On the Analysis of DC Network Dynamics of VSC-based HVDC systems

In this thesis, the dc network dynamics of VSC-HVDC systems is investigated through eigenvalue and frequency domain analysis. The eigenvalue analysis has been used to iden-tify the factors that have an impact on the system stability. It has been determined that instability in the form of sustained oscillations can take place, and that the operating point, the dc side electrical characteristics, the strength of the ac system and the controller struc-ture, are the major factors that impact the stability of the system. A frequency domain approach is proposed in this thesis in order to explain the instability that occurs in the system. A two-terminal VSC-HVDC system is modelled as a Single-Input-Single-Output feedback system, and the VSC-system and the dc grid transfer func-tions are defined and derived. The VSC-system transfer function has been interpreted as an admittance, whose conductance is positive or negative, depending on the direction of the power. The main characteristic of the dc grid transfer function is the resonance peak, which appear as a result of the RLC characteristic of the dc transmission line. When the re-sonance phenomenon takes place at a frequency in which the VSC conductance is negative