Asymmetric Virtual Impedance for Improving Transient Stability of Grid-Forming Inverters Based on Virtual Reluctance Torque

The grid-forming inverters have received much attention due to the great voltage and frequency support ability. However, a lot of transient instability phenomena have been reported to the grid-forming inverters. In order to improve the transient stability of grid-forming inverters, an asymmetric virtual impedance is proposed to simulate the reluctance torque of synchronous generator to increase to power transfer ability. Furthermore, the design of virtual impedance is considered from the dimensions of d axis and q axis respectively, and the effect of reluctance torque on the transient stability is shown from the perspectives of increasing power transfer margin and providing larger deceleration area. Finally, the simulation results are presented to validate the effectiveness of the proposed method.