Design and control of the mechanical-hydraulic hybrid transmission system in wind turbines

As wind turbines become larger and move into deeper sea, their operating environment worsen. The torque fluctuation inside the drive chain is aggravated, which leads to the premature failure of the wind turbines. To improve the transmission stability of wind turbines, the mechanical-hydraulic hybrid transmission system (MHHTS) has been applied. However, existing research has issues with structure and hydraulic control. An MHHTS structure was proposed to meet the requirements of structural simplicity and high efficiency, corresponding to the design principles summarized in the article. Based on this structure, we have selected a hydraulic control mode, the constant displacement pump variable displacement motor (CPVM), that satisfies the reasonable utilization of displacement and the stability of control. This mode was chosen by comparing the displacement and pressure variation rules of three hydraulic control modes that meet the maximum power point tracking (MPPT) control principle of the wind turbine. In this study a co-simulation model of a 10 MW MHHTS wind turbine was built using Simulink and AMEsim. The correctness of the structural and control mode ware verified by co-simulation with a uniformly varying wind. Moreover, a method that can suppress fluctuations in output power by controlling the motor displacement was proposed. Different-step winds response analyses were conducted to explore the relationship between the displacement response speed and the output power fluctuations. The non-linear relationship between the displacement and the wind speed can also have an impact on the previous relationship. Finally, the feasibility of the control method was verified through simulation of an actual turbulent wind. These achievements should guarantee the future application of the novel transmission system in wind turbines.