The development of a novel electricity system integrating hydropower, wind, solar, and energy storage requires enhanced rapid-response capabilities from hydropower units, which are expected to undergo prolonged transitional processes. Pelton turbines, equipped with a dual regulation system comprising needles and deflectors, offer significant advantages in maintaining stability during transitional processes. However, the transient flow and hydrodynamic characteristics of Pelton turbine during load rejection remain inadequately understood. This study employs a one-dimensional method of characteristics coupled with three-dimensional computational fluid dynamics to numerically investigate the load rejection process of a Pelton turbine. The results reveal that deflector-jet interference causes pressure fluctuations in the water supply system, with a maximum amplitude reaching 5% of the gross head. The deflector alters the jet flow direction, which simultaneously exerts a braking effect on the runner's rotation and causes a significant increase in the radial force on the runner, by an order of magnitude. Moreover, the peak rotational speed and maximum radial force on the runner are observed at a similar deflector opening, independent of the initial load and deflector closure scheme.
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