Experimental Investigation of Pressure Pulsations in High-head Francis Turbine During Load Acceptance

Abstract

In the past few decades, renewable energy sources like wind and solar have seen an unprecedented growth in the share of energy production. However, being intermittent in nature, the power production from solar and wind shows large variability even within short period of time. This makes the grid unstable and result in power swings. Hydropower plays a key role in balancing the grid, as it can shift its operating point in very short time. Such transient operations, however, compromise the stability of the turbine and life of the runner. The current work focuses on the pressure pulsations in the Francis turbine during load acceptance. For startup, the guide vanes were rapidly opened from 6.9 % to 76.9 %. During the process, down surge pressure was observed at the upstream of the runner from rapid guide vane opening. In this region, the pressure pulsation from blade passing was significant. In the vaneless space, the first (97.18 Hz), second (194.36 Hz), and third harmonics (291.54Hz) of the blade passing frequency were present showing the impact of rotor-stator interaction. The first harmonics was the strongest frequency component, with the second and third harmonics showing magnitudes reduced by 2.21 and 12.5 times compared to the amplitude of the first harmonic. Resonance can occur if the blade passing frequencies coincide with the natural frequency of runner. In draft tube, there were significant pressure pulsations at lower opening which could have been associated with vortex rope. The spectrogram of pressure signal from this region showed the presence of 137.22 Hz frequency component which could have originated from the standing waves. Such pressure pulsations induced considerable stress on the turbine components, which introduces fatigue cracks and shortens the lifespan of the components.