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.
| Date of Award | 28 Jul 2025 |
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| Original language | English |
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| Awarding Institution | - University Of Strathclyde
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| Sponsors | EPSRC (Engineering and Physical Sciences Research Council) |
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| Supervisor | Agusti Egea Alvarez (Supervisor) |
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