On-board measurements at a 100 MW high-head Francis turbine

Modern sources of energy, like solar and wind, force hydropower machines to change their operational behaviour. Rapid changes in the availability of renewable energies lead to instabilities in the grid, which need to be stabilized by flexible machine operation (Lowys et al. [1]). This leads to a significant change in hydropower operations, e.g. to an increasing number of start-stop events (Seidel et al. [2], Robert et al. [3], Coutu and Chamberland-Lauzon [4]). The four ternary machine groups at the Grimsel 2 power plant have been designed under completely different circumstances in the late 1970's and are suffering from this change in operation. After only 35'000 hrs of operation all runners show fatigue cracks at the trailing edges close to the hub which needed to be repaired effortful. As a part of these repairs, the chamfer between the blade and the hub was increased from about 5 mm to 12 mm with local welding and heat treatment to reduce stresses. Apparently, this seems to work out fine but, since the source of the damage was not detected, this situation was not satisfying for the plant owner. The turbines were running at smooth conditions most of the time. Since deep part load could be eliminated as a source of those cracks, it was assumed that either start-up, shutdown or some unknown phenomena lead to the cracks. From recent literature (Gagnon et al. [5] & [6]), it is assumed that the start-up process can be responsible for the observed kind of cracks. To get more information, extensive measurements have been performed on the Group 2 of the power plant. The overall goal of this research is to advice the plant owner concerning the operation of the machines to increase the lifetime of the existing runners