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 pumpedstorage station, the Sshaped characteristics of the pumpturbines can result in relatively large waterhammer 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 guidevane closing schemes for reducing the maximum transient pressures, including the waterhammer 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 Sshaped 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 pumpturbine operating points (defined as trajectory slopes) and the rigid watercolumn pressure, which approximates the waterhammer pressure considering compressibility. Then, based on the characteristics of the rigid watercolumn pressure during the transient process and the effects of guidevane closure on the trajectory slopes, the selection method for a twophase guidevane 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 pumpturbines were discussed under different working regions and guidevane openings (GVOs). Considering the characteristics of the pulsating pressures and the runaway speed during the transient processes, the advantage of threephase valveclosing schemes in controlling the pulsating pressures and the runaway speed was clarified. Finally, a series of model tests were conducted on a pumpedstorage station model and the measured data fully validated the correctness of our analyses in this work.