Oscillation stability analysis of grid-forming converters in strong and weak grids based on frequency-coupled impedance model

The application of grid-forming (GFM) control technology in power systems has been increasingly widespread. However, its oscillatory stability mechanisms under strong and weak grid conditions remain insufficiently understood. This paper investigates and compares the oscillatory stability characteristics of grid-forming converters under varying grid strengths using an aggregated impedance model. First, an aggregated impedance network comprising the grid-forming converter and the AC grid is established, and an oscillatory stability criterion based on the zero-crossing property of the determinant of the aggregated impedance matrix is introduced. Then, the determinant-based comparative analysis of the aggregated impedance is carried out to examine the differences in oscillatory stability of the grid-forming converter under strong and weak grid conditions. Finally, a single-machine infinite bus simulation model is developed in PSCAD to validate the theoretical findings. Simulation results indicate that the grid-forming converter demonstrates superior oscillatory stability in weak grids, with no risk of oscillatory instability even under extremely weak grid conditions where the short-circuit ratio (SCR) approaches 1. Conversely, in strong grids, the converter is more prone to oscillatory instability, including sub/super-synchronous oscillations.