| Rectifier is widely used, and the power level is becoming higher. Phased andnon-control method is mainly used for rectifier. So it causes many harmonies to theelectrical grid. PWM is the best idea for rectifier now. It does not only cause lessharmony to the grid, but also pass the power through both sides. Because of theadvantages PWM rectifier has, it is not only to replace other rectifiers, but also has itsused in special field. For example it's used in active filter, wind or solar electricalenergy generation, electrical energy storage system.Traditional control for PWM rectifier has two control loops under balanced voltages.With the development of the control theory, unbalanced voltage condition has beenconsidered.This paper has done the main work that is displayed below.We set up the mathematic model for the system. Simplify the equations underbalanced voltage condition. Then we designed the voltage and current loop with thegrid voltage fixed on d axis.When research on high voltage sags and transformer, we find out typical faults occurat the user terminal. We make 3 assumptions about the voltage sages. So the PWM isworking under 3-phase 3-line grid with no zero-sequence.The problems of the PWM rectifier under unbalanced voltages have been considered.New control method contains positive and negative sequence current trace under theirown synchronous axis. The improved method has good performance underunbalanced voltages. It shows regulation in dc voltage and sinusoidal in ac current.Filter and T/4 shift separating method are discussed in the paper. There are both usedto get the positive and negative sequence. These two methods have been compared inmany ways, and finally find that T/4 shift separating method is better.In the end of this paper, we set up Simulation models under balanced and unbalancedvoltage condition. And we also do some experiments under unbalanced voltagecondition. The results approve that the control method is correct and efficient. |
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