Energy-Shaping Control Strategy Of Doubly-Fed Wind Power System

As a clean power supply form, wind power has been significant to optimize the energy structure and implement the low-carbon economy strategy for our country. Among various kinds of wind power systems, doubly-fed induction wind power system becomes mainstream of large-scale ones for its low converter cost, wide power range, and active and reactive power independent adjustment. Since the doubly-fed wind power system (DFWPS) is a multivariable nonlinear system with strong coupling, applying modern control theory to search out novel advanced control methods for improving system performance and power quality is getting widespread attention. This thesis focuses on the research of novel and robust control strategies for machine-side and grid-side control of the DFWPS.A novel perspective, energy-shaping perspective, fully behaves the physical essence of dynamical energy conversion in the wind power system and gets rid of the traditional control principle, that is, the system is solely considered as a signal-processing device that transforms certain inputs into outputs. Breaking down the traditional thought pattern that a wind power system is an active system which generates the electrical energy, we consider the wind power system as a passive system with dissipation on condition that the wind energy is treated as one of external input energies. Thus, passivity theory can be applied to describe the processes of energy storage, consumption, and transmission. Then energy-shaping control method will be appropriate for the study of wind power control strategies. The main contents include four parts: new modeling of the DFWPS, novel energy-shaping control strategy for feedback interconnection, control strategies for machine-side and grid-side of the DFWPS based the energy-shaping control strategy, and coordinated control of the DFWPS based the energy-shaping control strategy.Applying port-controlled Hamiltonian (PCH) modeling method to construct the model of the DFWPS is a brand new way with regard for the energy flow process of the system. Considering the complexity of the system, it is proposed that the whole system is divided into four subsystems: the mechanical subsystem, the electromagnetic subsystem, the DC-link subsystem and the grid side subsystem, according to the energy storage components. The energy stored in each component is taken as the energy function of each subsystem. The port variables of the four subsystems are elaborately chosen so as to determine the interconnected structure among the subsystems. In terms of the port variables, the PCH model is designed for each subsystem. Based on this, the machine-side and the grid-side PCH models of DFWPS are further derived. These models pave the way for designing the machine-side and the grid-side energy-shaping control strategies. Also, the clear physical meanings of the models satisfy the essence rule of the energy transmission in the DFWPS and make the design process of the control strategies more clear and intuitive.Due to the subsystems of the DFWPS are connected by the feedback pattern, the advantages and disadvantages of the present energy-shaping control strategies which aim at the feedback interconnection structure, are studied. Two favorable properties are considered to be obtained when the PCH systems connect in the feedback way: one is dimension expansion for the PCH systems; the other is energy shaping control realization for the controlled system. Sequentially, a new energy-shaping control strategy, named"energy-based control", is proposed. This strategy is no restriction on the forms of the controller and the port variables, and no need for introducing Casmir function. Moreover, the strategy has definite physical meaning during its control process, and provides physical implications as well as preliminary design principles for the matrices J a, Ra in the energy matching equation.Based on the machine-side and the grid-side PCH models of the DFWPS and the novel energy shaping control method, the energy-based control strategies for the machine-side and the grid-side converters of the DFWPS are designed in terms of control objective analysis, desire equilibrium assignment and energy matching equation solution. Simulation shows that not only the energy-based machine-side control is able to realize the maximum wind energy capture, but also the energy-based grid-side control can make the DC-link voltage stable rapidly and ensure the unity power factor under the turbulent wind speed. To evaluate the controlling performances of the machine side and the grid side, comparisons between the energy-based controller and the classical PI controller are carried out. The results show the energy-based controller is more robust and faster convergent, whose characteristics are more suitable for the DFWPS with nonlinearity and multi-disturbances.The respective control for the machine-side and the grid-side of the DFWPS will cause the instantaneous energy difference between the two sides, thus large voltage fluctuations will exist in the DC-link. To avoid this drawback, the machine-side converter and the grid-side converter are regarded as an energy-related and coordinated unity from the energy-transmission point of view. Therefore, for the control of the machine-side and grid-side converters, a thought is proposed which is that incorporating each other's control information and matching the energy between the rectification and invertion, so as to strengthen the coordination for the energy extraction and supply at the machine side and the grid side. The entire PCH model with eight-dimension is constructed for the DFWPS and the energy-based coordinated control strategy is designed based on it. Compared with the energy-based respective control, simulation demonstrates that utilizing the coordinated control strategy, the objectives of the machine-side and the grid-side control in the DFWPS are both well accomplished, moreover, the superiorities of less DC-link voltage fluctuations and faster dynamic response to the wind speed are further obtained.This thesis comprehensively investigates the PCH modeling method and the energy-shaping control strategy for the DFWPS. Focusing on physical properties rather than the mathematical properties and combining tightly with the energy flow process, the nonlinear and robust energy-shaping control has well satisfied the requirements of the DFWPS with nonlinearity and multi-disturbances. The implementation of the proposed control strategy will provide a theoretical basis for the robust and nonlinear controller design of the large-scale wind power systems and have significance to alter the status that our country lacks the independent intellectual property rights of the high-performance wind power control system.