On the Transient Overcurrent Mechanism of Grid Forming Converter in Fault Recovery Stage

The grid forming (GFM) converter may undesirably output a higher current during recovery stage than fault duration due to its transient angle swing, which would cause the power devices to face the risk of overcurrent or even damage in both fault stage and also recovery stage. In this letter, the current power angle plane is presented for current trajectory description and the mechanism of the recovery overcurrent (ROC) issue is revealed through the proposed concept of critical recovery angle (CRA). On top of that, the power angle relationship in critical ROC state is described in detail and the ROC boundary is further characterized accurately. Moreover, an analytical calculation method of CRA is proposed to identify whether the ROC phenomenon occurs, which is well validated by a series of experiments. The proposed method depicts the inherent relationship between the current behavior and power angle motion of GFM converters quantitatively and intuitively, and it may provide some new insights into the transient analysis and control optimization of GFM converter.