Unbalanced Voltage Compensation using Interfacing Converters in Hybrid AC/DC Microgrids

  • Author / Creator
  • Today, conventional power systems are evolving into modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energies and storage elements (SEs). The hybrid AC/DC systems with DC and AC sources/loads are considered to be the most likely future distribution or even transmission structures. For such hybrid AC/DC microgrids, control strategies are one of the most critical operation aspects. Also, unbalanced voltage caused by ever-increasing unbalanced distribution of single-phase/unbalanced loads, single-phase/unbalanced distributed generations (DGs), and remote grid faults has raised serious concern about such hybrid microgrids due to the adverse effects on the power system and equipment. In hybrid AC/DC microgrids, the high penetration level of power electronics interfacing converters creates great ancillary services potential such as unbalanced voltage compensation. These interfacing converters (IFCs) are the interfacing converters of DGs/SEs, and AC and DC-subsystems IFCs (they can also be called solid-state transformers). These IFCs can be single three-phase IFCs, parallel three-phase IFCs (when larger power and current capacity are needed), or single-phase IFCs. However, the operating interfacing converters under unbalanced voltage will introduce some adverse effects such as output power oscillations, DC link voltage oscillations (especially when the DC link capacitance is designed to be small for a three-phase converter), and the output peak current enhancement. Therefore, designing suitable control strategies for these IFCs for operation under unbalanced AC voltage or even to compensate for the voltage unbalance with consideration of the above adverse effects is very important. Moreover, since the compensation is realized through the available rating of IFCs, it is equally important to consider the effectiveness of the control strategy for unbalanced voltage compensation. The purpose of this research work is to develop control strategies for the interfacing converters for unbalanced voltage operation or compensation in hybrid AC/DC microgrids. Three interfacing converters configurations, single three-phase IFCs, parallel three-phase IFCs, and single-phase IFCs, are considered. Specifically, for single three-phase IFC control, the focus is on the minimization of adverse effects such as DC link voltage ripple and the effectiveness of the unbalanced compensation, while providing an adjustable level of unbalanced compensation ability. For parallel three-phase IFCs, which usually have the same DC link (the DC link of the DG, SE or the DC subsystem), since adverse effects can be multiplied by the number of IFCs, the control focus is to optimally utilize the parallel IFCs to minimize their adverse effects on each other. For this purpose, either one IFC can focus on the adverse effects minimization, or all IFCs can share it. For single-phase IFCs, the focus is to coordinate multiple IFCs to compensate the unbalanced condition while providing an adjustable level.

  • Subjects / Keywords
  • Graduation date
    2017-11:Fall 2017
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
    • Department of Electrical and Computer Engineering
  • Specialization
    • Energy Systems
  • Supervisor / co-supervisor and their department(s)
    • Yunwei (Ryan), Li (Electrical and Computer Engineering)
  • Examining committee members and their departments
    • Ali Khajehoddin (Electrical and Computer Engineering, University of Alberta)
    • Mojgan Daneshmand (Electrical and Computer Engineering, University of Alberta)
    • Hao Liang (Electrical and Computer Engineering, University of Alberta)
    • Yasser Abdel-Rady I. Mohamed (Electrical and Computer Engineering, University of Alberta)
    • Martin Ordonez (Electrical and Computer Engineering, University of British Columbia)