Dynamic load on a Francis turbine runner from simulations based on measurements

The frequency of the Nordic power grid has become more volatile during recent years. This gives rise to two effects in synchronous machines. Firstly, as grid frequency changes the rotational speed of synchronous machines must change likewise. Secondly, the Nordic grid uses speed droop operation extensively as the primary governing; hence the power produced is a function of the grid frequency. These two coupled effects will lead to the runner and axle on synchronous machines having to cope with a varying level of torque. Even if the unit is supposedly operating at steady state via a fixed set point for the production, the influence of the varying grid frequency is that the torque in not steady at all. Recent years' new high head Francis runners in Norway have shown a tendency towards experiencing fatigue to a greater extent than what seem to be the case for new runners decades ago. Leading to this paper, measurements have been made of the rotational speed; generator power; main servo motor position and grid frequency at a Francis turbine unit. Based on these measurements simulations that include the hydraulic domain have then been performed. From these simulation results a property is constructed which is intended as a qualitative measure of the material stresses induced in the rotating masses of the unit, and is representative of the dynamical loads on the material of the rotating masses. The work is a part of a longer term goal, namely identifying the stress oscillations in a Francis turbine runner operating at speed of rotation oscillating because of grid frequency variations.