Power Electronic Topologies with High Density Power Conversion and Galvanic Isolation for Utility Interface

Abstract

The past decade has seen a significant increase in the number of applications where power electronic converters play a major role. Renewable energy systems such as wind turbines, solar photovoltaics, etc. employ power converters to interface with the utility grid. More and more power converters are being used in transportation sector such as in electric vehicles, locomotives, aircrafts, ships and submarines. Advancements in power converter topologies and devices have constantly pushed the limits and standards applicable in different markets towards better efficiency, lower cost and higher power density. Especially for large power systems such as wind turbine generators, adjustable speed drives, locomotives, etc., achieving smaller footprint at low cost and high efficiency has become a major challenge. These factors generate the major impetus towards the research undertaken in this dissertation.

In applications that require integration with the utility grid, the bulkiest components are usually the transformers, inductors and DC electrolytic capacitors. Instead of using a line frequency transformer to interface any power electronic system with the utility grid directly, it is possible to use a power converter to transform the line frequency AC into a higher frequency AC that can be fed to a medium or high frequency transformer. These transformers are much smaller and lighter compared to line frequency transformers. This dissertation elucidates these concepts in detail in the first section as well as at the beginning of each subsequent section, along with a summary of such techniques already proposed in the literature.

The sections in this dissertation propose and discuss several architectures (approaches) adhering to the earlier stated concepts that enable higher power density energy conversion for applications such as wind turbines, adjustable speed drives, data centers, energy storage systems, etc. Detailed operational analysis, design example, control strategy, simulation results and experimental results are shown for each concept or topology. The advantages and drawbacks are also discussed.

Finally in this dissertation, the medium or high frequency transformers that can be used in the proposed approaches are analyzed in detail using ANSYS Maxwell software in terms of material, saturation, loss and size. Further, these numbers are used to estimate the relative size advantage and efficiency that can be achieved using higher frequency transformer compared to a line frequency transformer for utility interface applications. It will be shown that for many high power applications, medium frequency transformer based circuit designs can be more efficient and simpler alternatives for high frequency transformer based approaches. The specific contributions along with future research opportunities of the proposed concepts are summarized at the end.