Operational modal analysis and wavelet transformation for damage identification in wind turbine blades

Abstract

The presented study demonstrates an application of a previously proposed modal and wavelet analysis-based damage identification method to a wind turbine blade. A trailing edge debonding was introduced to a SSP 34 m blade mounted on a test rig. Operational modal analysis (OMA) was conducted to obtain mode shapes for undamaged and damaged states of the blade. Subsequently, the mode shapes were analyzed with one-dimensional continuous wavelet transformations (CWTs) for damage identification. The basic idea of the method is that structural damage will introduce local mode shape irregularities which are captured in the CWT by significantly magnified transform coefficients, thus providing combined damage detection, localization, and size assessment. It was found that due to the nature of the proposed method, the value of the identification results highly depends on the number of employed measurement points. Since only a limited number of measurement points were utilized in the experiments, valid damage identification can only be obtained when employing high-frequency modes.

Damage detection in an operating Vestas V27 wind turbine blade by use of outlier analysis

“…However, under more realistic conditions, these methods become inapplicable as the direct changes of the aforementioned features typically will be concealed by environmental and operational effects. For instance, it is documented in [8], [9] how these effects in general can account for up to at least 5 % shifts in eigenfrequencies, which, as documented in [9], [10], cannot be expected to be exceeded by damage-induced eigenfrequency changes.…”

“…In this regard, it is noticed that the actuator excitation, as originally suggested in [14], serves to excite the high-frequency vibration interval. This is of general interest as studies have proved that the low-frequency content, which is typically excited in ambient vibrations, is insensitive towards blade damages [7], [10].…”

Vibration-based damage detection in a wind turbine using 1 year of data

“…The last damage scenario (D3) is related to blade damage. The majority of published studies refer the small sensitivity of the natural frequencies of the blade modes to small common damages . Even so, the ability of the system to detect blade damage was assessed.…”

Damage Sensing in Blades

“…Simple analyses using the eigenfrequencies are valid for controlled laboratory tests, but under more realistic conditions these methods are unable to detect damages, since the modal property variations caused by them are the same order as the ones created by the environmental effects and noise contamination (Salawu 1997). Therefore, sophisticated and robust methods, such as continuous wavelet transformation, have to be used along with modal analysis in order to be deployed as a SHM application (Ulriksen et al 2014). Figure 3.2 depicts a modal measurement setup.…”

“…The advantages of Modal Analysis are that it is a mature technology widely used for gearbox and bearing faults, feasible, well-proven and low cost. The disadvantages are small sensitivity (detection of relatively big damages), so there is a necessity to have a fine measurement density (more sensors) and the impossibility to install sensors close (Ulriksen et al 2014) to the tip due to the small section since they should be placed inside the blade for SHM applications.…”