Date Available

5-13-2024

Year of Publication

2024

Degree Name

Master of Science in Electrical and Computer Engineering (MSECE)

Document Type

Master's Thesis

College

Engineering

Department/School/Program

Electrical and Computer Engineering

First Advisor

Dr. JiangBiao He

Abstract

In the recent decade, ultra-fast wide bandgap switching devices such as Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) have been increasingly utilized in power electronic converters for renewable energy power generation systems due to their advantages of enabling high energy efficiency and high power density. However, the much higher voltage slew rate (i.e., dv/dt) with SiC power converters may induce voltage reflection and reliability concerns in machine-converter systems, especially for medium-voltage systems with long cable connections. In wind-turbine power generation systems, generators are typically located in the nacelles, and medium-voltage power converters are mostly placed at the bottom of the tower for the convenience of maintenance, resulting in long cables (e.g., 200-300 feet) interconnected between generators and power converters. Consequently, the high dv/dt caused by fast-switching SiC devices coupled with the long cables induces high-frequency overvoltage ringing across the generator’s stator windings. To mitigate such reflected overvoltage across the generator windings and improve the reliability of the wind turbine system, an innovative mitigation solution will be developed and presented in this thesis. In this thesis, the voltage reflection mechanism with its main formation factors, including high dv/dt caused by fast-switching SiC devices, long cables interconnected between the generator and power converter, and high-frequency surge impedance mismatch between the cables and the generator are thoroughly investigated. The conventional methods to mitigate the reflected overvoltage will be reviewed, including surge impedance matching, dv/dt mitigation, and integrated machine-drive solutions. Furthermore, high-frequency modeling for generator-converter systems is indispensable to studying the voltage reflection phenomenon versus different cable lengths and semiconductors’ rise time. The surge impedance measurements for cables and generators are carried out to accurately model the high-frequency distributed circuit models. The uneven voltage distribution across different coils is investigated, confirming that the first few coils withstand most of the reflected voltages. Targeting mitigating the surge voltage stress across the first few coils, an innovative solution named the smart coil concept is developed, simulated, and tested to verify its efficacy. This proposed smart coil solution mainly includes a bi-directional Gallium Nitride (GaN) transistor, capacitors, diodes, and the related control logic. Therefore, it can be conveniently integrated into the generator junction box due to its ultra-compact footprint. Based on a lab-scale generator-cable-converter system, the proposed mitigation solution is thoroughly verified in the simulation studies.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.225

Funding Information

This study was supported by the US National Science Foundation (no: 2135543) in 2022 and 2023.

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