Research On Mitigation Strategy For Subsynchronous Control Interaction Of Wind Power Grid-connected System

The grid integration of large-scale wind power through series compensation has caused the problem of subsynchronous control interaction,which carries a risk for safe operation of the grid.The suppression measures based on the use of FACTS devices increase the investment and fail to fully explore the potential of the converter control of wind turbine generators.The subsynchronous damping control,which is based on the filtering and phase-compensation,cannot maintain satisfactory performance under operating conditions far away from the prescribed resonant frequency.It is difficult for subsynchronous damping control to address the time-varying characteristics of subsynchronous control interaction and the nonlinearity and complex operating conditions of wind power system.Therefore,this thesis conducts related research on suppression strategies for doubly-fed induction generator-based wind power system and direct-drive permanent magnetic synchronous generator-based wind power system.The main contents are as follows:1)Establish the model of doubly-fed induction generator-based wind power system and permanent magnetic synchronous generator-based wind power system to lay the foundation for the design of mitigation strategy.A feedback linearization sliding mode control is designed by using appropriate coordinate transformation and nonlinear state feedback,so that the nonlinearity of doubly-fed induction generator-based wind power system is compensated.For the transformed linear system,select linear switching manifold and exponential reaching law to design sliding mode variable structure control.Feedback linearization control method relies on the accurate modeling of the studied system,and this limitation is overcomed by sliding mode control.Based on the IEEE first benchmark model,the effectiveness of feedback linearization sliding mode control is verified through aggregated impedance model analysis,eigenvalue analysis and time domain simulation.2)It is difficult for feedback linearization sliding mode control to quickly eliminate subsynchronous control interaction under severe fault conditions.Therefore,fractional-order sliding mode control strategy is designed.Fractional-order sliding mode control is a combination of sliding mode control and fractional calculus.The additional degree of freedom,given by fractional operator,is utilized to realize rapid suppression of subsynchronous control interaction.Simulation results based on IEEE first benchmark model and multi-machine system verify that fractional order sliding mode control can achieve rapid suppression of subsynchronous control interaction under a variety of series compensation degrees and external disturbances.3)For doubly-fed induction generator-based wind power system,a finite-time control strategy based on fractional-order sliding mode control and direct power control is designed.It is proved that doubly-fed induction generator-based wind power system can reach the steady state within a finite time under the designed control strategy.Simulation results verify that the finite time control can achieve rapid suppression of subsynchronous control interaction under a variety of operating conditions,and enhance the robustness of the controlled system.4)From the perspective of energy,energy shaping-L₂ gain control strategy is designed to suppress subsynchronous control interaction in doubly-fed induction generator-based and the permanent magnetic synchronous generator-based wind power system.First,Hamiltonian model of the wind power system is obtained.The energy shaping method is employed to make the expected system energy function remain a local minimum at the equilibrium point.Based on L₂-gain method,the excitation control voltage is adjusted to attenuate the disturbance.The design process of energy shaping-L₂gain control is clear and simple.Simulation results show that energy shaping-L₂ gain control can effectively suppress the subsynchronous control interaction,and the performance is better than the subsynchronous damping control.