Abstract. Francis turbines operate in many cases out of its best efficiency point, in order to regulate their output power according to the instantaneous energy demand of the grid. Therefore, it is of paramount importance to analyse and determine the unstable operating points for these kind of units. In the framework of the HYPERBOLE project (FP7-ENERGY-2013-1; Project number 608532) a large Francis unit was investigated numerically, experimentally in a reduced scale model and also experimentally and numerically in the real prototype. This paper shows the unstable operating points identified during the experimental tests on the real Francis unit and the analysis of the main characteristics of these instabilities. Finally, it is shown that similar phenomena have been identified on previous research in the LMH (Laboratory for Hydraulic Machines, Lausanne) with the reduced scale model.
IntroductionOne of the advantages of using large hydraulic turbines for power generation is that they output power can be adjusted depending on the instantaneous energy demand of the grid. In order to regulate the output power, these units have to operate out of its design conditions. Particularly, many Francis units, which are nowadays the most installed hydraulic turbines in power plants, have to work out of its best efficiency point for long periods of time. Nevertheless, some operating points can be unstable, limiting the effective operating range of the unit. For this reason, it is of paramount importance to accurately predict (during the design phase of the unit) and to determine (during the operation of the installed unit) these particular points.Inside the HYPERBOLE project (FP7-ENERGY-2013-1; Project number 608532) a large Francis unit has been investigated by means of numerical models [1, 2] and experimentally on both the reduced scale model of this unit [3][4][5][6] and the real prototype [7].The real unit has a rated power of 444MW and it is located on the British Columbia in Canada. The tests on the reduced scale model (complete model with scale 1/16) were performed in the Laboratory of Hydraulic Machines (EPFL) in Switzerland.The unstable points of this unit were firstly predicted based on the numerical and experimental studies made on the reduced scale model. As a final step of the HYPERBOLE project, a measurement campaign on the real unit has been performed. One of the goals of this campaign was to determine and to investigate the unstable operating points of the real prototype of the investigated unit.In this paper, the experimental data obtained during the prototype tests are analyzed. The main focus of the analysis is firstly to determine and to analyze the main unstable operating points of the unit. Finally, it is observed that similar phenomena was found in the tests made on the reduced scale model.