Converter interactions in VSC-based HVDC systems
Sammanfattning: The main objective of this thesis is to perform stability and control studies in the area of VSC-HVDC. A major part of the investigation focuses on the explanation of poorly-damped conditions and instability that are linked to dc-side resonances. Initially, a frequency domain approach is considered, applied to a two-terminal VSC-HVDC connection that is modeled as a Single-Input Single-Output (SISO) feedback system, where the VSC-system and dc-grid transfer functions are defined and derived. The passivity analysis and the net-damping criterion are separately applied, demonstrating the superiority of the latter as an analysis tool. Furthermore, it was discovered that the net-damping of a system and the damping factor of its poorly-damped dominant poles are correlated in an almost linear way. The occurrence of poorly-damped conditions is further analyzed from an analytical perspective, where the eigenvalues of a two-terminal VSC-HVDC system are approximated by closed-form expressions. This offers the benefit of a deeper understanding in the way selected parameters of the system can affect the frequency and damping characteristics of its eigenvalues. The Similarity Matrix Transformation (SMT) method is introduced in this thesis and applied to the reduced 4th order state-space model of a two-terminal VSC-HVDC system. The results show that the SMT offers improved accuracy in approximating the actual eigenvalues of the system, compared to the already established LR method. Finally, studies are performed in VSC-MTDC grids, with the main objective of proposing advanced control strategies that can offer robust performance during steady-state and transient conditions, with improved power flow and direct-voltage handling capabilities. The advantageous properties of the proposed controllers are proven through simulations of four- and five-terminal MTDC grids, in which their benefits compared to their conventional counterparts are shown.
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