Author(s): Giacomo Zanetti; Francesco Nascimben; Giovanna Cavazzini
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Keywords: No Keywords
Abstract: The global energy consumption has increased markedly over the past decade, intensifying the need for the exploitation of renewable energy sources. Within this context, hydropower remains the backbone of renewable electricity production, contributing more than 50% of global renewable generation and approximately 14% of total electricity supply. However, climate change is profoundly reshaping hydrological regimes, with several European regions experiencing prolonged droughts and increasingly irregular precipitation patterns. As a result, many run-of-river hydropower plants frequently operate far from their design conditions and are often forced to shut down when river discharge drops below the technical minimum of the turbines. Francis turbines, the most widely adopted technology for medium-head hydropower installations, typically exhibit a minimum operational flow of about 40% of the nominal discharge. Below this threshold, severe instabilities arise, including draft-tube vortex breakdown, large pressure oscillations and strong radial forces that can compromise structural integrity and significantly limit operating time (GI. Krivchenko (1986)). In this context, Variable Speed Generation (VSG) has emerged as a promising strategy to extend the operating range of reaction turbines. By decoupling the generator rotational speed from the grid frequency through power electronics, VSG enables turbines to run at reduced rotational speed during partial-load operation, thereby decreasing swirl intensity at the draft-tube inlet and generally smoothing the flow patterns throughout the hydraulic passages (G. Amul (2023), J. Schmid (2022), I.Iliev (2019)). However, the influence of the velocity variation on the turbine performance and behaviour at deep partial loads, particularly regarding the minimum operational discharge, is not sufficiently clear.
Year: 2026