Revisiting Voltage and Synchronization Stability Analysis in Converter-Integrated Weak Grids: Insights from Non-Minimum-Phase Zeros

TL;DR

This paper introduces a unified stability assessment framework for converter-interfaced generators in weak grids, based on non-minimum phase zeros, bridging the gap between theory and practice.

Contribution

It reveals the role of non-minimum phase zeros in stability issues and extends the short-circuit ratio concept into a more comprehensive NMP-Z factor for multi-converter systems.

Findings

NMP zeros in the grid Jacobian determine voltage and synchronization stability.

The SCR is a special case of the proposed NMP-Z factor.

The method enables stability margin assessment across various operating points.

Abstract

The increasing penetration of converter-interfaced generators (CIGs) intensifies concerns over small-signal voltage and synchronization stability. While existing theories treat these two stability issues distinctly, practical wisdom in contrast employs a unified and static metric, short-circuit ratio (SCR), to assess both in weak grids. This paper aims to bridge this theory-practice gap by introducing the insight of non-minimum phase (NMP) zeros. First, we demonstrate that the two stability issues in weak grids originate from NMP zeros in the grid Jacobian transfer matrix: a zero at the origin corresponds to voltage instability, while low-frequency zeros impose fundamental constraints on synchronization dynamics. The traditional SCR is proven to be a special case of our proposed novel stability metric, NMP-zero (NMP-Z) factor, evaluated at the rated operating point. This establishes the theoretical foundation for the empirical success of SCR. Building on this insight, we then develop a unified stability assessment method for multi-converter systems. The method retains the simplicity of SCR, requiring only the NMP-Z factor together with individual CIG dynamic models and enabling stability margin assessment under various operating points. Our work provides a simple yet theoretically rigorous framework for stability analysis in CIG-integrated weak grids, with all theoretical findings and the proposed method validated through detailed time-domain simulations.