Characterization and Damping of Oscillations in Microgrids and Multi-Microgrids with Hybrid AC Generation

  • Adedayo Aderibole

Student thesis: Master's Thesis

Abstract

Renewable energy resources have become an increasingly popular choice for meeting the world's growing energy demand. More attention is being paid to green-house gases (GHGs) emission and their impact on climate and the environment. The most common method of integrating renewable energy resources is by connecting them to the distribution network of existing power systems, and as such, they are generally referred to as distributed generation (DG). The rise in the participation of distributed energy resources (DERs) in electricity production has led to the development of 'microgrids'. A major challenge in all power systems is to ensure stability during steady-state and transient conditions. Since the interconnection of large conventional power systems in the 1950s, power systems oscillations are a well-known phenomena studied by researchers over the years. Microgrid reliability and security are being improved by interconnecting microgrids to form multi-microgrid (MMG) structures. Because of the similarity between conventional power systems and microgrids, oscillations are expected to be present in microgrids and MMGs. However, due to diversity of DGs which make up MMGs, the nature of oscillations inherent to MMGs differ from conventional power system oscillations. For microgrids comprising of inverter-based and synchronous-based distributed generation, the impact of equal and unequal active power sharing conditions on the dynamic stability is analyzed. Small-signal stability analysis is employed to characterize the stability of the microgrid. The sensitivity of low-frequency eigenvalues to changes in active power droop gains is studied and a 'domain of stability' region is proposed to assess the stability of a microgrid under different power sharing conditions. The different operating regions within the domain of stability are defined. Based on the sensitivity analysis and the domain of stability, the optimal droop gain for the diesel generator to improve the stability of the microgrid is determined. For MMGs, the small-signal stability of a MMG consisting of inverter-based and synchronous-based distributed generation is evaluated. Local and inter-microgrid oscillatory modes present in the MMG are characterized by modal analysis. Local and inter-microgrid oscillatory modes similar to conventional interconnected power systems oscillations are observed. In addition, a different 'dominant' oscillatory mode caused by interactions between the synchronous generators and inverters in the MMG is identified. Parametric studies show that MMG oscillations exhibit undesirable damping characteristics for certain conditions and trade-offs must be made between achieving good transient and dynamic performances. Consequently, power system stabilizers (PSS) are developed for the DGs to improve the transient and dynamic performance of the MMG. From the studies carried out in this work, it is concluded that low-frequency oscillatory modes arise in microgrids and MMGs as a result of the interactions between the DGs.
Date of AwardMay 2017
Original languageAmerican English
SupervisorHatem Zein El din (Supervisor)

Keywords

  • Hybrid Air Conditioning
  • Renewable energy sources
  • Power systems
  • Distributed generation
  • Microgrids
  • Synchronous generators.

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