Conference paper

Monitoring of a Steel-Lined Pressure Shaft using Global Indicators and the Frequency Response Function”

Due to high peak energy demands, existing hydropower plants operate nowadays under rough conditions to regulate the discharge and power with relatively fast and repeated opening and closing of turbines and pumps. The local deterioration of the mechanical properties of the steel-lined pressure wall induces a change of the two global indicators: the water-hammer celerity and the wave attenuation. This deterioration may arise from the weakening of the backfill concrete and/or the surrounding rock mass. In-situ measurements of the pressure shaft of the Grimsel II pumped-storage plant in Switzerland have been carried out to monitor these global indicators and to determine its frequency response function. The prototype measurements use dynamic pressure sensors placed at both ends of the pressure shaft. The data are acquired continuously and accessed on-line via internet. Different approaches to estimate the wave speed and wave attenuation coefficient generated inside the pressure shaft during start-up and shut-down of pumps and turbines have been applied. The wave speed was assessed from the Fourier transformation spectrums while the attenuation coefficient was determined by computing the root mean square of the signal followed by an exponential regression fitting. Monitoring charts have been established based on the statistical quality control method. The control limits and the overall behavior of the pattern of future measured points will be used for on-line monitoring of the shaft. The magnitudes of the frequency response function of the system and the coherence of pressure signals have been also defined. In spite of many difficulties encountered during the in-situ measurements, the results show a stable monitoring scheme of the wave speed and wave attenuation coefficient. Longer acquisition data series are needed to consolidate the control limits. The response identification function of the pressure shaft was not very efficient in this special case because of the reflection of the major part of the wave by the junction located between the pressure and surge shafts of the plant.

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