal gas. The pressure loss in the pipeline and the dynamic characteristic of the pipeline are neglected for a short pipeline length. The bulk modulus and density of hydraulic oil are constant.According to the principle shown in Figure , we establish the flow and outlet pressure equations of the hydraulic accumulator, the flow equation of the proportional valve, the flow equation and torque balance equation of the variable motor, and the torque balance equation of the synchronous generator. …”

“…Liu et al studied the variable‐speed‐input–constant‐speed‐output control technology of a 600‐kW hydraulic energy‐regulated wind generator closed system. They proposed a double closed‐loop control strategy that could fully utilize the energy storage system to control the motor to provide a constant speed output during wind energy fluctuations …”

“…They proposed a double closed-loop control strategy that could fully utilize the energy storage system to control the motor to provide a constant speed output during wind energy fluctuations. [34][35][36] The constant speed control of the "fixed displacement pump-variable displacement motor" system mostly adopts the volumetric control method, and the speed is adjusted by changing the displacement of a variable motor. This strategy results in high regulation efficiency but a slow response speed, which is suitable for high-power hydraulic systems.…”

Analysis of Dynamic Characteristics of a 600 kW Storage Type Wind Turbine with Hybrid Hydraulic Transmission

“…The model of a 600 kW closed hydraulic wind turbine comprising variable pump and motor was completed and the speed control study of the wind wheel and synchronous generator was carried out. Because the currently available variable pump cannot work well in the working speed range of the wind turbine, this closed hydraulic wind turbine cannot now be put directly into production [21][22][23].…”

“…Besides, the hydraulic wind turbine principles in refs. [20][21][22][23] cannot be directly applied to the design of 600 kW storage type wind turbine experimental prototype for the reasons noted. A storage type wind turbine composed of closed loop and open loop hydraulic transmissions was first advanced to deal with the problem, as shown in Figure 3.…”