构网型变流器中多模态低频振荡的相互阻尼排斥机制研究

The coupling between the synchronization loop (SC) and DC voltage control loop (DVC) of a virtual synchronous generator (VSG) controlled grid-forming converter (GFM) is prone to cause multi-modal low-frequency oscillations (LFO). These multi-modal oscillations consist of SC dominant LFO (SC- LFO) and DVC dominant LFO (DVC-LFO). In this paper, we investigate the mutual damping repulsion effect of multi-modal LFOs and reveal the mechanisms behind such phenomena by means of the damping torque method in the frequency domain. Analytical studies have found that, as the self-damping levels of SC and DVC increase, they exert incremental negative damping torque on each other. This interaction leads to an inverse variation in damping regulation of SC-LFO and DVC-LFO during the tuning of SC or DVC dynamic, ultimately causing the mutual repulsion phenomenon. Moreover, it has been proved that virtual damping in SC is not universally effective for LFOs stabilizing in GFM converter. Increased virtual damping ensures SC-LFO stability but may induce instability of DVC-LFO due to mutual repulsion. Finally, the analytical results are corroborated by both eigenvalue and time domain analysis.