Experimental investigation of a free surface oscillation in a model pump-turbine

Hydro power plants provide balancing services to the power grid and the amount is expected to increase in the near future. One of these services is the compensation of reactive power and the regulation of the power factor. Therefore, hydro power units equipped with synchronous machines are operated in so called synchronous condenser mode. During this operation the runner is dewatered and rotates in air above a free surface at the draft tube cone. As a result, a free surface oscillation is excited under some conditions. This paper deals with the experimental investigation of this flow phenomenon. Therefore, model tests are conducted at the hydraulic laboratory of the Institute of Energy Systems and Thermodynamics at TU Wien. The investigated model pump-turbine is equipped with a draft tube cone made of acrylic glass for optical investigations. In addition to the standard instrumentation, fast response pressure transducers are installed at different planes and positions. The machine setup and measurement conditions aim at the investigation of the free surface oscillation and shall prevent other influences. The experiments are performed for both runner rotation directions, different initial water levels at the draft tube and different densimetric Froude numbers. High speed camera recordings are conducted and the amplitude of the water formation is determined by post processing the captured images. In periodic state the frequency of the free surface oscillation is extracted out of the pressure signals. Finally general trends and dependencies are deduced from the acquired data.