Research On High-order Sliding-mode Control Strategies Of DFIG-based Wind Power Systems

To solve the problems of shortage of fossil resources and the pollution of theenvironment, many countries are making efforts to develop the wind energy.Doubly-fed induction generators (DFIG)-based wind power system is themainstream in the current wind power market. Therefore the research on the controlstrategies of the DFIG-based wind power systems has significant meaning. In thisthesis, high-order sliding-mode based controllers are designed to control thedoubly-fed generator systems.The current status and trends of the development of wind power are surveied.Both the DFIG-and the PMSG-based wind power generation systems utilizing thevariable speed constant frequency (VSCF) technique are analyzed. The sliding-modecontrol theory applied in the thesis has two main advantages: the equivalentlow-pass filter designed in the control law can effectively eliminate the chattering,and the nonsingular terminal sliding-mode control can overcome the problem of thesingularity in terminal sliding-mode control.The operating principle of back to back PWM converters is analyzed. Themathematical modle of grid-connection converters is simplified based on the powerbalance between the AC side and the DC side. High-order sliding-mode controllerfor the DC bus voltage is designed to realize the convergence of the square of theDC bus voltage, and the high-order sliding-mode controllers for currents of theconverters are designed. The rotor-side converter is controlled using the maximumpower point tracking (MPPT) strategy. High-order sliding-mode controllers for boththe speed and the currents of the DFIG are designed to achieve the maximum windpower tracking. Utilizing no-load grid-connection mode, the high-ordersliding-mode controllers for the currents are designed to achieve the flexiblegrid-connection for the doubly-fed generators. Low voltage faults are the mostcommon faults of DFIG-based wind power systems. Research on low voltageride-through (LVRT) control strategies is critical to ensure the stability of theoperation of the systems. Based on the analysis of the low voltage faults, thehigh-order sliding-mode controllers of the currents which take the dynamic processof stator fluxes into consideration are designed to realize the control of the systemsunder the low voltage condition.The simulation results validate the proposed methods. Simulation results of PIcontrol and high-order sliding-mode control are compared, with the result that thelatter shows the advantages of strong robustness, fast convergence and high trackingprecision. Simulation results of traditional and high-order sliding-mode controllers are also compared, with the result that the latter has the ability to eliminatechattering, achieving perfect both dynamic and static performances of the system.