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Voltage Stability and Control of Electrical Distribution Systems with High Penetration of Power Electronic Converters

Dionysios Moutevelis

TL;DR

This thesis addresses voltage stability in distribution networks with high penetration of power electronic converters. It advances stability assessment via bifurcation analysis that includes converter capacity limits and LTCs using smooth approximations to non-smooth phenomena. It proposes a circuit-inspired, virtual admittance loop to provide simultaneous active and reactive voltage support, including a quasi-stationary variant to improve transient performance. It also introduces a complex-frequency based taxonomy to quantify how different converter control schemes influence local bus frequency, complemented by a recursive secondary controller and experimental validation in the SEIL facility, enhancing practical voltage regulation with limited communications.

Abstract

Power systems are currently undergoing a rapid paradigm change in their operation. Centralised energy production is being replaced by a number of Distributed Generation (DG) units that are placed at different locations and voltage levels in power networks. These distributed units are mostly based on renewable energy technologies, like wind turbines and photovoltaic cells and are commonly interfaced to the grid via power electronic converters. These sources reduce energy system dependency on conventional generation units based on fossil fuels. At the same time, this shift introduces technical challenges for the safe and reliable operation of electricity network since DG sources do not inherently provide the grid regulation services of conventional, centralised generation units. Moreover, the increased penetration of renewable energy sources and their converter-based interfaces is creating voltage deviation and voltage stability issues in distribution networks. These issues range from overvoltages during hours of peak renewable generation, reverse power flows and sudden voltage drops due to the variable nature of renewable energy production. All of the above jeopardise the reliable operation of the distribution networks that were not originally designed to accommodate for these effects. The objective of this thesis is to propose novel techniques for the accurate assessment of the DG impact on voltage stability in distribution net works and investigate how the control capabilities of converter-based interfaces of DG units can be harnessed to improve stability margins and overall system robustness and performance.

Voltage Stability and Control of Electrical Distribution Systems with High Penetration of Power Electronic Converters

TL;DR

This thesis addresses voltage stability in distribution networks with high penetration of power electronic converters. It advances stability assessment via bifurcation analysis that includes converter capacity limits and LTCs using smooth approximations to non-smooth phenomena. It proposes a circuit-inspired, virtual admittance loop to provide simultaneous active and reactive voltage support, including a quasi-stationary variant to improve transient performance. It also introduces a complex-frequency based taxonomy to quantify how different converter control schemes influence local bus frequency, complemented by a recursive secondary controller and experimental validation in the SEIL facility, enhancing practical voltage regulation with limited communications.

Abstract

Power systems are currently undergoing a rapid paradigm change in their operation. Centralised energy production is being replaced by a number of Distributed Generation (DG) units that are placed at different locations and voltage levels in power networks. These distributed units are mostly based on renewable energy technologies, like wind turbines and photovoltaic cells and are commonly interfaced to the grid via power electronic converters. These sources reduce energy system dependency on conventional generation units based on fossil fuels. At the same time, this shift introduces technical challenges for the safe and reliable operation of electricity network since DG sources do not inherently provide the grid regulation services of conventional, centralised generation units. Moreover, the increased penetration of renewable energy sources and their converter-based interfaces is creating voltage deviation and voltage stability issues in distribution networks. These issues range from overvoltages during hours of peak renewable generation, reverse power flows and sudden voltage drops due to the variable nature of renewable energy production. All of the above jeopardise the reliable operation of the distribution networks that were not originally designed to accommodate for these effects. The objective of this thesis is to propose novel techniques for the accurate assessment of the DG impact on voltage stability in distribution net works and investigate how the control capabilities of converter-based interfaces of DG units can be harnessed to improve stability margins and overall system robustness and performance.

Paper Structure

This paper contains 39 sections, 10 figures.

Table of Contents

  1. Introduction
  2. General Background and Motivation
  3. Distributed Generation and Future Power Systems
  4. Power System Stability - Traditional and Contemporary Approaches
  5. Voltage related Issues in Active Distribution Networks
  6. Power Converter Control for Distributed Generation
  7. Main Objective and Scope of the Thesis
  8. State-Of-The-Art
  9. Power system stability analysis methods
  10. Bifurcation Analysis of Power Systems
  11. Bifurcation Analysis for Traditional Power Systems
  12. Bifurcation Analysis of Converter-Based Devices
  13. Bifurcation Analysis of Systems with high Converter Participation and Microgrids
  14. Circuit-Inspired Control Loops
  15. Voltage Control for Distributed Generators
  16. ...and 24 more sections

Figures (10)

  • Figure 1: Comparison between the existing and future power networks. Reference: Graphic: Bartz/Stockmar, CC BY 4.0.
  • Figure 2: Power system stability classification hatziargyriou2020definition.
  • Figure 3: Schematic representation of reverse power flows from low voltage to higher voltage systems gabash2016variable.
  • Figure 4: Suitable representations of (a) a GFL converter and (b) a GFM converter, using their electrical circuit equivalents rocabert2012controlrosso2021grid.
  • Figure 5: Representative distribution grid with high penetration of power converters that was used as a benchmark for the stability. It contains different devices that regulate voltage and affect voltage stability.
  • ...and 5 more figures