Analysis and Damping of Sub-Synchronous Oscillations for Cascaded Grid-Forming Converters Considering DC-Link Dynamics

Grid-forming (GFM) converters have received much attention in renewable energy applications because of their voltage support capability. Existing studies generally assume constant dclink voltage for cascaded GFM converters, which may lead to the stability challenges under some occasions with dc-link dynamics. Thus, this paper firstly analyzes the small-signal stability of GFM converters considering dc-link dynamics. It is found that dc-link exhibits inherent instability risk due to the non-minimum phase characteristics of the dc-link capacitor, which may lead to subsynchronous oscillations on the ac side. To solve this issue, dc-link dynamics are introduced into both active and reactive power loops through proportional controllers. This method does not increase the order of the controller, and it is easy to implement digitally. Compared with the conventional damping method that only introduces dc-link dynamics into the active power loop, the proposed method can dampen the sub-synchronous oscillations caused by dc voltage fluctuations, and it can enhance the transient response performances of ac side. The experimental results validate the correctness of the small-signal model and the effectiveness of the proposed damping method.