Analysis and mitigation of instabilities originated from dc-side resonances in VSC-HVDC systems

Instabilities related to dc-side resonances can occur in Voltage Source Converters based HVDC (VSC-HVDC) when resonances originated in dc-transmission links coincide with a non-passive behavior of one of the VSCs within the VSC-HVDC system. Through Nyquist stability criterion, it is shown in this paper that, when the VSC that controls the direct voltage has a non passive behavior, the size of dc-side resonance peak has also a negative impact on the instability, i.e. the higher the resonance peak, the greater the risk of instability. Then, the impact of transmission link configurations on the dc-side resonance characteristic is investigated, including overhead transmission lines and dc-side filters. Based on the analysis presented in this paper, it is proposed that, around the dc-side resonance frequency, the magnitude of the VSC subsystem transfer function should be limited to the inverse of the resonance peak at highest. Finally, time-domain simulations show the effectiveness of the proposed countermeasure in stabilizing the system