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The pumped-storage hydropower station is the most reliable, economic, long-term, large capacity, and mature energy storage technology in the power system, and it is an important component of renewable energy. Cavitation and water column separation of a pumped storage unit are important and widely researched factors in the safe and stable operation of a unit. This study focused on the evolution of water column separation of a pump turbine and its relationship with the pressure distribution of the cross section of a draft tube as well as the pressure pulsation characteristics of different measuring points in the cross section of the draft tube. A pumped storage experimental platform that can realize water column separation is established, and experiments with different opening angles are carried out. The results show that there are three factors that impact water column separation and cavitation: gas nucleus, vaporization pressure, and duration of vaporization pressure. Water column separation is the development and continuation of cavitation. The difference between the center pressure of the vortex rope and the wall pressure is large, reaching 2.23 m at a large opening. The pressure fluctuation amplitude of the wall measuring point is greater than that of the other measuring points in the same cross section, but the frequency characteristics are the same. In the transition process, the pressure pulsation amplitude of the liquid column bridging is the largest, and the largest pressure pulsation amplitude can reach 4.18 m at a small opening. [ABSTRACT FROM AUTHOR]

During the transient processes in load rejection in a pumped-storage system, water column separation (WCS) can occur in the draft tube when the local pressure is less than the vapor pressure. The reverse water hammer due to water column bridging affects the safety of the unit and the tailrace tunnel. However, what are the conditions that trigger WCS? What is the physical mechanism? These questions have not been elucidated experimentally. Therefore, it is necessary to investigate the conditions that lead to WCS and the impact of the reverse water hammer generated by WCS bridging on the stability of the unit. In this study, a semi-open test rig was modified by installing a variable-frequency pump to reduce the static pressure, resulting in a cavitation vortex in the draft tube. The static pressure and the water hammer pressure derived from load rejection cause the pressure at the draft tube wall to fall below the vapor pressure for some time, interrupting the flow in the draft tube. This is a typical case of WCS in a pump turbine. If the pressure in the draft tube does not remain at the minimum pressure for long enough, imperfect water column separation may occur with bubble groups. The amplitude of the pressure pulsations during the initial and development stages was (−0.2 m, 0.2 m). It was 4.96% of the initial static pressure. Thus, the pressure fluctuations have a negligible effect on WCS. [ABSTRACT FROM AUTHOR]