An Improved Virtual Impedance Method for Single-loop Controlled Grid-forming Inverters to Suppress Low-frequency Oscillations

In order to build voltage and frequency for power-electronics based power system, the grid forming inverters have received much attention in recent years. However, it is found that the grid-forming inverter will also encounter low-frequency oscillation under various grid conditions. Thus, the issue of how to analyze this kind of low frequency stability problem should be studied. To give physical insight of the phenomenon, complex torque analysis is utilized, and it is revealed that the decrease of damping torque is the main cause of system instability. Subsequently, an improved virtual impedance combining a low pass filter is proposed to enhance the damping torque and mitigate the low-frequency oscillations, thereby enhancing the adaptability of grid-forming inverters. Finally, the proposed approach is validated through simulation results.