Vibration caused by unsteady flow usually leads to a problem of stability in operation for large hydraulic turbines. This paper reports the recent investigation and analysis of vibration and pressure fluctuation problem on a prototype 700MW Francis turbine unit at the Three-Gorges Hydropower Plant in China. The site investigation were carried out for a wide load range of 200–600MW at the condition of water head 57–76m. Significant vibrations were observed on head cover when the turbine was operated at load of 540–542MW with water head 68.3m. To identify the causes, investigations were done by measuring oscillations of the guide-bearing, pressure fluctuations under the head cover and in the draft tube. A special vibration zone at frequency of 4.56fn (fn is the runner rotation frequency) was found by further spectrum analysis for the vibrations and pressure fluctuations, while this special zone was not found in other turbines in the same site with same configurations. On the other hand, the low-frequency vibrations caused by pressure oscillations of vortex rope in draft tube were also measured and analyzed.

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