UBC Theses and Dissertations
Optimization of a grid-tied inverter : an application-oriented for designing multilevel converters Di Tullio, Luccas
Recent developments in energy systems, including the rapid adoption of renewable energy sources and expansion of microgrids have been introducing new challenges and opportunities for the power electronics industry. Of particular interest to this thesis is the increase of grid-connected DC systems. As a means to reduce cost of copper infrastructure these systems favour the utilization of higher DC link voltages. To accommodate higher voltages, a variety of multilevel converters have been proposed, which generally can be built without specialized components, present lower dV/dt losses and synthesize AC signals with better power quality. Irrespective of the application, the power electronics industry has traditionally relied on generic rules and practices to quickly design converters. Rarely does the development cycle allow for thorough investigation of the converter design, which could enhance performance and give an edge over competitors. This thesis proposes using optimization techniques to aid power electronics engineers in the design of multilevel converters. The Neutral Point Clamped (NPC) with its variant the Active NPC (ANPC) were selected for the exercise presented. Chapter 2 of this thesis explains the operation and modulation of the topologies. From the analysis, the conduction and switching losses of each device can be predicted. A description of three semiconductor technologies is presented with their characteristics and source of losses. Lastly an equation to size the filter inductor is introduced. All this information is packaged into a model used in the optimization. Chapter 3 introduces the optimization strategy. Given the complex nature of power electronics, four objective functions were adopted: efficiency, loss distribution, inductance and cost. These functions were combined through a weight system which allows priorities to be asserted. Next, design variables are introduced along with their respective impacts on the objective functions. Experiments performed with a hardware platform showed the model closely predicts the impact of the design variables on the objective functions. Confident in the model, the optimization was carried out for various scenarios. Single objective optimization led to converters that excel in one aspect but were often not practical. When optimizing with multiple objectives a good compromise was reached with a practical converter.
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