Rapid Estimation of CCT for GFM and GFL Inverters Integrated Systems Using the Direct Method

The integration of grid-forming (GFM) inverters can provide voltage and frequency support for the system and enhance system stability. However, existing transient stability analyses for GFM and grid-following (GFL) inverters integrated systems face challenges in rapidly estimating the stability domain, limiting their suitability for online applications. To address these issues, this paper quantifies the system’s transient stability domain using the critical clearing time (CCT). By considering the Q-V characteristics of the GFM and the power output characteristics of the GFL, the structure-preserving transient energy function (TEF) is derived based on the energy conservation law. Then, an improved iterative potential energy boundary surface (IPEBS) method is proposed to evaluate the CCT, which reduces estimation errors and prevents misjudgments while ensuring a conservative CCT estimation. Finally, simulations verify the method’s efficiency, accuracy, and conservatism.