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) systems due to undesirable interactions between its dc-transmission link and the VSC dynamics. Previous works have shown that this instability takes place when DC-side resonances coincide with non-passive behavior of one of the VSCs within the VSC-HVDC system, typically, the VSC that controls the direct-voltage. Through Nyquist stability criterion, it is shown in this paper that also the resonance peak of the dc-side resonance has also a negative impact on the instability, i.e. the higher the resonance peak, the greater the risk of instability. 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 is limited to the inverse of the resonance peak at highest. Finally, time domain simulations show that the proposed measure stabilizes the system.