Radial dampers impact on shaft vibration at resonance

Abstract

During commissioning of a refurbished mid-size hydropower unit in Sweden, high shaft vibration occurred due to resonances. The refurbishment included a runner upgrade from 38 MW to 45 MW, a new stator and refurbished rotor poles. One of the requirements was that the dynamic behaviour of the machine should not be affected in a negative way despite a reduction in runner mass. In spite of the requirements, severe resonance problems became apparent during re-commissioning of the unit. Extended commissioning measurements were conducted to identify the cause of the high shaft vibrations. It was found that the resonance problem occurred when the machine reached 90% of its synchronous speed and the vibration levels increased with increased rotor speed; i.e., it was not possible to test the protection equipment for runaway. The runner was identified as the excitation source with an increased excitation frequency in the range of 5.4–7.4 Hz. Linear rotodynamic analysis of the rotor system was performed by the turbine and generator manufacturer as well by the owner of the unit. The analysis showed that the system has several natural frequencies with low damping in the 5.4–7.4 Hz range. This can cause instability problems if the excitation frequency coincides with the eigenfrequencies of the rotating system. It was not possible to eliminate the excitation force from the turbine, and, due to the bandwidth of the excitation force, it was not likely that enough change in frequency for critical eigenmodes could be achieved by adding mass and stiffness to the rotating and support structures. The measure most likely to have a positive effect on the resonance problem was to add damping to the system. Extended numerical simulations were performed to investigate the effect radial dampers would have on the unit if they were mounted between the upper bracket and the concrete structure. The simulations were performed in software developed for transient simulation of vertical machines with non-linear hydrodynamic guide bearings and non-linear dampers. The radial dampers were found to reduce response in the range of 5.4–7.4 Hz, in accordance with the simulations. Radial dampers were installed in the machine, and it was possible to finalize commissioning with acceptable vibration levels.