Optimized operation framework of pumped storage power stations with fixed- and variable-speed units sharing a diversion tunnel: Efficiency optimization and transient characteristics analysis

Variable-speed units (VSUs) offer faster response and greater flexibility than traditional fixed-speed units (FSUs). However, their high cost limits large-scale deployment. To balance flexibility and cost, pumped storage power stations (PSPSs) can adopt a hybrid configuration where VSUs and FSUs share a diversion tunnel. However, this configuration may induce unfavorable hydraulic interference between units, posing transient safety risks. This study first develops a generalized efficiency optimization method for a VSU across the full operational range and identifies the best efficiency points (BEPs) along with the corresponding speeds. Subsequently, a transient simulation model of a PSPS with two FSUs and a VSU sharing a diversion tunnel is developed. The low-pressure phenomenon in the draft tube of the VSU during load rejection and its underlying mechanism are revealed. When the VSU operates at the BEPs, its optimal speed deviates significantly from the rated speed, resulting in a substantial difference in steady-state conditions compared to the FSUs. During load rejection, hydraulic interference owing to the increased discharge of the FSUs aggravates the decrease in the discharge of the VSU, causing a sharp decrease in the draft tube pressure of the VSU. Finally, an optimized control strategy to improve transient performance by applying differentiated guide vane closing laws to FSUs and VSU is demonstrated. Compared with the original scheme, the maximum spiral case pressure is reduced by 12.09 m and the minimum draft tube pressure is increased by 27.99 m. The proposed optimized framework provides theoretical support for the efficient and safe operation of hybrid PSPSs.