Research On Control Strategy Of DC Dual Controllable DFIG System

The DC double-controlled DFIG system could be connected to the DC grid of offshore wind farms with low cost effectively.However,the nonlinear,multi-input and strong coupling system characteristics make system control effect unsatisfied for the traditional control scheme,and nonlinear control strategy is an effective solution.Therefore,this paper focuses on the control strategy and control structure optimization for DC double-controlled DFIG system and shore-based converter system.The innovative work is as follows:1)To solve the problems of external disturbance and system parameter perturbation in double-controlled DFIG system,this paper proposed a cooperative SMC control strategy with double-loop cascade control structure for DC double-controlled DFIG system based on stator flux orientation.First,in order to improve the approaching speed,an integral sliding surface is designed according to the system control target.Second,in order to improve the motion quality of sliding mode motion,the system switching voltage control rate is designed by equivalent control and exponential reaching law,and the system stability is proved by Lyapunov function.Finally,the feasibility and effectiveness of control strategy are verified by system simulation and experiment.2)Aiming at the problems of complex control structure,difficult parameter matching and control delay in a double-loop cascade SMC control strategy,this paper proposed a cooperative SMC control strategy with single-loop control structure for DC double-controlled DFIG system based on stator flux orientation.First,in order to establish the direct relationship between rotor speed and system control rate,the system motion equation is differentiated.Second,in order to reduce the design difficulty of sliding mode surface for second-order system,the integral-discretization method is used to reduce the system order and complete the design of single-loop SMC controller.Finally,the feasibility and effectiveness of control strategy are verified by system simulation and experiment.3)Aiming at the problems of traditional double-loop cascade control structure for shore-side converter system,such as difficult parameter matching and control delay,this paper proposes a single-loop MPC control strategy based on voltage orientation.First,according to the power conservation principle,in order to establish the direct relationship between DC voltage and switching voltage control rate,the capacitor voltage equation is differentiated.Second,in order to solve the problem of second-order model prediction accuracy,an integral-discretization method is proposed and a single-loop prediction model is completed.Third,in order to improve system control precision and ensure system operation safety,the state variable delay compensation function and current limit function are added in the controller respectively.Finally,the feasibility of control strategy is verified by system simulation and experiment.4)Aiming at the problem of steady-state error caused by neglecting switching devices loss of shore-side converter system single-loop MPC controller.In this paper,two system optimization schemes are proposed: First,according to the slow time-varying characteristics of switching power consumption,this paper completes the design of single-loop MPC controller based on switching device loss delay observation compensator by using effective system model and system sampling data;Second,in order to improve the robustness of switching device loss observer,a single-loop MPC controller based on switching device loss SMC observer is designed.Finally,the feasibility and effectiveness of control strategy are verified by system simulation and experiment.