Direct Transient Stability Assessment of Grid-Connected Voltage Source Converters: A Transient Energy Functions Perspective

The integration of renewable energy sources (RES) into the grid predominantly utilizes voltage source converters (VSC) with a grid-following (GFL) connection strategy. This approach, however, often faces instability issues in low-inertia grid regions. To enhance grid resilience in renewable-dominated power systems, grid-forming (GFM) connection strategies have been introduced, with the virtual synchronous machine (VSM) technique based on GFM control being particularly prevalent. Consequently, transient stability investigations of power systems employing both GFL and GFM connection techniques have gained increasing relevance. This paper proposes an innovative fast direct transient stability analysis (DTSA) approach that leverages Lyapunov's direct method to assess the transient stability of a GFM with VSM control technique, incorporating the influence of parallel-connected VSC with GFL control strategy. The proposed DTSA approach models the GFL as a current source with parallel internal reactance and the GFM as a voltage source with series internal reactance. Detailed analytical derivations of the DTSA approach using transient energy functions, along with the DTSA criterion for the interconnected system, are presented to determine the critical clearing time (CCT) of the GFM-employed VSC. Furthermore, Matlab/Simulinkbased electromagnetic transient (EMT) time-domain (t - d) simulation responses are utilized to validate the accuracy and computational effectiveness of the proposed fast DTSA approach. In addition, the proposed DTSA approach offers considerable benefits for efficiently assessing and managing power systems that heavily rely on RES in terms of operational planning and contingency management.