Rotor Vibration Analysis
Determination of mass and aerodynamic imbalance
A well-balanced rotor, as well as accurately adjusted blade angles are fundamental requirements for the optimal operation of wind turbines, through their positive impact on energy and life cycle costs. Early and periodic imbalance checks and corresponding technical counter measures help to optimise availability and yield. This also effectively prevents a reduction of turbine lifetime caused by rotor imbalance.
Expenses for periodic rotor balancing usually amount to a mere fraction of the costs associated with the repair and yield loss caused by rotor imbalance. There are two different primary causes of rotor imbalance, namely mass imbalance and aerodynamic imbalance.
• Mass Imbalance is caused by uneven mass distribution along the blades.
• Aerodynamic Imbalance is a consequence of differing
aerodynamic pro-perties between rotor blades, often caused by blade
Often, rotors exhibit both types of imbalance in combination.
Statistics from long-standing imbalance measurements on more than
2000 turbines show that on average 4 out of 5 turbines are affected
by rotor imbalance beyond acceptable limits
Mass imbalance as well as aerodynamic imbalance cause an unwanted increase in turbine vibrations and thus are one of the main causes of dynamic problems of today's wind turbines. With years of experience and extensive research BerlinWind can reliably detect both types of imbalance by analysing their vibrational signature and initiating appropriate technical counter measures.
For state-of-the-art imbalance assessment our engineers have years of experience measuring not only axial and lateral nacelle vibrations, but also tower top torsional vibration. This allows aerodynamic imbalance to be reliably identified and eliminated and hence, prevents the falsification of mass imbalance measurements as a consequence of the superposition of aerodynamic effects.
Figure left: Mass imbalance causes nacelle vibrations in axial and lateral directions. Thanks to a sophisticated 3-point sensor system, aerodynamic imbalance can be reliably identified in the complex 'mixture' of vibrations | Figure right: Impact of successively taken measures for vibration reduction of a wind turbine on level of axial, lateral and torsion vibrations.
After correcting the aerodynmic imbalance by adjusting blade angles,
which is usually done while our engineers are on-site to perform
vibration-based validation for quality control, the mass imbalance
is determined. For this purpose, several defined states of mass
imbalance are generated by attaching test weights to the rotor. The
vibrational signature for each state is recorded while the turbine
is in operation.
Given the results of several measurement runs for different imbalance levels, the magnitude and position of the mass imbalance are determined and validated. For quality control, a final vibration measurement is recommended to validate the final counter weights mounted on the rotor. Based on the measurement results, BerlinWind gives specific and comprehensive recommendations for appropriate technical measures to ensure safe, profitable and low-wear operation of the turbine.
Our in-house developed measurement systems and procedures combined with our experience conducting measurements on more than 110 different wind turbine types, ranging from kW, up to and exceeding 10 MW, form the basis upon which suitable measurement procedures for reliable assessment of new turbine types with complex control systems can be developed.
Since 2013 BerlinWind has been developing and publishing quality criteria for reliable rotor balancing, e.g. based on the international standard on in-situ balancing of large rotors (DIN ISO 21940-13:2013). We stand for high-quality vibration measurements and high diagnostic safety in the challenging field of turbine rotor balancing and participate in related committees, e.g. for the guideline VDI 3834 on vibration measurement of wind turbines, in which part 1-2015 containes an annex on rotor balancng.