Reducing Vibration Issues at Small Vertical-Axis Wind Turbines on Buildings (Grunwald et al. 2013)

Small wind turbines (SWT) are rotating machines, therefore the rotor speed and its harmonics produce excitation frequencies leading to resonance issues if natural frequencies of the SWT, its sub structure and/or the building are in the operational frequency range. This is often not addressed during design and siting of SWT because of the complexity of rotor and structural dynamics and/or because natural frequencies of the building are unknown. The unexpected resonance issues threaten the SWT project by noise and vibration, accelerated wear and damages and production loss from standstill. At five recently installed vertical-axis SWT installed on buildings in Berlin, vibration measurements with 17 sensors and a root cause analysis was carried out to address resonance issues and building vibration. For vertical-axis SWT, the dynamics are far more complex than for horizontal-axis SWT, since more harmonics occur due to upwind and downwind blade-mast interaction. Therefore, resonance at SWT components and/or the building may occur even at low wind idling. It turned out that long braces, used to avoid periodic maintenance efforts for guy wires pre-tension control, had resonance frequencies in the same range as the steel framework building. Dampers installed to prevent generator vibration propagation into the tower formed an additional vibration system, and the damper’s natural frequencies were excited by the harmonics. The heavy vibrations wore down the dampers fast. Rotor mass imbalances which had been several times the limit value were successfully reduced by advanced two-plane rotor balancing despite the strong rotor speed fluctuations.

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