A major cause of the dynamic problems found in wind turbines (WT) today is Rotor Imbalance (RI). Independent engineers state that up to 35% of installed WT have a rotor imbalance above the limit value. This causes increased vibrations, accelerated wear and lifetime consumption and at the same time loss of profit through increased O&M costs and stand still hours. First measurements on Offshore WT prove that RI is an important issue for Offshore WT's, especially Aerodynamic Imbalance (AI) caused by blade angle deviation. Initial costs for Offshore WT are much higher, working conditions and accessibility are more difficult, hence RI has a bigger impact on the WT's availability and O&M costs. Therefore, checking the WT for rotor imbalance should be part of the commissioning procedure and the inspections since imbalance increases during operation. Blade angle measurements for determination of AI are performed reliably using a specially developed laser measuring system since there is no solid ground for positioning a camera for blade angle photos. The mass imbalance (MI) can be determined by measurements of the tower-nacelle vibrations and frequency-selective evaluation, which is as well used for validation of the correct blade angle adjustment. Test mass mounting spots are required in the blades for easy and safe on-site calibration of the MI measurement. Since AI and MI superimpose in the measured vibration signal, a sophisticated balancing procedure requires a strict logic order of measurements and actions taken to achieve a significant reduction of RI and the vibrations.
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