Guide-Vane Closing Schemes for Pump-Turbines Based on Transient Characteristics in S-shaped Region

Abstract

During the transitional processes of load rejection in a pumped-storage station, the S-shaped characteristics of the pump-turbines can result in relatively large water-hammer and pulsating pressures. These pressures and the high runaway speed during transient processes may directly damage the penstocks and shorten the life of the turbine. In this study, different guide-vane closing schemes for reducing the maximum transient pressures, including the water-hammer and pulsating pressures, and runaway speed were investigated, and the principles for improving the closing schemes were theoretically analyzed based on the transient characteristics in the S-shaped region. First, an analytical expression for the rate of change of relative water head during the transitional processes was deduced based on a simplified mathematical model. It reveals the relationship between the slopes of the trajectory at the pump-turbine operating points (defined as trajectory slopes) and the rigid water-column pressure, which approximates the water-hammer pressure considering compressibility. Then, based on the characteristics of the rigid water-column pressure during the transient process and the effects of guide-vane closure on the trajectory slopes, the selection method for a two-phase guide-vane closing scheme was proposed. The method included the technique for choosing the coordinates of the turning point and the closing speed of the guide vane. Furthermore, the pulsating pressures of pump-turbines were discussed under different working regions and guide-vane openings (GVOs). Considering the characteristics of the pulsating pressures and the runaway speed during the transient processes, the advantage of three-phase valve-closing schemes in controlling the pulsating pressures and the runaway speed was clarified. Finally, a series of model tests were conducted on a pumped-storage station model and the measured data fully validated the correctness of our analyses in this work.

Optimization of Guide Vane Closing Schemes of Pumped Storage Hydro Unit Using an Enhanced Multi-Objective Gravitational Search Algorithm

“…OGVCS plays an important role in ensuring the security and stability of the power grid, and several methods have been proposed to deal with the OGVCS problem. Based on the characteristics of the rigid watercolumn pressure during the transient process, Ref [10] proposed a two-phase guide-vane closing scheme and three-phase valve-closing schemes to control the pulsating pressures and the runaway speed. Ref [11] analysing the effect of valve closure on the water-hammer pressure and laid the theoretical foundation for improving the turbine guide-vane closure.…”

Pressure Fluctuations in the S-Shaped Region of a Reversible Pump-Turbine

“…In the associated transient processes of pump-turbines, the S-shaped characteristics of the performance curves in turbine mode usually induce instability problems. The most common problems are difficulties in synchronizing with power grids during turbine start-ups, and unstable performance during turbine load rejections [3,4].…”

Effects of Pump-turbine S-shaped Characteristics on Transient Behaviours: Experimental Investigation

“…Detail theoretical analyses of guide vane closing scheme considering the pump turbine S-shaped characteristics were conducted in the paper [13]. Here, three types of closing schemes (shown in Fig.6) based on the theory were compared through experiments.…”

“…Due to the S-shaped characteristics, the dynamic processes of a pumped storage station is complicated: 1) The unstable pump turbine S-shaped characteristics make it difficult to measure the Sshaped curves, which can, therefore, be obtained by the transient method [9,10]; 2) pump turbines often show an unstable runaway and no-load operations, which are close to the form of S-shaped curves [5][6][7][8]; 3) in the load rejection process, the water-hammer pressure is also related to the Sshaped curves; 4) in some extreme conditions, such as OAA load rejections, numerical simulations have shown that the water-hammer pressure, especially in the draft tube, is fairly significant [11,12]; 5) to control the water-hammer pressure and runaway speed during the load rejection process, multiphase closing schemes can be used to guarantee the safety of a pumped storage station [13]. Therefore, based on the transient issues described above, the relevant verification experiments were conducted to clarify the effects of S-shaped characteristics on hydraulic transients.…”