Research On Modeling And Control Of Three-level PWM Rectifier

With the development of rectification technology, various converters which are widely used in industrial production have improved efficiency greatly, but also a large number of harmonics and reactive power are injected into the grid. Currently, the main way to solve the problems of serious pollution to grid is introducing the PWM technology to control the rectifier, in order to achieve unity power factor operation. So the research on modeling and control of three-level PWM rectifier has important theoretical significance and application value.This thesis builds the model of three-level rectifier under certain approximate derivation, based on the principle of analyzing the four-quadrant operation of PWM rectifier and the idea of switching function, obtaining the mathematical model in the three-phase stationary coordinate system. In order to solve the problem that there is coupling among neutral current and three phase current in the model of three-phase stationary coordinate system, the paper has introduced the idea of coordinate transformation to transform the model from three-phase stationary coordinate system to two-phase rotating coordinate system which reduces the order of the system, according to the principles of grid voltage vector orientation. For designing the controller by the engineering approach of univariate linear system, the model in two-phase rotating coordinate system has been treated approximately under certain conditions, and ultimately we get the compact four-order linear model, which is prepared for designing the controller. Based on strategy analysis of the control of three-level rectifier, we have made the solution that design the structure of the rectifier system into the double closed-loop which is consist of voltage loop and current loop, in order to realize the purpose that the power factor of network side keeps 1 and the voltage of dc side keeps stability, according to the control objectives of both the network and DC sides. Completing the decoupling of the state feedback of current, we design the inner current controller and the outer voltage controller respectively, as well as verifiy the approximate equivalent condition, according to the engineering design methods of making the atypical system typical. By analyzing the results of Matlab simulation, we have proved that the dynamic indicators of the designed system coincide with the expected targets. For dealing with the problems of DC bus voltage caused by the disturbance of load current, we have introduced the feed-forward compensator to improve the anti-jamming performance of the system, and finally verify the anti-jamming performance by simulating the feed-forward and feedback complex control system.Finally, to further discuss how to improve the anti-jamming performance and dynamic performance of the system, we have introduced the generalized predictive methods. Based on in-depth analysis of generalized predictive control theory, we design the voltage controller of the system using the model with no simplifying. To verify the characteristics of the system based on generalized predictive algorithm, we run the procedures of generalized predictive algorithm in Matlab simulation environment and analyze the results of simulation on both dynamic performance and anti-interference performance sides. It turns out that the generalized predictive control can better overcome the interference of uncertainties to enhance the robustness of the system, and the methods of engineering design have significant advantages in improving the dynamic performance of the system.