Fault-tolerant Partial-resonant High-frequency AC-link Converters and Their Applications

Recently, the demand for high-power-density converters with high efficiency and enhanced reliability has increased considerably. To address this demand, this dissertation introduces several low, medium, and high power converter topologies with high-frequency ac links and soft-switching operation, both with and without galvanic isolation. These converters can be in ac-ac, dc-ac, ac-dc, or dc-dc configurations to transfer power from the utility, energy storage systems, or renewable/alternative energy sources (e.g., photovoltaics, wind, and fuel cells) to stand-alone loads or the utility. The advantages of these topologies include soft switching at turn-on and turn-off of all the semiconductor devices, exclusion of short-life electrolytic capacitors in the link, step-up/down capability, and the use of a smallsized high-frequency transformer for galvanic isolation. The proposed converters are also able to generate output waveforms with arbitrary amplitude and frequency as well as achieving a high input power factor in the ac-ac and ac-dc configurations. Moreover, some of the introduced topologies have fault-tolerance capability, which may allow the converter to run even with one or more faulty switches. In this case, a partial failure will not result in the converter shutdown, and thus system availability is improved.

The high-frequency ac link of the introduced converters is composed of an ac inductor and small ac capacitor. The link inductor is responsible for transferring power, while the link capacitor realizes soft-switching operation. As the link components have low reactive ratings, the converters exhibit fast dynamic responses. The inductor can be replaced by an air-gapped high-frequency transformer to achieve galvanic isolation without the need for any snubber circuits. Due to operation at a high frequency, the link transformer is substantially smaller in size and lower in weight compared to conventional line-frequency isolation transformers. In this work, the proposed power topologies are explained in detail, and their comprehensive analyses are given to reveal their functioning behavior in various working conditions. Simulation and experimental results at different operating points are also presented to verify the effectiveness of the introduced power converters.