Design And Research Of Three-Level Rectifier Based On Direct Current Control

At present, in the fields of direct-current power supply, active power filter, reactive power compensation and power drive, controllable PWM rectifier has been used widely. While in the high-voltage and high-power applications, multi-level rectifier, especially three level neutral point clamped rectifier, is becoming the focus of research with its sinusoidal voltage output, low THD and low voltage the switch tube withstanding.In this paper, three-phase stationary coordinate system mathematical model of the main research object, which is called three level neutral point clamped rectifier, has been reduced. Also, the basic principle of the rectifier has been researched by analyzing the steady-state vector. The mathematical model in synchronized rotating coordinate system has been reduced by introducing the concept of coordinate transformation. Especially in that process, the design of PLL control system with a feed-forward control, which is used to obtain voltage phase angle, has been given. Because of the model of rectifier in d-q coordinate simplifying the design of the control system, this paper analyzes the principle of direct current control system, combined with the best engineering design parameters, and gives the methods as well as steps of controller parameters design. Meanwhile, the origins and physical sense of the parameters in the method described in detail has been explained.Three-level PWM modulation algorithm has been a key research of three-level converter. Firstly, analysis of the impact of any vector working on the three-level PWM rectifier has been done while the direction of alternating current fixed. Secondly, when deducing the conventional three-level SVPWM algorithm, the implementation method of it has been given. Finally, a practical and simplified algorithm of three-level SVPWM, as well as the neutral voltage balance strategy, in the g-h coordinate has been given under the idea of previous scholars.In addition, the hardware and software design of three-level rectifier was studied in detail. In the part of power circuit design, a practical method of setting the DC target voltage and the lower limit of the inductor has been given. In the part of software design, the paper adopts the design ideas that using the interrupt service routine as much as possible. Under that idea, the system is divided into sampling and processing functional unit, double-loop control systems, as well as communications service, while the main function is only responsible for simple task allocation. And also the software design flow chart under that idea is given.Finally, system simulation is conducted by the use of Matlab/Simulink software first to study the performance of three-level rectifier at rated load and power return. And also the results of simulation show the validity and feasibility of the control system include phase-locked loop, SVPWM algorithm and dual loop control system. Afterwards, a small prototype is built for a series of experiments such as diode rectification, the open-loop rectification, current closed-loop rectification, double loop rectification and neutral voltage balance control strategy.