Term of Award

Summer 2020

Degree Name

Master of Science, Electrical Engineering

Document Type and Release Option

Thesis (open access)

Copyright Statement / License for Reuse

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.

Department

Department of Electrical and Computer Engineering

Committee Chair

Masoud Davari

Committee Member 1

Rocio Alba Flores

Committee Member 2

Weinan Gao

Committee Member 3

Seungmo Kim

Committee Member 3 Email

seungmokim@georgiasouthern.edu

Abstract

As the integration of renewable energy on the grid increases, the number of voltage-source converters (VSC) installed also does. VSC controls both switch turn-on and turn-off, allowing a dc voltage source to be switched between phases. For the converter to accurately synchronize with the grid, a phase-locked loop (PLL) is used for the frequency measurements of the grid. However, the implementation of PLL with measurement delay introduces harmonics, noise, high frequency, and voltage oscillation to the system due to its dynamics. The dynamics introduced to the grid can be ignored under stiff grid conditions, but power from renewable sources decreases the grid inertia creating a weak-grid condition. Older grids accommodate this by using generators that compensate for the rate of change of frequency (RoCoF). Modern grids have less generator to accommodate the RoCoF, so there is a desideratum to implore a robust controller that responds quickly to the RoCoF, disturbance/ distortion rejection, and noise immunity to the grid. In recent literature, the effect of the PLL dynamics on a weak grid has been of great concern because of its unmodeled dynamics that destabilizes the converter under the weak-grid condition. This thesis proposes showing the impact of the weak-grid on the VSC as the dynamic of the grid changes. It also provides remedies to the grid instability and high-power injection levels. The detailed PLL dynamics model, including the ac-bus voltage dynamics with constant frequency, is developed and linearized. Even at a fixed frequency, various factors play a role in grid instability, and this tremendously affects the ability of the VSC to control the grid efficiently. The effect of the PLL gain under the weak-grid condition is analyzed.

Research Data and Supplementary Material

No

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