@ARTICLE{10.21494/ISTE.OP.2026.1482, 

TITLE={Uncovering Weakly Damped Nonlinear Inter-Aria Modes in High-RES Scenarios: A Koopman-Based Analysis of Tunisia’s Future Grid}, 
  
AUTHOR={Yassine BOUSSAA
, Khadija BEN KILANI
, Fethi GHODHBANE, }, 
	
JOURNAL={Entropy: Thermodynamics – Energy – Environment – Economy }, 
	
VOLUME={7}, 

NUMBER={Special issue CIER}, 
	
YEAR={2026}, 

URL={https://www.openscience.fr/Uncovering-Weakly-Damped-Nonlinear-Inter-Aria-Modes-in-High-RES-Scenarios-A}, 
	
DOI={10.21494/ISTE.OP.2026.1482}, 
	
ISSN={2634-1476}, 
	
ABSTRACT={The increasing penetration of renewable energy sources (RES) significantly modifies power system dynamics by reducing inertia and amplifying nonlinear interactions. Conventional eigenvalue-based small-signal stability methods capture linearized behavior but fail to reveal hidden nonlinear oscillatory components emerging under renewable-rich conditions. This paper investigates the oscillatory stability of the Tunisian power grid projected for 2030 under a peak-load scenario with 35% RES penetration. Using a detailed nonlinear PSAT model, Classical Linear Modal Analysis (LMA) is combined with Koopman Modal Analysis (KMA), a data-driven operator-based approach extracting spatiotemporal dynamic structures from simulation data. Results show that LMA identifies the dominant inter-area mode near 0.8 Hz, while KMA reveals additional weakly damped nonlinear modes within the critical [0.7–1 Hz] band. Koopman mode shapes highlight a dominant North–[Centre + South] oscillation driven by the 400 kV transmission backbone, with time-domain reconstructions confirming long decay times. The study proposes targeted measures including PSS improvement, transmission reinforcement, PMU-based wide-area monitoring, and advanced control strategies. KMA is demonstrated as a powerful complement to classical modal analysis for future renewable-dominated grids.}}