Thesis
Modelling, control and analysis of HVDC voltage-source converters in weak AC grids
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2023
- Thesis identifier
- T17417
- Person Identifier (Local)
- 201884397
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- An increase in renewable and distributed electricity generation, long-distance transmission, and interconnected power grids means that the prevalence of converter-based systems in the power network has increased rapidly over the last 30 years, and is expected to rise further over the coming decades. The high penetration of converter-based generation in the UK and in many other power systems worldwide presents a challenge for power system control because the high impedance of these systems represents a reduction in the local short circuit ratio (SCR) of the grid. A low SCR, known as a weak grid, in turn presents challenges for the stability and control of additional power converters that are connected to the AC network. The AC power grid of the future will require converter control that can operate safely, maintain stability, and provide rated power transfer even in these weakening grid conditions. This is particularly pertinent for high-voltage, direct-current (HVDC) transmission for two reasons. Firstly, because HVDC is used to transmit power over long distances from a high-supply, low-demand region, one or both converter stations are likely to be in a particularly weak grid area with little or no synchronous generation. Secondly, HVDC transmission almost always involves very high power transfer, which decreases the effective short circuit ratio ‘seen’ by the converter and exacerbates the control problem. This thesis therefore addresses some of the key challenges surrounding the control of voltage-source converter (VSC)-HVDC in very weak AC grids. This work presents modelling techniques, stability analyses and comprehensive assessment methods for different grid-following and grid-forming of voltage-source converter controllers. Linearized small-signal models are developed for conventional vector current control, power synchronization control and a simple virtual synchronous machine controller. Other linearised modelling approaches such as impedance models and the Jacobian transfer matrix model are also discussed. To validate and extend the results of linearised analysis, a number of time-domain simulation and modelling techniques are also presented. These include an averaged model in MATLAB/Simulink and switching models running in a control hardware-in-the-loop (CHiL) setup on two types of real time digital simulator (RTDS). The purpose of the small-signal and time-domain analyses is to investigate the absolute stability limits of each type of control and to describe the interactions that occur between different controller elements when operating in very weak AC grids. The stable operating space for the controller tunings of vector current control and power synchronization control is described under a number of dynamic performance and grid strength constraints, giving tuning recommendations and maximum performance limits. The stability analysis developed is then used to propose a standardized assessment framework for any VSC controller in a very weak AC grid. The objective of this framework is to provide a standardized set of time and frequency domain tests that can be applied to any grid-connected VSC. The assessment should be applicable to any type of weak grid-connected VSC controller at any level of implementation (e.g. small signal analytical model through to operational controllers implemented in hardware). This comprehensive assessment framework is used to compare the vector current control, power synchronization control and virtual synchronous machine controllers using small-signal models, time-domain simulations and control hardware-in-the-loop RTDS experiments to demonstrate the versatility and robustness of the proposed framework for controllers with very different structures. The final area of research in this thesis is the concept of dual-infeed VSC-HVDC in weak grid areas. The increasing number of HVDC infeeds inevitably means that high-power converter systems are operating in increasingly close electrical proximity. Interactions between controllers on different infeeds, and the implications that this has for optimum tuning are investigated. The effect of the strength of the coupling between infeeds (modelled by varying tie-line impedance) and the impact of high frequency resonances in dual-infeed systems are also investigated. Both small-signal analytical modelling and control hardware-in-the-loop RTDS experiments are used to perform these analyses, with additional discussion on the modelling of time delays in analytical dual-infeed systems.
- Advisor / supervisor
- Egea-Alvarez, Agustı
- Ahmed, Khaled
- Resource Type
- DOI
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