Study On DFIG Wind Energy System Under Low Voltage Ride Through

In recent years, as the development of technology in wind turbines, the proportion of wind power increases rapidly in power supply, and more and more attention is paid to the failure in the grid of wind power systems. As there will be a voltage sag when a failure appears in the grid, the wind turbines on the farm will be isolated from the grid, and can bring unstable in system.Moreover, it may also cause partial or complete breakdown of the grid. In order to maintain the stability of the power system, more and more countries pay more attention to wind turbine and grid. The requirements of low voltage ride through(LVRT) are aroused. Hence, the study on this issue has important practical and theoretical significance.The dissertation firstly reviews the current research results on wind turbines at domestic and abroad. The control strategy of high order sliding mode scheme is employed in this dissertation to tackle the problem of LVRT in wind power system. Firstly, the sliding mode control is theoretically investigated, and the design method of sliding mode controller are studied, as well as the method of how to eliminate in the conventional control strategy. These establish a basic fundamental for this for this dissertation.Secondly, it analyses the operational principle of the doubly-fed induction generator (DFIG) wind generator. DFIG has some advantages about capturing wind energy source suitably and capacity control flexibly in a wide speed range, and it can be used in a large varying velocity and constant frequency wind power system. And then, it creates a two-phase dq ordinate set of mathematical model of DFIG power system in theoretical derivation which aims at doubly-fed wind power generator mathematical model, coordinate transformation, and operating characteristics.In case of taking off grid, this dissertation will do some study about low voltage through control technology.It has analysed the response characteristics of DFIG in voltage dip, and then raise a strategy when in the condition of grid voltage dipping. Based on the control principle of sliding mode control, the dissertation designs a high-order sliding mode controller. The simulation studies with Matlab show the advantages of the proposed control strategy over the conventional PI control.At last, the analysis is made for the structure and control principle of the double pulse width modulation type converter in DFIG for the purpose of elimination of the effect of the DFIG wind system grid-side converter when a failure occurs in power grid. The grid-side converter in modeled in the two phase rotating coordinate system, and the control strategy of high-order sliding mode scheme is proposed. It not only makes it come true that controlling the AC unit power factor and DC link voltage, but also for outside input disturbance and internal parameters change has strong robustness. The effectiveness of this control strategy has been verified with simulation.