Study on pressure pulsation and internal flow evolution of pump turbine during low-head start-up process

Abstract

[Objective] During the low-head start-up of pumped-storage units, units are prone to entering the unstable reverse-S zone of the full characteristic curve, which can lead to failure in grid connection and severely affect the safe and stable operation of the units. [Methods] A three-dimensional numerical simulation was conducted to study the low-head start-up process of the full-flow system in a pumped-storage hydropower plant's pump-turbine units, focusing on analyzing pressure pulsations in the volute and guide vane impeller regions, as well as internal flow characteristics. [Results] The simulation result showed that during the low-head start-up process of the pumped-storage units, in the stage of guide vane opening, vortices and blocked flow paths at the impeller inlet and within the impeller blades reduced the flow, generating a water hammer effect that rapidly increased the upstream pressure of the impeller. As the high pressure caused by the water hammer dissipated, the flow increased rapidly, followed by a slight negative water hammer that caused the pressure to drop. After the guide vanes stopped moving, the distribution of vortex structures at the impeller inlet and outlet decreased. As the impeller torque continued to decrease, the units entered the reverse-S zone and approached the no-load operating condition, with pressure pulsations starting to intensify. Unstable vortex structures reappeared at the leading and trailing edges of the impeller blades, and the increased water flow velocity at the impeller outlet caused the vortex belt in the tailwater pipe to extend downstream, forming a double-layer vortex structure at the center and near the wall. [Conclusion] Some vortex structures are the main cause of the instability of the units' external characteristic parameters at the beginning of the start-up process. As the process continues, pressure pulsations begin to increase, and unstable vortex structures reappear within the impeller flow passages, [Results] ing in the unstable hydraulic characteristics of the pump turbine under no-load conditions. The analysis of pressure pulsations and internal flow characteristics in the volute and guide vane impeller regions aims to provide a reference for achieving stable low-head start-up of the pumped-storage units.

Keywords

pumped-storage power station / pump turbine / reverse-S characteristic / low-head start-up / pressure pulsation / flow characteristics