Analysis of extremely low water hammer pressures of draft tubes for double units in pumped storage power stations under successive load rejection conditions

Abstract

With large-scale integration of intermittent energy into power systems, the operating conditions of pumped storage power stations (PSPSs) change frequently, thereby intensifying the risk of load rejections. For multiple units sharing a water conveyance system, when one or more units reject the load, other units also reject the load after a few seconds; this is defined as successive load rejection (SLR). During SLR conditions, pumped storage units produce extreme water hammer pressures, jeopardizing the safe operation of PSPSs. This study reveals the generation mechanism of extremely low draft tube pressure (DTP) during SLR, and clarifies the generating unit and corresponding occurrence moment characteristics of the minimum DTP. The results indicated an extremely low DTP, attributed to the sharp decline in the discharge owing to increased rotational speed in unit's S-shape region and the hydraulic interference owing to the increased discharge of the other unit. When the guide vane closing time (GVCT) was short, the minimum DTP occurred in the subsequent load rejection unit (SLRU) near the first peak of its rotational speed, with the discharge of initial load rejection unit (ILRU) in the rising stage. Conversely, for a large GVCT, the minimum DTP occurred in the ILRU near the second peak of its rotational speed, with SLRU discharge in the rising stage. Finally, engineering measures to improve the extremely low DTP were proposed. Overall, the findings underscore crucial engineering insights for ensuring the safety of PSPSs under extreme conditions.