  This study explores the practical viability of repurposing aging Kaplan
turbines into variable-speed propellers by employing full-size frequency
converters. The motivation behind this approach is to improve the provision of
\emph{Frequency Containment Reserve} (FCR) while reducing fatigue in the Kaplan
blades servomechanism. We evaluate the performance of these modified Kaplan
turbines against the one of another hydro asset composed of the same Kaplan
turbine hybridized with a \emph{Battery Energy Storage System} (BESS).
Experiments are conducted on a one-of-its-kind reduced-scale model testing
platform. Our findings reveal that Kaplan turbines repurposed as variable-speed
propellers exhibit similar dynamic response characteristics compared to the
standalone Kaplan operation, with the added benefit of effectively eliminating
blade movements. Furthermore, the ability to control the speed increases the
hydraulic efficiency for certain operating points. In summary, investment in
variable speed technology emerges as a viable alternative to BESS-based
hydropower hybridization.
