On the robust stability analysis of VSC-HVDC systems
The VSC-HVDC system with a distributed parameter DC-cable model can be described by two cascaded blocks. The first block is a transfer function that will be different, due to which input and output variables that are considered. The second block is a feedback loop, where the forward path is a rational function and the return path is a dissipative infinite dimensional function, that remains the same in all cases. The stability is then analyzed using the Nyquist criterion in a straight forward manner. Examples with different operating points P20 and different SCRs of the connected AC-grids have been studied, showing that the VSC-HVDC system with a single Pi-section cable model is sufficient to evaluate system stability, independently of the DC-cable length and impedance density.
Based on the mixed small gain and passivity theorem, this thesis provides a theoretical method to evaluate a sufficient stability condition for a two terminal VSC-HVDC system with respect to the connected AC-grid. The result is that, for the frequency band where the converter admittance matrix is not passive, the negative closed loop system is stable if the loop gain is strictly less than one. On the basis of such a theorem, the sufficient stability conditions are provided, showing that at the DC-voltage controlled converter side, the system robustness can be increased by designing iref = Pref/E0 instead of iref = Pref/E. In addition, the active power controlled converter can be designed to have passive converter admittance for all frequencies and thus the system is stable under all kinds of AC-grid.
small gain theorem
symbolic eigenvalue expressions
Nyquist stability analysis
distributed parameter cable model
Chalmers, Signaler och system, System- och reglerteknik, Reglerteknik
Y. Song, C. Breitholtz, VSC-HVDC system robust stability analysis based on a modified mixed small gain and passivity theorem
Y. Song and C. Breitholtz, On passivity based measures of robustness for a VSC-HVDC system connected to weak AC-grids
Annan elektroteknik och elektronik
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4192
Chalmers tekniska högskola
KB-salen, Kemigården 4, Chalmers
Opponent: Adjunct Professor Lennart Harnefors, Department of Power Electronics Royal Institute of Technology and ABB, Stockholm, Sweden