Date of Graduation

8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor

Homer A. Mantooth

Committee Member

Juan C. Balda

Second Committee Member

Roy A. Mccann

Third Committee Member

Qinghua Li

Keywords

AC-DC Converter, High Power, Magnetic Design, Microgrid

Abstract

Microgrid concepts are gradually becoming more popular because they are expected to interface with renewable energies, increase end users’ reliability and resiliency, and promote seamless integration of distributed generators (DG) and energy storage units [1]. Most units are connected through power electronics interfaces, such as ac-dc, dc-dc, and dc-ac converters. The converter design and control are critical to the stability and efficiency of a microgrid.

A microgrid may operate in either gird connected mode or islanded mode [1]. In terms of stability, the grid connected mode is less challenging compared to the islanded mode of operation due to the nearly infinite ac bus having a very small equivalent impedance. This results in negligible interference between multiple converters. High power converters [2] operating in islanded mode encounter stability problems due to their relatively small impedance. One of the aforementioned instability cases is demonstrated in a microgrid testbed built at the University of Arkansas.

To mitigate the instability, modeling and control methods of high power voltage source converters are reviewed. Traditional methods of designing low power ac filters may not expand to high power design directly. Most academic papers designed ac filter inductors which have a fixed inductance value. This dissertation proposed a variable inductor whose inductance value changes by a factor of three from low current to peak current. The variable inductor approach gives many benefits with regard to high power microgrid applications. The design process of the inductor is described and simulation tools are used to verify the feasibility before final prototyping of the inductor.

A start-up control algorithm is important for a high power ac-dc converter, otherwise inrush current caused by the dc capacitor bank may trigger over current protection, induce system oscillation, or even result in a system collapse. The reason of inrush current is analyzed in details. An improved soft-start control algorithm is proposed and the inrush current is greatly reduced which is validated in both simulation and experimental results.

A microgrid hardware testbed prototype is proposed and tested successfully. The rating of the power converter described here is greater than 1 MVA.

Available for download on Friday, September 21, 2018

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