Abstract: Based on the water transmission and power generation system at the Changlongshan pumped storage station, this article presents numerical simulations of the load rejection condition using two sets of complete characteristic curves with different specific speeds, and summarizes the transient characteristics of pump-turbines by resorting to the unit operating trajectories on these curves. We give a definition of the S characteristic angle, a parameter that can be used for evaluating the influence of the inverse S region of the curves on the transient process. To verify this angle, we compare the influences of different inverse S shapes on the parameters of transient process, and examine the causes of the differences. The results show that for the load rejection of a unit, all the parameter extremes occur in a time interval from its runaway point to zero-flow point. A smaller S characteristic angle of the characteristic curve in this region leads to the pressure extremes farther away from the reverse pump zone, along with larger flows at the extremes and a lower flow change rate, thus lowering the pressure extremes greatly.

Key words: water conservancy and hydropower engineering, pumped storage station, complete characteristic curve, pump-turbine, S characteristics, transient flow