Abstract
The transient characteristic of the power-off process is investigated due to its close relation to hydraulic facilities’ safety in a pumped storage hydropower (PSH). In this paper, power-off transient characteristics of a PSH station in pump mode was studied using a three-dimensional (3D) unsteady numerical method based on a single-phase and volume of fluid (SP-VOF) coupled model. The computational domain covered the entire flow system, including reservoirs, diversion tunnel, surge tank, pump-turbine unit, and tailrace tunnel. The fast changing flow fields and dynamic characteristic parameters, such as unit flow rate, runner rotate speed, pumping lift, and static pressure at measuring points were simulated, and agreed well with experimental results. During the power-off transient process, the PSH station underwent pump mode, braking mode, and turbine mode, with the dynamic characteristics and inner flow configurations changing significantly. Intense pressure fluctuation occurred in the region between the runner and guide vanes, and its frequency and amplitude were closely related to the runner’s rotation speed and pressure gradient, respectively. While the reversed flow rate of the PSH unit reached maximum, some parameters, such as static pressure, torque, and pumping lift would suddenly jump significantly, due to the water hammer effect. The moment these marked jumps occurred was commonly considered as the most dangerous moment during the power-off transient process, due to the blade passages being clogged by vortexes, and chaos pressure distribution on the blade surfaces. The results of this study confirm that 3D SP-VOF hybrid simulation is an effective method to reveal the hydraulic mechanism of the PSH transient process.
Highlights
To maintain the balance between increasing demands for energy and environment protection, more and more renewable energy resources, such as wind and solar power, have been exploited, even though their intermittent electric output will cause surges in power network [1]
Wecomputation developed an innovative method combining single considerations, we developed an innovative simulation method combining single phase andtank the volume of fluid (VOF)
To eliminate influences caused by computational grids, three different meshing schemes are employed in the computational fluid dynamics (CFD) simulation: the corresponding mesh grid size is about 2.5 million, 3.5 million, and 4.5 million respectively
Summary
To maintain the balance between increasing demands for energy and environment protection, more and more renewable energy resources, such as wind and solar power, have been exploited, even though their intermittent electric output will cause surges in power network [1]. There is a larger peak–valley difference of electric load owing to the increasing net capacity [2]. Pumped storage hydropower (PSH) stations, as the most efficient energy storage facilities [3], play an important role in the power grid, such as peaking load shifting, frequency and phase modulation, and as an emergency reserve. Most pump-turbines have the unsteady S-shape region in synthetic characteristic curves [6], which can result in more risks than normal hydropower stations in transient processes
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