Abstract: During startup, severe pressure fluctuations usually occur in a pump-turbine; complex flows and pressure pulsations in its runner are two main causes for the strong vibration of its stationary parts. In-depth research is necessary on the physical mechanism of startup. This paper presents numerical simulations of a large pumped-storage power station, coupling a one-dimensional characteristic method hydraulic model of its pipeline system and a three-dimensional calculation model of the whole water passage of its pump-turbine unit. We focus on the analysis of the instantaneous flow characteristics of the unit and the internal correlation between its runner’s radial force and axial hydraulic thrust and its external characteristics during startup. The results show that when the unit is in no-load state, due to its small guide vane opening and a rapid increase in its rotational speed, a high-speed water ring is formed in its vaneless area, resulting in a sudden drop in its flow discharge and runner torque. The corresponding variations in the radial force and axial hydraulic thrust are complex and directly impacted by the variations in the guide vane opening and rotational speed. The trend of radial force roughly follows that of guide vane moving, while the trend of axial hydraulic thrust is nearly the same as that of the unit speed, both gradually increasing on the whole during startup. This study provides an in-depth understanding of the physical mechanism of the startup process and its variations of pumped-storage units and high-head Francis turbines.

Key words: pump-turbine, startup process, one-dimensional and three-dimensional coupling model, radial force, axial hydraulic thrust