Interaction Analysis and Damping Control of Sub-Synchronous Oscillation and Medium- Frequency Oscillation in HVDC-Connected Offshore Wind Farm

The rapid exploitation of offshore wind energy has led to an increasing adoption of high voltage direct current (HVDC) transmission systems and permanent magnet synchronous generator (PMSG)-based wind energy conversion systems (WECSs). Accompanying this development, numerous power converters may result in significant electromagnetic oscillation within offshore wind farm (OWF). Currently, most studies mainly investigate the oscillation mechanism and oscillation characteristic, neglecting the potential interaction between independent oscillations. To fill this gap, a novel mechanism for potential interaction between the sub-synchronous oscillation and medium-frequency oscillation is explored. This mechanism elucidates, for the first time, that the presence of the grid-side converter (GSC) instigates interaction between the sub-synchronous oscillation and medium-frequency oscillation, generating new secondary oscillations. Modal analysis and Nyquist stability criterion verify that, in HVDC-connected OWF, there are not only principal oscillations, i.e., sub-synchronous oscillation and medium-frequency oscillation but also secondary oscillations induced by their interaction. Furthermore, two practical oscillation damping methods are proposed, tailored for the operation and planning of OWFs, respectively. By implementing a supplementary damping controller or tuning GSC parameters, the multiple-mode oscillations can be effectively suppressed without adding additional equipment.