Optimizing surge tank layout for highly flexible hydropower

Abstract

This article considers high-head hydropower schemes and pumped storage schemes that usually are considered as flexible power plants. Surge tanks are the key structural element in the hydropower scheme for flexible operation in power generating and pumping of hydro power plants with pressure tunnel systems. Surge tank layout and components can be further optimized to improve flexibility on new projects or for upgrading of existing plants. Therefore, intensive and ongoing research activities lead to major benefits since the hydraulic behaviour is complex, interdependent and therefor the resulting design for each project is very site specific. Surge tanks have been constructed for thousands of years to enable secure and flexible water supply using pressurized conduits [1]. Due to electrification and first high-head hydropower schemes with conduits in the 1880`s surge tanks have become more and more sophisticated to allow for the operational needs. The surge tank development and research was heavily influenced by the work of Johnson [2], and Thoma [3] in the beginning of the 1900’s. Additionally to the water hammer protection is the need of governor stability during power plant operation. Surge tanks smartly combine these demands; it reduces the water hammer and allows speed and power governing of the hydropower plants. The Thoma criterion [3] is widely used for stability analysis. This criterion determines the necessary size of the surge tank to ensure stable dampening of the mass oscillations in the surge tank. This criterion was extended by Jaeger [4] with an empirical security factor taking the destabilizing impact of pressure shafts on the oscillation into account. Furthermore, the stability criterion needs to be verified in terms of 1D numerical simulations. Summing up, the purpose of surge tanks is basically to allow hydraulic machines to be accurately synchronized with the electrical grid in high-head schemes with significant kinetic energy in water masses due to long tunnel systems. The surge tanks mitigate negative interference of the large water mass acting on the power production and oscillating frequency in the grid. At the same time surge tanks mitigate the water hammer effect on the pressure shaft and pressure tunnel. Due to topographic and geological conditions the design is very site specific as it is typical for most hydropower schemes in general. In contrast to the well-chosen place of the power cavern the surge tank needs to be positioned close to the surface and is more likely in the vicinity to face altered rock with limited overburden compared to the acting internal pressure. Fig. 1 shows a schematic longitudinal section of underground works for a pumped storage scheme with a vertical shaft possibly concrete lined. The maximum and minimum pressure lines visualize the effect of the surge tank. Due to a throttle at the headrace surge tank the max. pressure is above the free surface level.