Research On Three-phase PWM Rectifier Control Strategy Under Unbalanced Grid

Unbalanced matter appeared in electronic current transformation devices is hard to avoid, for load fluctuations and external disturbance. If three-phase voltage type rectifier takes no special measures to avoid unbalanced grid, harmonic would be caused in grid-side current or voltage of direct current side, which would pollute harmonic in grid and lower quality of DC power supply[1].Unbalanced grid exists in following two situations: First, asymmetrical three-phase VSR parameters lead to unbalanced system operation; second, there is serious unbalanced matter in grid itself. While in the design, the first situation is always equivalent to the second, that is, assuming three-phase VSR parameters are symmetrical. Now, solution to the first situation is: separate voltage and current in positive and negative sequence to eliminate harmonic in grid-side current or voltage of direct current side. Therefore, research on three-phase PWM rectifier control strategy under unbalanced grid focus on calculating instructions in positive and negative sequence and designing double closed loop controller[2-3]. In this thesis, a positive and negative sequence separation algorithm is proposed, meanwhile a direct power control scheme with improved sliding mode variable structure(SMVS) is also applied to acquire better dynamic and static performance.In this thesis, firstly, the author introduces situation and development trend of three-phase PWM rectifier, harm to traditional control strategy by unbalanced grid, and its overseas and domestic research status. Secondly, mathematical model and operating principle of three-phase PWM rectifier under unbalanced grid is recommended in-depth. The controller is quite difficult to design, because mathematical model in abc three phase static coordinate has a strong nonlinearity and coupling. Static coordinate should convert into dq two phase rotating coordinate, then, feed forward decoupling control strategy should be applied to control component in two phase coordinate separately, which would make the design much easier.Next, separation of components in positive and negative sequences is very important in three-phase PWM rectifier control strategy under unbalanced grid. Harmonic content of acquired components in positive and negative sequences will influence efficacy and stability of control, so this thesis analyzes merits and demerits of traditional positive and negative sequences separation method, and the improvement made by positive sequence estimator, which enhances immunity from interference. The largest impact in double closed loop controller based on SMVS is buffeting in system. Therefore, on the basis of traditional SMVS controlling method and MATLAB/Simulink simulation, the author makes a comparative analysis on the traditional one, and proves the improved one does affect the system. Improved SMVS controlling method guarantees the rapidity and also weakens the buffeting at the same time.Finally, to prove the feasibility of the methodology comprehensively, the author designs and makes the experimental prototype of PWM rectifier, and makes an introduction of its hardware circuit design and software control method in detail. Combination of theories and practice testifies that this arithmetic is totally correct.