Abstract

In most line-interfaced power converter applications the ac mains voltage is first rectified into a dc voltage or current, which is subsequently converted into voltages and currents of appropriate amplitude, frequency and shape to meet the load requirements. The front-end rectifier must satisfy three main requirements: (a) Minimum harmonic injection into the ac mains should comply with limits imposed by recommended standards such as IEEE-519, IEC-555. (b) High input power factor to reduce the reactive power requirements. (c) High efficiency and reliability and low cost to ensure competitiveness on the market. The challenge is therefore to provide a conversion scheme which delivers high quality output waveforms without distorting the ac mains and without drawing any reactive power. Successful application of PWM techniques to forced commutated converters has prompted recent investigation in finding more suitable topologies for ac to dc conversion. Two structures have evolved based on the characteristics of the dc link: the current source topology and the voltage source topology. This thesis investigates these two topologies and proposes a number of control schemes to achieve unity displacement factor operation and fast response. For the current source topology two control methods, a closed loop and a feed-forward scheme, are proposed. The feed-forward scheme is based on phase shifting and gating patterns of individual switches to compensate the effect of the input filter and load operating point. Furthermore, the feed-forward scheme is combined with a control strategy to eliminate the need for damping resistors. For the voltage source topology, a simple control strategy is proposed to obtain a near unity power factor input stage for voltage source inverter based ac drive applications. Also, performance of current controlled voltage source type rectifiers in rotating and stationary frames is investigated. Small signal models are developed for both topologies and different transfer functions are derived for each structure. The theoretical considerations are verified by simulation and by experiments on laboratory prototypes.