NTNU research quantifies grid asymmetry as stability margin predictor

A team from the Norwegian University of Science and Technology (NTNU) has published a paper establishing a direct, quantifiable link between system-level asymmetry and stability margins in converter-grid systems. The work, led by Chirag Ramgopal Shah, Marta Molinas, Sjur Føyen, and Roy Nilsen, introduces the Asymmetricity Quantification Index (AQI), derived from the sequence-domain representation of interconnected converter-grid impedance. By defining symmetrical matrices as a benchmark, the researchers show that increased asymmetricity—dominated by control loops like DC-link voltage control, phase-locked loops, and power synchronization loops, as well as the inherently asymmetric topology of two-level voltage-source converters—directly correlates with reduced stability margins. This finding moves beyond purely control-focused asymmetricity to consider the whole system, including operating points, as the root cause of instability.

The analysis was validated in both control-hardware-in-the-loop (CHIL) and power-hardware-in-the-loop (PHIL) environments, covering grid-following and grid-forming control structures. The practical implication is clear: reducing system asymmetricity—without compromising controller functionality—can improve stability margins in power grids with high converter penetration. This research offers engineers a measurable diagnostic tool (AQI) to predict instability before it occurs, enabling proactive design adjustments. As renewable energy and HVDC links increase converter penetration, quantifying asymmetry becomes critical for grid stability. The paper is available on arXiv under arXiv:2606.11373.