Design And Research On The Three-level Voltage PWM Converter Control System

With the rapid development and wide application of the modern power and electronic technology, the harmonic pollution of the power grid caused by the common diode converter is increasing seriously. In order to improve the power factor of power grid, the multilevel rectifier gets more and more attentions from the society.Three-level voltage PWM rectifier could eliminate the pollution of harmonic wave in the power grid, while running on the state of the unity power factor. It can also achieve the energy flow between AC side and DC side. At the same time, the DC voltage of the rectifier DC side could be adjusted stably in a certain range. This paper takes the control system of the diode clamed three-level PWM converter as the study object and analyzes the principle and topological structure of the three-level PWM converter. Based on this condition, this paper deduces mathematical model in different coordinates. Through the detailed analysis of the traditional SVPWM algorithm, this paper designs a new SVPWM algorithm based on60degrees coordinate and makes the simulation contrast test by the two algorithms.In this paper, it adopts two different methods to design the control system:the direct current control and the direct power control. In the direct current control, it chooses the most common control method which is the double closed-loop PI control based on feed forward decoupling. This system includes the outer loop of the DC voltage and the inner loop of the current. The outer loop makes the DC voltage keeping constant, while the inner loop controls the current in accordance with the current instruction given by the voltage outer loop. In this way, it achieves the sine wave current control of the unity power factor as well as the energy feedback. In the direct current control, this paper completes simulation tests of MATLAB and makes the prototype tests which improve correctness and feasibility of the control method.The direct power control takes the active power and the reactive power as the control object. Compared with the direct current control, the advantages include simple control structure and fast dynamic response. This paper analyzes the instantaneous power theory, establishes the direct power mathematical model of three-level voltage PWM rectifier, designs the power hysteretic comparator and makes the switch table by the principle of maximum instantaneous power. In the direct power control method, this paper establishes the mathematical model of the direct power on MATLAB, completes the design of the system and simulation tests. It improves correctness and feasibility of the control method and provides theoretical basis for future design of the actual control system.