A Study on Damping Controller for Grid-Forming Inverter Power Synchronization

The application of grid-forming inverter(GFM) that can control voltage and frequency similar to synchronous generators is a key for a weak grid with a high penetration level of renewable energy sources (RES). However, the GFM connected to the high short circuit ratio(SCR) power system can cause instability in the synchronization process. This is because the closer the electrical distance is, the more sub-synchronous oscillation occurs in GFM. Therefore, GFM operation may not be possible due to oscillation occurrence and control instability when GFM is initially integrated into high SCR. In this paper, to get the mechanism of GFM which is unstable control according to SCR, we propose a basic structure of GFM small model. A small-signal model of GFM based on droop control with a SISO system that reduces the coupling of current and voltage has been utilized for the stability analysis. Through controller stability analysis, we proved that the unstable pole point of GFM at high SCR and that controller converges to a stable condition when the proposed damping control is applied to the existing droop control. The verification of the proposed damping control method and approach has been performed by using the electromagnetic transient (EMT) simulation.