New NMP-Z factor unifies grid stability analysis for weak power systems

Researchers from multiple institutions (Fuyilong Ma, Lidong Zhang, Wangqianyun Tang, Waisheng Zheng, Huanhai Xin, Linbin Huang, Lennart Harnefors) have published a paper on arXiv (2605.13061) that addresses a long-standing gap in power system stability analysis. As converter-interfaced generators (CIGs) proliferate in weak grids, small-signal voltage and synchronization stability become critical. Historically, these two phenomena are treated separately in theory, yet practitioners use a single static metric—short-circuit ratio (SCR)—to assess both. The authors show that both issues originate from non-minimum-phase (NMP) zeros in the grid Jacobian transfer matrix: a zero at the origin signals voltage instability, while low-frequency zeros limit synchronization dynamics.

The paper proves that the traditional SCR is a special case of their proposed NMP-zero (NMP-Z) factor evaluated at the rated operating point, providing theoretical justification for SCR's empirical success. They then develop a unified stability assessment method for multi-converter systems that retains SCR's simplicity but adds theoretical rigor. The method requires only the NMP-Z factor plus individual CIG dynamic models, enabling stability margin assessment across various operating points. All findings are validated through detailed time-domain simulations. This work offers a practical yet mathematically sound framework for grid operators managing increasingly renewable-heavy power systems.