Grid converter study reveals hidden non-uniform PAC convergence

A new paper by Haoxiang Zong, Chen Zhang, Yu Zhang, Xu Cai, and Marta Molinas (arXiv:2607.08214) digs into a critical but overlooked flaw in virtual admittance-based grid-forming (VA-GFM) converters. These devices are essential for connecting renewable energy sources to the power grid, and their transient synchronization stability (TSS) analysis relies heavily on the current-constrained power-angle curve (PAC). The PAC formulation assumes a quasi-steady-state: that active power converges to steady-state quickly and stably across the entire angle space. While intuitive, that assumption had never been rigorously validated—until now.

The team performed an eigen-sweep analysis of the full-order VA-PAC model, which includes detailed controls, and discovered a new phenomenon: non-uniform convergence of the PAC. They theoretically proved its existence and then mapped out the exact open-loop stability and response-rate conditions required to guarantee convergence. These findings show that under certain conditions the assumed convergence fails, potentially leading to unexpected instability. For engineers designing next-gen grid converters, this work provides essential guidelines and prompts a re-evaluation of TSS analyses used today.