Review of positive slopes on pump performance characteristics of pump-turbines

Positive slopes in the performance characteristics of pump-turbines are an indicative feature of unstable behaviour in pump mode operation, and are additionally conducive to instability problems during pump start-up. With the rapid development of pumped-storage power plants, this instability has become increasingly critical to the safe and stable operation of the entire unit. Thus, this topic has been attracting attention in the pump-turbine design and research fields. In this study, the relative achievements with regard to positive slopes, which have been obtained by various investigators over the last two decades, were reviewed. Such studies, including flow characteristics, and influence parameter, related to positive slopes were summarized. The generating mechanism, accompanying unsteady phenomena, and corresponding hysteresis phenomena, are also discussed. Investigations on the influence of geometric and operating unit parameters are presented. Finally, precautions and countermeasures for the improvement of positive slopes, and various perspectives regarding future work are proposed. The present review provides theoretical guidance for the design and operation of pump-turbines, in terms of the stability of the pumped-storage powers plants.