Electromagnetic Transient Modeling, Simulation, and Characteristic Analysis of a High-Head Hydropower Unit Islanded via Flexible DC Transmission System

• • High-head units exhibit weaker short-term stability than conventional units due to penstock dynamics; this necessitates refined elastic water hammer models (n=1) rather than rigid models (n=0) for accurate EMT simulation, as the latter underestimates hydraulic inertia in long conduits. • • The hydraulic-electrical coupling between the high-head unit and MMC-HVDC can trigger ultra-low-frequency oscillations (below 0.1 Hz), posing a risk to islanded system frequency stability; this requires damping control in the governor or MMC controllers. • • A significant phase difference exceeding 180° exists in the [30, 50] Hz band, indicating a subsynchronous oscillation risk that could excite torsional interactions or converter control instability, demanding detailed impedance-based analysis. • • Under sending-end three-phase faults, conventional hydropower units demonstrate superior stability compared to high-head units, highlighting the need for enhanced fault ride-through strategies in high-head islanded systems.