Imbalance in Wind Turbine Rotors
Efficient operation of wind farms demands a long lifespan for each wind turbine. As many turbine defects can be related to system vibrations, the investigation of vibrations caused by rotor imbalances is receiving increasing attention. The increase in the size of new wind turbines leads to a more flexible structure, resulting in even larger vibrational amplitudes. Additionally, the operation of large wind farms requires careful defect monitoring. One of the common problems faced by wind turbines is imbalance in the main rotor, which can lead to excessive vibrations and mechanical damage. A well-balanced rotor will prevent premature failure and ensure a safe and economical operation of the wind turbine. Imbalances affect the drive train components, as well as the structural health of the turbine. Therefore, early detection and elimination of imbalances are of vital importance.
In this article, we will present the types of imbalances that occur in the main rotor, the symptoms of each one, and some necessary conditions for detecting the signature of main rotor imbalances.
Figure 1: Crack in a wind turbine blade.
There are two main causes of imbalance in wind turbine rotors: mass imbalance and aerodynamic imbalance.
Aerodynamic imbalance occurs when one or more of the blades have a different aerodynamic efficiency than the others. This can be caused by different aerodynamic profiles among the blades, erosion on the leading edge, or, in most cases, due to a deviation of the blade angle from its ideal position, also known as pitch misalignment. The figure below illustrates aerodynamic imbalance in the main rotor of a wind turbine.
The consequences of aerodynamic imbalance include fluctuations in the speed of the main rotor and an increase in loads supported by the main bearing and other components, which in turn result in higher vibrations in the axial direction at the fundamental components of the main rotor and blade passing frequency. Additionally, aerodynamic imbalance can lead to a reduction in energy production, as the rotor blades become less efficient in capturing wind energy. Consequently, continuous occurrence of aerodynamic imbalance can shorten the turbine's lifespan and reduce the Annual Energy Production (AEP).
The figure below shows a comparison between the spectra of two turbines, one of them with aerodynamic imbalance caused by pitch misalignment of the blades (on the left), and the other diagnosed as balanced (on the right). In this figure, several symptoms are evident, indicating the presence of aerodynamic imbalance.
· Predominance of vibration at the main shaft frequency (1x main shaft) in the axial direction compared to the radial direction. In other words, in the turbine with pitch misalignment, the vibration in the main rotor component in the axial direction is 4 times higher than in the radial direction, whereas this factor is less than 2 times in the balanced turbine.
Vibration of 40 mm/s in the axial direction of the turbine with pitch imbalance, while in the balanced turbine, it is only 15 mm/s.
Therefore, it is essential to ensure that the rotor blades are constructed and maintained with high quality and that the aerodynamic balance of the turbine is regularly monitored and adjusted.
One of the main causes of aerodynamic imbalance is pitch misalignment. Pitch misalignment refers to a difference between the angles of attack of the blades concerning the ideal attack position. During blade installation, deviations in the angle of attack may be introduced due to mechanical tolerances, human errors, etc. Additionally, issues in the pitch system (bearings, actuators, etc.) can also cause pitch misalignment. Some manufacturers recommend a maximum pitch misalignment between the blades of 0.3°; however, achieving these values in the industry may require turbine shutdown, resulting in production losses. By using continuous monitoring systems, it is possible to track the vibration of the main bearing in real-time and obtain diagnoses of aerodynamic imbalances.
On the other hand, mass imbalance results from non-uniform mass distributions caused, for example, by erosion, dirt or ice accumulation on the blades, fractures at the blade tips, among others. This type of imbalance elevates the vibration component at the rotor frequency in the radial direction and can affect the turbine's efficiency in capturing wind. The figure below illustrates mass imbalance in the main rotor of a wind turbine.
From the explanation above, it becomes clear that for monitoring imbalances in the main rotor, the use of two accelerometers is necessary, with one capturing vibration in the axial direction and the other in the radial direction. These accelerometers are commonly installed on the upwind main bearing, closest to the main rotor.
The conditions for capturing the signature of rotor imbalance are related to the main rotor's frequency. Considering turbines where the main rotor rotates at around 15 RPM, the fundamental component of the main rotor is at approximately 0.25 Hz. Therefore, two conditions need to be met to effectively capture the signature of the imbalance:
· The frequency response of the accelerometer must include the main rotor's frequency. Since this frequency is in the sub-hertz range, specific accelerometers with a frequency response that includes the main rotor's component are required. Additionally, the vibration magnitudes of the main rotor can be quite low, necessitating the use of accelerometers with higher sensitivity. Commonly, accelerometers with a sensitivity of 100 mV/g and a frequency response of 0.5 Hz to 10 kHz are used. However, for monitoring the main rotor, an accelerometer with a lower frequency response and higher sensitivity is required. An appropriate choice that meets both conditions is the accelerometer with a sensitivity of 500 mV/g and a frequency response of 0.17 Hz to 10 kHz.
· Furthermore, for diagnosing the signature of imbalances in the main rotor, the signal recording duration must be long enough to represent the main rotor's components in the spectrum. This typically requires a recording duration of at least 30 rotor revolutions, which corresponds to a recording duration in the order of minutes.
By fulfilling these conditions, it becomes possible to effectively capture the signature of rotor imbalance and ensure efficient monitoring of wind turbine performance. In this article, we have explored the types of imbalances in the main rotor of wind turbines, their main causes, and how to detect them. Finally, we have presented conditions for capturing the signature of imbalance. The use of a CMS (Condition Monitoring System) allows for monitoring vibrations in the main rotor of the turbine and identifying these types of imbalances. This enables timely maintenance and helps avoid unplanned shutdowns.
AQTech's CMS, OneBreeze, offers an advanced diagnostic matrix tool that supports the detection of major failure modes in wind turbines, including imbalances in the main rotor, as well as other failure modes affecting the rotor and main bearing region, such as main bearing and pitch bearing defects, misalignment in the gearbox coupling with the main shaft, among others. Using automated algorithms, this tool identifies frequencies associated with the failure mode of interest, generates notifications at different attention levels, and displays these occurrences to the user.
If you would like more information about failure modes in wind turbines or would like to know more about AQTech's OneBreeze, please feel free to contact us.