Abstract
Theoretically, a three-phase boost rectifier can be controlled to achieve unity power factor as well as clean sinusoidal current at the input and zero voltage ripple at the output. Hence, it is now gradually replacing the traditional thyristor rectifiers to avoid power pollution problems. However, the ideal characteristic of the boost rectifier will be lost when the boost rectifier is operated under unbalanced three-phase voltage source. Hence, many control methods were proposed to solve this problem in the existing literature. Nevertheless, these methods, although can be used to achieve unity power factor, do not consider the minimization of the output voltage ripples. In view of this drawback, it is the major motivation of this thesis to solve this dilemma. Basically, the contributions of this thesis may be summarized as follows.First, general closed form analytic duty ratio control expressions of the three-phase boost rectifier are derived under unbalanced three-phase input voltage condition for further theoretical analysis. Second, based on the derived mathematic expressions, the corresponding necessary condition for the converter to achieve unity power factor is obtained. Third, two closed form output voltage-ripple expressions are derived under unbalanced three-phase input voltage condition . One is derived under the condition of allowing the existence of both positive and negative sequence currents. The other is derived by assuming having only positive sequence current. Fourth, based on the consideration of achieving better efficiency, a minimum output voltage ripple control strategy is proposed. In addition, a prototype is also constructed toverify the effectiveness of the proposed control.