Impedance-based DC Side Stability Assessment of VSC-HVDC Systems with Control Time Delay

In this paper, an impedance-based assessment is used to analyse the stability at the DC side of VSC-HVDC systems. The assessment applies the passivity theory and the Nyquist stability criterion on the frequency response obtained from the VSC-HVDC system. Via this method, it is possible to predict potential instability of a MMC-based VSC-HVDC link. Further, it is demonstrated that the control dynamics of the MMC causes regions of non-passivity at the kHz-range which can interaction in a undesirable way with the system resonances. A wideband model of the VSC-HVDC system is employed to accurately capture these unstable interactions. Hence, the averaged dynamic model and the frequency-dependent phase model of PSCAD/EMTDC are chosen for representing respectively the MMCs and the transmission lines. Both the stability of submarine cable and overhead line connected VSC-HVDC links are analysed and the impact of the connection type on the system stability is investigated. Finally, a comparison is made between the time domain simulation of the system and the result of the impedance-based stability assessment.