Research And Design On High-power Three-level PWM Rectifier

This paper takes high-power three-level PWM rectifier as its research object. A series of study methods, such as theoretical analysis, software simulation and prototype experiment, are used to investigate mathematical model, control strategy, modulation algorithm and some related problems about the rectifier.This paper firstly analyzes the operating principle of diode-clamped three-level PWM rectifier. Then its mathematical model under three-phase static reference frame is derived. Based on the principle of constant amplitude reference transformation and constant power reference transformation, a-b-c model stated above can be converted to d-q model which provides convenience for the design of control system.Using feed-forward decoupling dual closed loop control strategy, simulation platform of NPC three-level PWM rectifier control system is constructed in Matlab. For the rotating angle needed by reference transformation, this paper analyzes characters of power grid under balanced and unbalanced conditions with grid voltage vector as d axe, and then provides phase-locked loop solutions to trace the angle. Synchronous pulse board is designed to realize zero-crossing phase-locked loop. Experiment results prove the effectiveness of phase-locked loop solutions when power grid is balanced.This paper emphasizes pulse width modulation algorithm applicable for NPC three-level rectifier. Based on the principle of traditional space vector modulation and from the aspect of harmonic component of AC voltage, a conclusion is made that even-order harmonic can be eliminated when P type and N type small vector are selected alternatively as start vector. In order to reduce the complex computing of traditional SVPWM in digital system, a simplified SVPWM based on two-level and another simplified SVPWM in g-h reference frame are demonstrated. Harmonic analysis is also made for these two methods and their result is same with the traditional SVPWM. Finally, a selective harmonic elimination pulse width modulation is proposed. This algorithm is applied in dual-closed loop rectifier control system successfully with specific order harmonics eliminated.The problem of neutral point balance about NPC three-level PWM rectifier is discussed in this paper. Some factors which may cause zero-point fluctuation are analyzed in theory. After making comparison of some solutions,f factor adjustment method is finally chosen to control the neutral point voltage.Based on DSP TMS320F28335, this paper develops a set of control solution about NPC three-level PWM rectifier with IPM modules. A human-machine interface of Siemens OP 177B touch panel is also designed for ensuring accurate transmission of control signals and display parameters.