Improved quality of rotor balancing measurements through criteria from the guideline VDI-3834-1:2015

Introduction

Independent field statistics presented on EWEA2013 revealing high shares of intolerable rotor imbalance and blade angle deviation at series wind turbines (WT) caused an intensified discussion in the wind industry. It is known from design simulations that e.g. a blade angle deviation of 1.5° increases the blade root damage sum by approx. 50%. There is also a strong impact of a wrong rotor parameterisation on fatigue loads. Further field measurement evaluation revealed that the RMS-value based method proposed in the guideline VDI 3834-1 for vibration assessment at wind turbines is insensitive even to severe rotor imbalance and blade angle deviation.
Therefore, the guideline’s committee decided to include an annex on in-situ rotor balancing at WT in the guideline’s revision VDI 3834-1:2015 (Bi-lingual version German/English). This provides basic knowledge and practical hints to obtain a high-quality measurement result despite the challenges in the field.

Approach

Criteria for in-situ rotor balancing at WT discussed in VDI 3834-1:2015 are presented. Their background is illustrated using field measurement results, that a non-vibration expert grasps the criteria’s benefit and the disadvantage emerging, if they are ignored by the applied method.

Main body of abstract

The international standard DIN ISO 21940-13:2012 dealing with large gas and steam turbines, ventilators, etc., states that any rotor which is finally assembled on site and where the balance state changes during operation, e.g. by erosion, requires in-situ balancing. According to WT design standards and guidelines, imbalance and blade angle devia-tion limits are design parameters for design fatigue load analysis. However, requirements for verification at the individual WT lack, which leads to the fact that more than 70% of series WT show intolerable mass imbalance, and/or blade angle deviation. Permanent vibration monitoring may serve for indicating that a WT is affected. However, exact diagnosis of the root cause and the amount of imbalance needs mobile in-situ measurements including calibration.
The main criterion is safety of the method since there is staff on site. Excess vibration has to be prevented - but may be caused by test speeds too close to the tower’s resonance or wrong balancing recommendations. Fixing test and permanent balancing masses has to be safe.
Next, the measurement procedure has to consider the total system be-haviour and all potential root causes for increased vibration to act in the right logical order. Aerodynamic issues and temporal imbalances, e.g. from blocked drain holes, have to be eliminated before mass balancing is reasonable.
Low measurement uncertainty requires suitable measurement systems. Only a few acceleration sensor types are suitable for the rotational frequencies below 0.5 Hz and very small target amplitudes below 0.5 mg (~ 5 mm/s²).
The correct individual limit values for the WT type have to be applied. Some require the high balance quality of a hard disk. The large rotor mass requires large calibration masses (100+ kg).
For a statistically safe result, long time runs (30+ min) are required. Evaluation by order analysis is imperative due to rotor speed varying with the changing wind. Else, result falsification of more than 30% may occur, and more test runs are necessary.

Conclusion

Available WT rotor balancing methods and procedures show a different level of complexity. The criteria mentioned in VDI 3834-1:2015 help judging the overall suitability and benefit of a method. The criteria are as well useful to develop a life-cycle-oriented approach for rotor balancing, which reduces lifetime consumption, stand still and overall O&M costs.


Learning objectives
Learn the relevance of the rotor design parameters, mass imbalance and blade angle deviation limit, for WT design fatigue loads.
Understand the challenges for mobile measurements used for rotor parameter deviation diagnostics.
Understand how to use criteria from VDI3834-1 to judge the quality and overall benefit of different measurement methods.