Research On Control Strategy Of Three-phase Voltage-source PWM Rectifier Under Unbalanced Source Voltage

The conventional rectifier section widely consists of diode-rectifier circuit and phase-control thyristor rectifier, which injects large amounts of harmonics into the power networks and produces much contamination. The three-phase PWM voltage source rectifier presented in this paper owns such feature as consistant output voltage, running under unit power factor and feedback of the power into the power networks. The three-phase voltage source PWM rectifier desinged based on the condition of balanced grid voltage will appear abnormal operating condition under unbalanced grid voltage. The negative sequence current results in current asymmetry, non-characteristic harmonic component and the total harmonic component, which makes dc voltage waveform distortion happens. Therefore, it is necessary to study the control strategy of the three-phase voltage source PWM rectifier under unbalanced grid voltage.First of all, the theory of PWM rectifier was introduced in details, which were mainly about the topology and working principles, in other words, constructing the high and low-frequency mathematical model under ABC stationary reference frame, a/3 stationary reference frame and dq rotation reference frame. In this paper, it mainly did some research on the three-phase voltage-source rectifier, and then decoupled its mathematical model in the dq reference frame.As regarding to the control techniques on PWM rectifier, the current control was proposed earliest compared to others. In this paper, it presented the indirect and direct current control, introduced space voltage vector-oriented control in the direct current control. Furthermore, it proposed the principal algorithm of the control strategy and established the simulation model. The direct power control was just proposed recently for three-phase PWM rectifier. After introducing the traditional direct power control, this paper presented the problems of that method, proposed the combination of space vector modulation and the DPC, and verified the effectiveness of such control strategy by means of simulation. Based on the mathematical modeling, two unbalanced control strategies are focused on. One is the traditional double current control strategy, and the other is the constant switching frequency direct power control. Meanwhile, two methods for the extraction of the voltage positive and negative sequence component. Lastly, in the above theory, the simulation of the control strategies under the balanced and unbalanced grid voltage are comparatively detailed in this paper.