Abstract: To explore the relationship of the full S characteristic curve of a pump-turbine and the minimum pressure in its draft tube, we conduct numerical simulations of successive load rejection which is most unfavorable to the draft tube flow for the Xinfeng pumped storage power station. By adjusting the opening curve slope near the unit’s operating trajectory, a comparative analysis and a summary are made on the influence of changes in the S zone shape on the extreme pressure in the draft tube under successive rejection conditions. We formulate a graphical indicator that is applicable in the early design phase of such stations to evaluate the impact of S-shape characteristics on draft tube pressure, and verify the calculation accuracy. The results show that for the first decrease of the unit’s flow caused by the rejection, the operating trajectory presents a complete inverted S-shape. This shape change will cause a change in the slope of the unit’s flow variations, and further result in a change in the extreme draft tube pressure that occurs near the S-shape inflection. The more the graphical indicator shape obtained approaches a vertically slender regular rectangular shape, the better the improvement of the corresponding minimum draft tube pressure.

Key words: pumped storage power station, full characteristic curve of pump-turbine, S characteristics, successive load rejection, minimum pressure in draft tube