Thesis
Analysis of converter small signal model limitations and oscillation propagation in medium voltage and high voltage DC systems
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2025
- Thesis identifier
- T17497
- Person Identifier (Local)
- 202061838
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- To combat climate change, the traditional electricity grids of the past with fossil fuel synchronous generation are being replaced with renewable based generation. The increasing penetration of inverter based resources (IBR) has resulted in the modern grid experiencing changes in dynamic characteristics caused by new devices, controls and time frames being introduced. This replacement of synchronous generation has resulted in several issues that need to be addressed to further progression in reaching Net Zero emissions. Some of the new technologies in the modern grid to enable the integration of renewable generation are grid-tied converters, state-of-the-art control topologies and Medium Voltage Direct Current (MVDC) and High Voltage Direct Current (HVDC) transmission and distribution. The fundamental topologies, future prospects and applications of MVDC and HVDC need to be understood to achieve the full potential of these technologies. Therefore, this thesis presents a comprehensive analysis and overview of MVDC uses, demonstration projects, converter topologies, architecture structures, and unique issues and operations. These new technologies are offering a wealth of solutions to technical challenges facing the integration of renewable generation, there are however, associated problems with each new evolution. Firstly, IBRs have been mainly controlled with grid following control, which requires the measurement of the grid voltage source to synchronise. This can contribute to grid issues and system instability. There have been several suggested amendments to classical GFL control to improve ancillary support to connected AC grids. For example, there has been discussion in the literature comparing the inertial response provided by grid forming and following controllers, showing grid following converters can provide similar inertial responses to grid forming under certain conditions. However, the effect of providing an inertia response on stability of grid following connected systems needs to be assessed, with this thesis investigating the tuning limitations that restrict the capability of grid following converters to provide the required inertial response in an appropriate time. With the differing control topologies being implemented, the rapidly changing grid composition is producing scenarios that are increasingly complex to model, as well as unwanted grid dynamics such as sub-synchronous oscillations (SSOs) that are not being predicted. The combination of new devices and controllers with faster time constants is resulting in an expansion of typically seen SSOs, from sub-synchronous resonances, device dependent SSOs, to sub-synchronous control interactions and subsynchronous torsional interactions. However, there are increasingly instances of SSOs persisting in the grid that were not accurately predicted. Capabilities and limitations of converter models need to be understood for recommendations to Transmission System Operators (TSOs), Original Equipment Manufacturers (OEMs), and project operators. Therefore this thesis assesses the impact of extreme parameter conditions on a series of common grid following and grid forming converter to grid small signal models of varying complexity, simplifications and assumptions. Additionally, comprehensive methods of assessment of dynamic transient and modal detail are presented to give an intuitive yet novel insight for showing capabilities of each model. This includes highlighting the effect of variations in low frequency modes on transient dynamic response, and a method of screening models for modal accuracy in identifying oscillatory SSOs, highlighting conditions where lower detail models do not represent low frequency oscillations when they are expected to. The importance of dynamic detail is essential in larger more complex systems. The difficulties of not recognising unwanted SSOs and harmonics are compounded with increasing grid complexity. The development of interconnected HVDC systems has been enabled with voltage source converters (VSC) and furthermore with modular multilevel converters (MMC), which provide large voltage step ups. HVDC-MMC systems have been stated to have an inherent “firewall” capability, which has been described as the prevention of unwanted oscillations between interconnected AC systems, because of the controllability on either side of the link. MMCs however have harmonics produced in internal circulating currents and sub-module voltages that need to be controlled, and require accurate modelling. Further to this, grid dynamics, accurate DC link representation, and converter control all need to be modelled to given a fully detailed representation of multiterminal MMC-HVDC systems and propagation between interconnected grids. Therefore, this thesis develops a highly detailed dynamic model of a point-to-point MMC-HVDC system for the effect of grid following and grid forming controls on the firewall capability of the system, assessing critical state dependencies that participate in oscillatory poorly damped modes. A firewall quantification metric is proposed to identify conditions that could cause oscillation propagation, and a comprehensive overview of the effect of changing system and control parameters on firewall capability.
- Advisor / supervisor
- Harrison, Sam
- Egea-Àlvarez, Agustí
- Resource Type
- DOI
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PDF of thesis T17497 | 2025-10-07 | Public | Download |