S-shaped characteristics on the performance curves of pump-turbines in turbine mode - A review

S-shaped characteristics on the performance curves of pump-turbines in turbine mode promote instability problems in associated transient processes, e.g., difficulties in synchronizing with power grid in turbine start-ups, unstable performance in turbine load rejections, etc. The present paper presents and discusses relative studies on the S-shaped characteristics, which have been carried out by various investigators. Studies include system instability analysis, experimental and numerical investigations on the flow mechanism of pump-turbines, studies on the influencing geometric and operating parameters of the unit, and precautions and countermeasures to improve the S-shaped characteristics and instability problems in associated transient processes.