Analysis, design and control of grid connected three phase pulse width modulated AC-DC converter

摘要

The increasing penetration of line-commutated power diode and thyristor rectifiers into the grid power system is becoming a problem in transmission and distribution lines due the harmonic and reactive currents they inject to the grid system. Therefore, three-phase pulse width modulation (PWM) ac-dc converters are becoming more and more attractive for replacing the line-commutated rectifiers in the utility-interface applications. With a proper control technique, the PWM ac-dc converter is able to reduce the harmonics in the line currents. This leads to the achievement of almost sinusoidal input currents and provides controllable de-link output voltage, unity power factor operation and regeneration capability. These features are not necessarily achieved under non-ideal operating conditions such as unbalanced, distorted and disturbed grid supply. -- This thesis investigates a virtual flux control for reducing the number of sensors in the direct power control (DPC) and the voltage oriented control (VOC) of a three phase PWM ac-dc converter. The use of input ac voltage sensors to measure the grid voltage for synchronization and estimation of input instantaneous active and reactive powers is avoided by applying a virtual flux concept in the new proposed control schemes. The virtual flux control technique is used to extract the grid voltage information from the converter switching states, de output voltage and line currents. -- A virtual flux direct power control (VFDPC) utilizing an improved virtual flux estimator and a newly designed switching look-up table, is proposed in this thesis. The switching look-up table is developed based on the instantaneous power derivative method which relies on the sign and magnitude of the change in instantaneous active and reactive powers. In this way, the switching table is able to choose the best converter voltage vector in order to ensure smooth control of active and reactive powers. -- Furthermore, a new virtual flux oriented control (VFOC) technique is proposed so that the ac-dc converter operates with a fixed switching frequency. The VFOC control structure is developed by using a newly derived mathematical model of the three phase ac-dc converter incorporating the estimated virtual flux components. Subsequently, the proposed VFOC is able to include the decoupling network and feed-forward control components to enhance the converter performance during the grid and load disturbances. -- It has been confirmed through simulation and experiment that the proposed VFDPC and VFOC are able to produce three phase sinusoidal input currents with low total harmonic distortion, near unity power factor and adjustable de-link output voltage under balanced and non-ideal conditions of the input voltage supply.