Impacts of AC/DC Coupling on Control Interaction Assessment in Power Systems with Black-Box Models

This paper demonstrates that focusing solely on AC-side dynamics may fail to predict the overall system stability, as exemplified by an AC weak-grid instability case where DC-side dynamics significantly shape the interaction mechanism. Moreover, even when a specific parameter appears to have a negligible impact on the standalone AC or DC dynamics, it can substantially alter stability margins through changes in AC/DC coupling pathways.

To reflect practical black-box conditions, this study adopts admittance-based modeling, which enables frequency-domain stability analysis without requiring full internal details of the system.

In addition, the paper shows that common mitigation strategies may unintentionally steer the design toward an inappropriate stabilization direction. This is illustrated through passivitybased assessment, which is frequently employed to guide robust converter integration into weak grids. Overall, the results emphasize that AC-only analysis is not sufficient for accurate stability assessment and mitigation, and that AC/DC coupled dynamics must be explicitly considered.