Research On Low Voltage Ride-through Strategy For Doubly-fed Wind Power System Based On Switched System Theory

Wind turbines are required to keep grid-connected to support the power system during voltage dips,which is called low voltage ride-through(LVRT)capability according to the modern grid coeds.As one of the common wind power systems,doubly-fed wind power system(DFWPS)is quite vulnerable to grid faults due to its structural characteristics.Therefore,to improve the LVRT capability of DFWPS is of great significance for further exploitation and utilization of wind energy resources.The switching phenomenon of DFWPS during a grid fault lead to more complex transient characteristics and more non-linear features.Thus,novel modelling methods to entirely depict the switching phenomenon and advanced nonlinear control strategies to achieve safe and smooth LVRT are in introduced in this thesis.This thesis starts with the analysis on transient characteristics of DFWPS during grid fault,which is the basis for the modelling of switched system and the design of LVRT strategy.The reasons that lead to overvoltage and over-current in DFWPS are investigated and the change trend of transient current is obtained based on the analysis of the transient process of the voltage,flux and current of doubly-fed induction generator(DFIG).And the results are validated by the comparison between values of calculation and simulation.To entirely describe the dynamic process of DFWPS during grid fault,switched system theory and energy control viewpoint are combined to establish the model of doubly-fed wind power switched system According to the change of control strategy and circuit structure,the operation of the system is divided into different modes.The complex dynamics of DFWPS in the process of LVRT are described by the switching actions between operation modes,and the physical properties of energy conversion,consumption and interaction of the system under different operating modes are depicted through the port-controlled Hamiltonian(PCH)structure,which is also the basis for the design of passive nonlinear control strategy based on energy viewpoint.To improve the grid-side control performance of DFWPS during grid fault,the damping matrix of grid side control is reconfigured based on the disturbance attenuation principle to suppress DC-link voltage fluctuation in mild grid faults.In the operation mode for severe grid faults,the reference value of reactive current is modified so that the DC capacitor and the grid side converter can supply the grid with reactive power to compensate the reactive consumption of generator.Finally,to realize the safe and smooth switching operation of DFWPS during grid faults,the switching control among the different operating modes of the switched system is designed with event-driven and logic-controlled switching rules,and the stability of this switched system is proved by multiple lyapunov function(MLF)method.The simulation results show that the designed control strategy can effectively realize the LVRT operation of DFWPS.