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Abstract. Extending the operating zone of Francis turbines toward low load is a major stake to allow the optimization of the electrical grid. The dynamic phenomena encountered at low load are potential sources of pressure fluctuations, power instability and runner fatigue. Traditionally, the peak to peak value of pressure fluctuations is used to assess these risks. However, this estimator is not sufficient to analyse separately the various dynamic phenomena and their impact on the stability of the turbine. In this paper the recent Spatial Harmonic Decomposition (SHD) method is used to analyse the pressure fluctuations through more relevant indicators. The evolution of these indicators along a load variation is compared with the associated runner strain measured with on-board gauges. It is shown that the use of the Spatial Harmonic Decomposition is a powerful tool to evaluate the risks for the industrial turbine and thus improve its behaviour and its reliability.

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