Abstract
The massive penetration of the electrical network by renewable energy sources, such as wind and solar, pushes the operators to extend hydropower plant units operating range to meet the transmission system operator requirements. However, in off-design operating conditions, flow instabilities are developing in Francis turbines, inducing cavitation, pressure pulsations and potentially resonance that can threaten the stability of the whole system. Reduced scale model testing is commonly performed to assess the hydraulic behaviour of the machine for industrial projects. However, it is not possible to directly transpose pressure pulsations and resonance conditions from model to prototype since the characteristics of the hydraulic circuits are different from model to prototype. In this paper, a methodology developed in the framework of the HYPERBOLE European research project for predicting the eigenfrequencies of hydropower plant units operating in off-design conditions is introduced. It is based on reduced scale model testing and proper one-dimensional modelling of the hydraulic circuits, including the draft tube cavitation flow, at both the model and prototype scales. The hydro-acoustic parameters in the draft tube are identified at the model scale for a wide number of operating conditions and, then, transposed to the full-scale machine, together with the precession frequency for part load conditions. This enables the prediction of the eigenfrequencies and resonance conditions of the full-scale generating unit.
Highlights
Hydraulic turbines operating in off-design conditions experience flow instabilities in their draft tube, promoting the onset of cavitation and the propagation of pressure fluctuations in the whole hydraulic circuit
In Francis turbines draft tube at part load conditions, i.e. with a discharge value lower than the value at the Best Efficiency Point (BEP), a cavitation vortex rope is observed at the runner outlet
The first eigenfrequency of a hydropower plant unit with a conventional Francis turbine operating at part load and full load, as well as the precession frequency of the vortex rope at part load, are predicted on the complete operating range based on a new approach
Summary
Hydraulic turbines operating in off-design conditions experience flow instabilities in their draft tube, promoting the onset of cavitation and the propagation of pressure fluctuations in the whole hydraulic circuit. Alligné et al [12] predicted the first eigenfrequency of a 444 MW full-scale generating unit at two different part load operating points They transposed the hydro-acoustic parameters of the draft tube cavitation flow identified on the reduced scale model by Landry et al [13]. Two additional sensors p5 and p6 are placed in the upstream pipes of the machine to evaluate the hydroacoustic response of the test rig At both part and full loads, pressure fluctuations measurements are performed within a wide range of discharge factor values for 4 different values of speed factor while the net head H and the Froude number F = ( H / D )0.5 are kept constant.
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