Nonlinear Control Strategy Of Voltage Source Converter Based High Voltage Direct Current Transmission System

With the rapid development of power electronics,voltage source converter based high voltage direct current transmission(VSC-HVDC)technology is becoming mature,and has been widely used in the field of transmission.VSC-HVDC system has become an indispensable part of wind power integration and grid interconnection.Because the VSC-HVDC system has the characteristics of nonlinearity,strong coupling and multivariable,the traditional PI control is difficult to achieve satisfactory control effect.Therefore this thesis focuses on the research of active disturbance rejection control and sliding mode control strategy of VSC-HVDC system.The main work is summarized as follows.First,the whole structure of VSC-HVDC is studied in detail,and the mathematical model of the system under symmetrical condition is described,including the three phase static coordinate system and the mathematical model in the dq0 coordinate system.Research on the VSC-HVDC overall control strategy,including system level control,converter station control and trigger control.The double closed loop structure controller is designed by using the traditional PI control.The outer loop controls the voltage and power to generate a command current as the input of the inner loop control,the inner loop PI controller outputs the converter's command voltage signal and then transferred to the PWM trigger unit,and the pulse signal is produced to control the bridge arms.Secondly,the VSC-HVDC system has the characteristics of nonlinearity,multivariable and strong coupling.,in the inner-loop controller,two controllers are designed,using integral sliding mode surface and integral terminal sliding mode surface,to realize the decoupling and tracking of the current and improve the robustness and response speed of the system.In the simulation,the control performance of two integral sliding mode controllers and PI controllers are simulated and compared,simulation results verify the effectiveness of the inner loop controller.The effectiveness and superiority of the proposed control method are verified.Thirdly,for the stable operation point of the system is changeable and system mechanical and electrical parameters perturbation frequently.In the inner loop control,a fuzzy active disturbance rejection controller is designed to achieve decoupling of the direct and quadrature axis currents.An extended state observer is designed to estimate system uncertainties and disturbances and compensating them to improve the system's robustness and anti-interference ability.And the fuzzy rules are designed to adjust the parameters of the extended state observer.Simulation are compared with the control performance of the PI controller The simulation results show that the fuzzy active disturbance rejection control method designed in this thesis is more robust and the anti-interference ability is significantly improved.Finally,a finite time sliding mode control strategy based on fuzzy extended state observer is proposed.In the inner-loop control,the integral terminal sliding mode controller is designed to realize current decoupling and fast tracking,and an extended state observer is designed to estimate system uncertainties and disturbances and compensate them.And fuzzy rules are designed to adjust the gain parameters of the extended state observer and adjust the control volume in real time.The effectiveness and superiority of the proposed control method are verified.