Reduction of vibration problems in small vertical-axis wind turbines on buildings (Grunwald et al. 2013)

Small wind turbines (SWT) are rotating machines, so the rotor speed and its harmonics generate excitation frequencies that cause resonance problems when the natural frequencies of the SWT, its substructure, and/or the building are in the operating frequency range. Due to the complexity of the rotor and structural dynamics and/or because the natural frequencies of the building are not known, this is often not taken into account in SWT’s planning and site selection. The unexpected resonance problems threaten the SWT project through noise and vibration, accelerated wear and damage, and production downtime due to downtime. Five recently installed vertical-axis SWTs installed on buildings in Berlin have had vibration measurements with 17 sensors and root cause analysis to investigate resonance issues and building vibration. In vertically oriented SWTs, the dynamics are far more complex than in horizontally oriented SWTs, as more harmonics occur due to the interaction between the wing and the mast in the upwind and downdraft. Therefore, even with low wind volumes, resonances can occur on SWT components and/or the building. It turned out that long guying used to avoid regular maintenance to control the prestressing of the guy ropes had resonance frequencies in the same range as the steel truss building. The dampers, which were installed to prevent the propagation of the generator vibrations into the tower, formed an additional vibration system, and the natural frequencies of the dampers were excited by the harmonics. The strong vibrations quickly wore out the dampers. Rotor mass imbalances, which were many times the limit value, were successfully reduced by advanced two-plane rotor balancing despite the strong rotor speed.

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