Stability Comparison of Grid-Forming Converters with Different Power Calculation Strategies

Abstract

Grid-forming (GFM) converters, such as virtual synchronous generators (VSGs), behave as voltage sources behind a reactance within a specified frequency range. Therefore, solutions to limit the rapid growth of the converter output current, which may cause over-current tripping during grid faults, are necessary. One effective solution is to use outer dual-loop voltage and current control so that current limitation can be realized by saturating the current reference. In the condition, electrical power (EP) of VSG can be calculated either with the converter output current or with the virtual current from the voltage loop, leading to different small-signal stability for the GFM converter. In this paper, small-signal stability of these two EP calculation methods are analysed and compared. Small-signal eigenvalues analysis shows that using virtual current from outer voltage loop for EP calculation contributes to better small-signal stability as compared to the conventional EP calculation approach, under both the weak grid and the strong grid conditions. Simulation results are provided to validate the effectiveness of the analysis.