Analysis and control of three phase ac/dc pwm converter under unbalanced operating conditions

The pwm ac/dc converter has been increasingly employed in recent years owing to its advanced features including sinusoidal input current at unity power factor and high quality dc output voltage with a filter capacitor of small size. These features are not necessarily achieved under the operating conditions of unbalanced input supply and unbalanced input impedances due to the generation of even-order harmonics at dc output and odd-order harmonics in input currents. The pwm ac/dc converter becomes more susceptible to unbalanced operating conditions as the size of reactive filters is reduced.; This thesis analyzed line side connected pwm ac/dc converters operating under generalized unbalanced conditions and proposed a new control method that can effectively eliminate input/output harmonics. A model of pwm ac/dc converters under generalized unbalanced operating conditions has been derived. Not only unbalanced input supply voltages but also unbalanced input impedances have been taken into consideration. The proposed technique nullifies the oscillating components of the instantaneous power at the poles of the converter so that harmonics in the output dc voltage can be eliminated under generalized unbalanced operating conditions. The proposed control scheme has relatively high current loop bandwidth with sufficiently low steady-state error for ac input current references.; It has been confirmed through simulation and experiment that the elimination of the even-order harmonics at the dc output side and odd-order harmonics at the ac input side makes it possible to reduce the size of the dc-link capacitor and ac inductors, thereby maintaining one of the major advantages of pwm ac/dc converters, small input/output filters. The input/output reactive filter components of reduced size in front-end converters are expected to play a significant role in shrinking down the total converter system. The diminished size of output dc link capacitor, mostly electrolytic types, can also lead to the enhanced life time of the entire system. The proposed control scheme also improves the fault ride-through capability of the pwm rectifier under voltage sag conditions.