Iterative Searching Method for Closest Saddle-Node Bifurcation Point in Multi-Infeed HVDC Systems

In multi-infeed LCC-HVDC (MIDC) systems, voltage collapse becomes a concern as DC power increases, and its proximity to the stability boundary is challenging to define precisely. Consequently, there is a growing focus on identifying the direction in which DC power destabilizes the MIDC systems most rapidly. To address the above issue, this letter introduces an iterative method for locating the closest saddle-node bifurcation (SNB) point in the MIDC power space, utilizing the concept of the generalized short-circuit ratio (gSCR) to represent grid strength. It is demonstrated that the right eigenvector corresponding to the gSCR possesses the capability to construct the normal vector of the stability boundary in the power space. When the iterative process converges, this normal vector aligns with the worst-case instability direction. Additionally, the method computes the Euclidean distance from the current operating point to the closest SNB point. Case studies are provided to demonstrate the effectiveness of the proposed method.