Beyond the S-region instability: A 3D blade reshaping strategy to unlock stable pump-turbine operation for enhanced grid flexibility

Driven by the global energy transition, the rapid shift toward renewable-dominated power systems has raised serious concerns over grid stability. Pumped storage hydropower, as a key regulation resource, faces operational risks due to pump-turbine instability in the S-region, characterized by severe flow separation and unstable vortex structures. These flow phenomena disrupt uniformity and induce hydraulic excitation and pressure fluctuations, threatening unit safety. To address this, a 3D blade reshaping strategy is proposed for a high-head model pump-turbine, using the circumferential angle of the runner blade camber line as the design variable. The optimization targets unit speed and vortex structure volume under three typical S-region conditions. A geometric parameterization method is adopted to analyze and suppress S-region flow instability. With increased blade twist, the optimized blades exhibit outward protrusions at the mid-span and near the lower band. Control points near the inlet and outlet at mid-span critically influence hydraulic performance. The redesigned runner eliminates the deviation of the characteristic curve toward lower rotational speed and improves speed uniformity across all conditions. At small openings, the curve satisfies dQ11/dn11 < 0; at large openings, it shifts toward higher speed, resolving the multi-valued behavior of one speed corresponding to three flow rates. Furthermore, the optimized blade shape effectively suppresses high-speed circulations, crossflows, and backflows, significantly reducing flow losses and pressure fluctuation amplitudes. These improvements enhance the S characteristics and hydraulic stability of the unit, offering practical guidance for pump-turbine design and optimization in support of grid flexibility.