Quantifying Damping Effect on Transient Stability of Grid-Connected Virtual Synchronous Generator With Current Limiting

The switching and saturation nonlinearities introduced by current limiting strategies not only reshape the power-angle characteristics but also change the damping properties of the virtual synchronous generator (VSG)-controlled grid-forming (GFM) inverters. However, the damping effect on the transient stability of GFM inverters with current limiting has not been intuitively and analytically investigated and quantified yet. This paper first analyzes the piecewise characteristic of the power-angle curve for the GFM inverter when current limiting is activated. Then concavity and convexity of the phase portrait under different operating modes of the GFM inverter are analytically investigated. An important proposition on the curvature of the phase portrait of GFM inverter with different current-limiting strategies is proposed and proven. Based on this, the dissipated energy of the GFM inverter is graphically illustrated and precisely estimated. Consequently, the critical clearing angle (CCA) and energy difference indicator of GFM inverters with virtual impedance and circular current limiter are analytically determined with low conservativeness. The effectiveness of theoretical analyses is validated by experimental tests.