Thesis

Small-signal dynamics in converter-permeated power systems : multi-machine interactions, probabilistic analysis, and variability quantification

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Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17441
Person Identifier (Local)
  • 202050827
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Novel emerging dynamic interactions in converter-permeated power systems are in urgent need of thorough investigation. Their potential for causing deterioration of power system stability and the introduction of previously unseen oscillations can lead to problems such as damage or tripping of equipment, load shedding measures, or even cascading failures leading to blackouts. The result of this is the stagnation of the critical uptake of renewable energy-sourced generation and the unwillingness to decommission fossil-fuelled synchronous generators. The wide-bandwidth control of converters-interfaced generators bring unconventional dynamics and the potential for interaction with other power system elements from electromechanical to electromagnetic timescales. A further consequence of this is the requirement for more detailed models of power system components such as dynamic representation of the electromagnetic elements of the network. It is well understood that traditional grid-following converter control approaches, which track the voltage angle at the connection point and output the specified power accordingly, will introduce significant challenges when they constitute high penetrations of the generation mixture of a power system. As such, grid-forming converter control is expected to be utilised to ensure the formation and maintenance of the grid voltage phasor in converter-permeated bulk power systems. In addition to this, the flexibility of converter control results in a plethora of possible implementations, especially for the grid-forming converters whose application to bulk power systems are less mature. Furthermore, the variability of the renewable energy sources driving the converterinterfaced generation magnifies the uncertainty and potential range of system operating points. Even greater complexity is compounded by the more distributed nature of converter-interfaced generation compared to traditional synchronous generators. Ultimately, we find a huge range of operating points, system layouts, converter control philosophies, realisations, architectures, and tunings, all of which increase the complexity of the system dynamics and obscure the pre-existing intuition of power system engineers, especially with regards to multi-machine (or multi-element) interactions. This is the reason for the focus in this thesis on investigative studies of multi-machine interactions and the development of analysis methodologies which take into account the complexity of converter-permeated systems. Furthermore, caused in part by this increased dependence on the converter controllers, traditional static grid strength metrics such as the short circuit ratio (SCR) are becoming less valid. In this context, small-signal modelling and analysis is utilised for interaction identification and characterisation studies in power systems, revealing the potential impact of the integration of converters for a range of system conditions and controller variations. Additionally, a framework is developed to enable characterisation of dynamic interactions in the context of probabilistic small-signal analysis. This offers an approach to understand any given system in terms of both the stability and involved elements for specific interactions across the full operating range. Finally, in response to the decreasing validity of static grid strength metrics as converters proliferate, a modal contribution metric is introduced which enables observation of the small-signal variability of voltage magnitude and/or frequency at different locations in response to standard disturbances. In theme with the rest of the work in this thesis, the corresponding approach also allows for the further investigation of the interactive characteristics of the specific modes contributing most to said small-signal variability. Ultimately, the investigations performed and analysis methodologies developed will help in the ongoing process of comprehending the dynamics of modern and future converter-permeated bulk power systems with a particular focus on small-signal multimachine interactions.
Advisor / supervisor
  • Egea-Àlvarez, Agustí
  • Papadopolous, Panagiotis N.
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