Design considerations of a novel three-phase multi-level inverter
- Countries around the world are committing to renewable generation and electrification targets in an effort to reduce greenhouse gas emissions. During this transition, power electronic inverters are becoming increasingly ubiquitous as the interface between different renewable sources and electrical loads. Today, utility-scale solar and battery energy storage systems mainly rely on single-stage central inverters to connect to the grid. Operating at a string voltage around 1kV, utility-scale solar often needs a medium voltage (MV) transformer to interface with the MV grid. Due to the MV transformer's cost, maintenance, and losses, multilevel inverters (MLIs) are investigated as a promising transformer-less alternative. Existing three-phase MLIs are designed with single-phase inverter modules or three-phase inverter modules as basic building blocks. Single-phase modules suffer from the need for large DC link capacitance to buffer the 120 Hz ripple for grid-tied applications. Three-phase modules do not require bulky input capacitance but rely on more complex topologies and result in poor efficiency. In this thesis, we propose a novel two-phase module-based MLI, that eliminates the need for bulky input capacitance while also enhancing the efficiency of a typical three-phase module. We achieve this by designing a power sharing scheme among inverter modules that allows the inverter module to operate at peak or zero power for most of the grid cycle. The proposed MLI design and power sharing scheme pose challenges for a distributed controller. This work leverages iterative and structurally constrained linear quadratic regulators (LQRs) to control the MLI system, and PLECS simulations are conducted to verify the efficacy of the controller. We build an experimental 110V DC input, 208 Vrms three-phase AC output, 1~kW 5-level prototype with > 95% efficiency to demonstrate the proposed MLI and controller design. With the same inverter module, we compare the efficiency of the proposed design to the conventional MLI design, where all modules share phase power evenly. We demonstrate that the proposed design consistently outperforms the conventional design in efficiency, with > 25% loss reduction at light load conditions. Lastly, we propose a formal way of optimizing the power sharing scheme for any multi-converter system based on the efficiency characteristics of the converter module.
|Type of resource
|electronic resource; remote; computer; online resource
|1 online resource.
|Dally, William J
|Dally, William J
|Degree committee member
|Stanford University, Department of Electrical Engineering
|Statement of responsibility
|Submitted to the Department of Electrical Engineering.
|Thesis Ph.D. Stanford University 2022.
- © 2022 by Tuofei Chen
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