Experimental Investigation of Rotating Stall in Reversible Pump Turbine at Turbine Brake Mode

Abstract

Integration of several intermittent energy sources causes instabilities on the grid as they create sudden frequency drops and overshoots. This necessitates the need of power balancing source, and pumped storage hydropower system, consisting of reversible pump turbine (RPT) is a proven technology for that. RPTs prevents frequency collapse during sudden wind drops by ramping turbine output, and RPTs absorbs excess energy while solar overshoots by pumping. However, their frequent mode-switching and off-design nature of operation possesses an operational challenge, which upon ignored causes detrimental effects in rotor-stator interaction leading to severe hydraulic instabilities. Normally, the RPT units are started at no load conditions and are provided with load after grid synchronization speed is achieved. To achieve this, the unit must traverse the S-shaped region, where slight speed increase at low discharge can cause the operating point to shift abruptly from turbine mode to turbine brake mode (TBM). In this study, an experiment has been conducted to study the flow effects in TBM of operation in different guide vane openings from deep part load to high load conditions. The pressure pulsations in different operating conditions were captured by the pressure sensors installed on different locations of the rig. The frequency analysis of the pressure data provided by the two pressure sensors of the vaneless space showed that the presence of rotating stall frequencies (in between 65% to 75% of the impeller rotational speed), particularly in upper part load conditions. The sensors placed at draft tube cone also captured this frequency, showing its severity. Notably, rotating stall was not observed during deep part load or high load operations.