Estimation of Stability Region for Grid-Forming Inverters Considering Switching Characteristics via Constructing Damping-Reflected Energy Functions

Grid-forming (GFM) inverters have been widely used as the interface between renewable energy sources and power grid. During low voltage ride through (LVRT) period, to limit the fault current, GFM inverters will experience the switching of control strategy, which makes the transient stability of GFM inverters exhibit different features from that of synchronous generators (SGs). In this paper, considering the switching characteristics induced by virtual impedance (VI), the framework of predicting the transient stability of GFM inverters during the fault period and after fault clearance is established from the perspective of energy. To reduce the conservativeness of the stability region, a uniform energy function considering the damping dissipation and the dynamics of reactive power control loop is constructed for the transient stability analysis of GFM inverters. Compared with existing approaches, the stability region estimated by the proposed energy function can intuitively show the effect of damping, and effectively reduce the degree of conservatism in transient stability prediction. Finally, simulation and hardware-in-loop experiments are performed to verify the effectiveness and accuracy of the proposed method in the transient stability prediction of GFM inverters with switching characteristics.