Dynamics and Sensitivity Analysis of a Voltage-Source Converter Connected to a Weak Grid

• Author / Creator

  Telmesani, Wael

• As the call to action on global warming is rising yearly, the integration of renewable and clean energy resources in the energy sector is gaining high interest. In grid-tied converter applications, power system stability becomes very sensitive in weak grid conditions. Due to the penetration level of the integration of renewable energy resources and the rapid increase of converters in power grid systems, preserving the overall system stability becomes a vital objective. In such demanding applications, controlling the voltage of interlinking DC-link capacitance is crucial to preserving system stability. Furthermore, the DC-link capacitor is a critical component because it determines the stability and dynamic performance in such systems. This critical element might face uncertainty due to loading/and unloading on the DC-side or changing in ambient temperature but not limited to these factors. Therefore, this thesis aims to develop a comprehensive analysis to investigate the dynamic interactions between the DC-link capacitance uncertainty and different grid strength conditions in inversion and rectification modes of operation. This study considers the influence of controller bandwidths, grid-angle variations, and AC-side faults under the dynamics of DC-link capacitance. In addition, the impacts of grid-side parameters that maintain the system stability in weak-grid conditions have been investigated. This thesis addresses the interaction dynamics by linearizing the nonlinear grid-connected voltage source converter (VSC) system around a certain equilibrium point. To capture the stability of the system dynamics, a small-signal state-space model is derived from the linearized model. To validate the analysis results, the small-signal was verified with the time-domain model under a MATLAB/Simulink environment. The results demonstrate that, at a stiff grid condition, the influence of the DC-link capacitance variations did not significantly affect system stability in inversion and rectification modes as it did in weak grid conditions. Furthermore, at weak grid conditions, increasing the DC-link capacitance and controller bandwidths enhanced system stability in the inversion mode and degraded stability in the rectification mode, except when increasing the bandwidth of the AC-voltage controller. Regarding the effect of the grid-angle variations at short circuit ratio (SCR) < 3, increasing the grid angle and decreasing the DC-link capacitance leads to worse stability conditions in the inversion mode and vice-versa for the rectification mode. In terms of fault assessment, the settling time increased when the DC-link capacitance decreased after the fault was cleared in
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  inversion mode at SCR < 3. At the same level of SCR, the system becomes less capable of overriding the fault when the DC-link capacitance increases in rectification mode. Regarding the grid impedance and AC-line inductance filter, grid inductance is not the only factor influencing system stability; the grid resistance also had an effect. In addition, the results revealed that a slight variation in AC-line inductance can significantly affect system stability, especially at a very weak grid condition. In conclusion, the effect of the DC-link capacitance uncertainty on system stability is higher in the rectification mode than in the inversion mode, in weak grid conditions. In addition, the selection of the DC-link capacitance depends not only on the ripple percentage in the DC-bus voltage but also on the grid strength level, which can affect system stability.

• Subjects / Keywords

  • Voltage source converter
  • Weak-grid
  • Small-signal modes
  • Power electronics

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-ht1x-7v58

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.