Improved Critical Clear Time Calculation for Grid-Forming Converter Based on Accurate Power Angle Characteristic

Grid-Forming (GFM) technique is a novel solution for renewable energy sources to mimic the conventional synchronous generator behavior and to provide inertia, frequency and voltage support for the electricity grids. The transient stability of GFM converters is a matter of concern and the fault clear time has significant impacts on the transient stability. However, in the traditional critical clear time (CCT) calculation under grid faults, the GFM converter is regarded as a synchronous machine and its low overcurrent ability is ignored, hence such study is inconsistent with the practical applications, resulting in inaccurate analysis of the CCT. To quantitatively analyze the transient stability margin of GFM converters with current limiting strategies, a CCT calculation method is proposed in this paper. The analysis results show that the proposed method has higher precision and wider applicability. The effectiveness of the theoretical analysis is verified by the Matlab/Simulink-based simulations.