Study On The Control Strategy Of The Hydraulic Double-Regulation Variable-Speed Constant-Frequency Wind Power Generation System With Short-Term Energy Storage Function

The variable-speed constant-frequency (VSCF) control technology is regarded as the optimal solution of the current wind power generation technology (WPGT), because the speed-increasing gearbox in the traditional VSCF wind power generation systems (WPGS) and the power electronic converter equipment have an obvious impact on the cost and reliability of the whole wind generating set, which restrict seriously the development of large-scale WPGS. While, the novel hydraulic VSCF wind power generation system not only has a simple structure and good robustness, but also makes the terrestrial power generation is possible by avoiding using the gearboxes and power electronic converter equipment. And thus, it has become one of the hot topics of next WPGT.However, for the hydraulic VSCF wind power generation technology, the small speed range, large overflow loss and low hydraulic transmission efficiency are hindering its further development. Accordingly, in this thesis, a double-regulation hydraulic VSCF wind power generation system with a short-term energy accumulator is put forward and investigated to enlarge its speed control range and improve its efficiency.Firstly, by analyzing the running characteristics, reliability and cost of various VSCF wind power generation technology, some problems existing in the current VSCF wind power generation systems and the single-regulation hydraulic VSCF wind power generation technology are summarized. Furthermore, the advantages of the proposed double-regulation hydraulic VSCF wind power generation system with a short-term energy accumulator are clarified from the point of view of the short-term energy storage characteristic of wind power generation and the matching between the hydraulic energy accumulator and hydraulic system.Secondly, based on the analysis of working principle and topological structure of the hydraulic VSCF wind power generation system, the corresponding mathematical model is established. Moreover, for two types of single-regulation hydraulic speed control circuits with different structure and application, the stability analysis is performed based on the principle of non-zero minimum phase, from which it is known that the stability condition of the single-regulation hydraulic VSCF wind power generation system is proportional to the rotary inertia, viscous friction coefficient and system leakage, but is inversely proportional to the liquid capacity of hydraulic circuit. Aiming to its shortages, especially the problem of the maximum power point tracking, a double-regulation hydraulic VSCF wind power generation system is proposed.Thirdly, in order to understand the effects of dynamic response of the displacement of hydraulic pump and hydraulic motor on control performances, some key parameters, such as load and its rotary inertia, pressure and rotary speed, are investigated to find their influences on the response speed and stability of the proposed double-regulation hydraulic VSCF wind power generation system. Due to the strong-coupling, essential non-linearity and uncertainty of the speed control circuit, a control law is presented to improve the robustness of wind power generation system combining the differential geometric feedback linearization with the sliding mode variable structure.Fourthly, for the maximum power point tracking approach used to the typical VSCF wind power generation systems, when it is used to the double-regulation hydraulic VSCF wind power generation system, it is found that the actual wind speed is hard to measure, the universality is poor the rapid control is difficult to implement, and the oscillation easily occurs at the maximum power point. Consequently, a novel maximum power point tracking control approach is presented to track the maximum power point under the disturbance working conditions, where the power disturbance is variable step-size. And then, the further theoretical analysis and simulation are carried out.Fifthly, considering the effects of the turbulence wind speed with micro-scale on the output power of wind turbine, an improving output energy approach is presented, which combines the short-term energy storage characteristics of a hydraulic energy accumulator with the constant rotary speed and the increasing the torque at the rated wind speed of micro-scale turbulence. And then, the comparison of two double-regulation hydraulic VSCF wind power generation systems with different configuration model of energy accumulators is performed. It is found that, comparing with the configuration model directly connected an energy accumulator to the hydaulic curcuit, the case connected an energy accumulator to the pump/motor is benefit for improving effectively the engery generated from the wind power generation system and making up its efficiency defect in some degree.Finally, the proposed double-regulation hydraulic VSCF wind power generation system with the short-term energy accumulator is validated to be feasible using the designed test bed especially. In addition, experimental results also show that the presented maximum power point tracking approach can meet the running demand of the corresponding system and the short-term energy storage can enhance the generated energy of WPGS largely.