Wind turbines have shown a maximization trend. However, most of the wind turbine blades operate in areas with a relatively poor natural environment. The stability, safety, and reliability of blade operation are facing many challenges. Therefore, it is of great significance to monitor the structural health of wind turbine blades to avoid the failure of wind turbine outages and reduce maintenance costs.
1. Wind Turbine Blade Fault Diagnosis Method
2. Non-Destructive Techniques
NDTs can conduct SHM on wind turbine blades to avoid serious accidents and ensure the safe operation of wind turbines. In addition, NDTs can also determine the cause of the damage. Some detection methods can detect the location and size of the blade damage to use for later maintenance and repair
[40][4]. At present, the non-destructive testing methods of wind turbine blades mainly include strain measurement, acoustic emission, ultrasonic, vibration, thermal imaging, machine vision, etc. Although these detection methods tend to be perfect and mature, few combine multiple detection methods for detection
[41][5]. Muñoz et al.
[42][6] believe that an NDT is applied to SHM systems to detect the internal performance of the material structure, which can reduce maintenance costs and prolong the service life of wind turbines. Gholizadeh et al.
[43][7] classified NDTs into contact and non-contact. This section focuses on the principle, working methods, advantages, and disadvantages of damage detection methods.
2.1. Strain Detection Method
The strain detection method can be applied to the monitoring of both onshore and offshore wind turbine blades. In the future, further exploration is still needed to reduce the development cost and improve detection accuracy.
2.2. Acoustic Emission Detection Method
The acoustic emission detection method has a good effect on crack damage detection and can also locate internal structural damage. However, there is often noise interference in the process of signal acquisition; eliminating noise interference will also increase the cost of the detection system, and it requires a data acquisition system with a high sampling frequency
[48,49][12][13].
2.3. Ultrasonic Testing Method
The ultrasonic detection method can continuously monitor the internal and surface of wind turbine blades. However, ultrasonic testing requires a long time to collect signals, and the signal data processing is also complex, which leads to the delay of damage judgment
[62][25]. Therefore, future research on artificial intelligence algorithms can improve the processing capacity of data.
2.4. Thermal Imaging Detection Method
The thermal imaging detection method is mainly applied to detect the change of thermodynamic properties of wind turbine blades by scanning the surface of that. When the micro-damage fault occurs, the temperature anomaly will occur, which can be utilized to detect and judge the fault
[57][20]. This technology requires accurate image processing. In real applications, it is difficult to eliminate the influence of blade damage on temperature and other factors
[57][20], making ambient temperature interference the key to accurately identifying damage. Doroshtnasir et al.
[63][26] used thermal imaging technology to carry out nondestructive testing on long-distance wind turbine blades and calculated the differential temperature of blades to eliminate signal interference reflection, to ensure the accuracy of thermal imaging technology in blade damage diagnosis. It concluded that the temperature difference near the hub is large, and there is the largest possibility of damage. Thermal imaging detection technology can identify the fault of wind turbine blades and extract the damage characteristics. Hwang et al.
[64][27] proposed thermal imaging technology using the continuous line later to visualize the damage of wind turbine blades under rotating conditions and extract the characteristics of damage. Avdelidis et al.
[65][28] applied infrared thermal imaging technology to wind turbine blade damage detection and summarized the advantages and disadvantages of this technology.
The thermal imaging detection method can detect the internal structure of the wind turbine blade without contact. However, the temperature change caused by damage is delayed, and it is easy for the environmental temperature to cause interference in the detection process. In future research, the influence of environmental temperature has to be reduced to improve the reliability and accuracy of this method.
2.5. Machine Vision Detection
At present, the research on the SHM of wind turbine blades based on the machine vision detection method is still in its infancy; the machine vision technology will have more extensive applications in the future. Although machine vision detection accuracy highly depends on image processing and data acquisition, its advantages are still obvious. The staff can remotely control the machine equipment to detect the wind turbine blades, which can improve the detection efficiency and protect their safety
[75,76][38][39]. In the future, the combination of the machine vision detection method and big data can realize earlier detection of the occurrence of damage, making it an important part of SHM.
3. Fault Diagnosis Method Based on Operation Data
In recent years, with the rapid development of artificial intelligence algorithms and big data analysis, artificial intelligence can imitate the learning skills of the human brain. At the same time, it combined with data analysis is widely used. The application of intelligent algorithms such as a neural network in the fault diagnosis of wind turbine blades has been well tested.
4. Fault Diagnosis Based on Vibration Signal
The fault diagnosis of wind turbine blades based on vibration signal is mainly through the selection of damage index and modal parameters. In the process of signal acquisition, it is necessary to eliminate the interference of environmental noise on the signal. The interference of the signal will reduce the accuracy of the vibration signal and the error of the modal parameters. Therefore, it is necessary to reduce the influence of the environment in the study of vibration signal damage identification.
The damage can be effectively identified by comparing the modal parameters before and after it occurs. Emilio Di Lorenzo et al.
[88][46] installed accelerometers on wind turbine blades to collect vibration data. By comparing the modal parameters before and after the buckling test, the occurrence of damage can be successfully predicted. In addition, the establishment of the finite element model can also be used to analyze the structural damage of the blade. Moradi et al.
[89][47] firstly installed intelligent sensors on wind turbine blades for experiments to obtain strain and vibration data and then simulated the structural state, and after the blade damage by finite element simulation, which can comprehensively detect the blade damage, this method can achieve a reliable SHM system. To exclude the influence of environmental noise, Abouhnik et al.
[90][48] used the empirical mode decomposition method to divide the vibration signal into basic components and built a model in the finite element software ANSYS to simulate the vibration of the wind turbine with three blades. At the same time, the crack damage was set on the wind turbine blade, and the vibration characteristics of the blade at different speeds were tested. By comparing the simulation and experimental results, the method can identify the location and extent of the blade damage. Gómez et al.
[91][49] proposed a supervised statistical method to solve the interference of uncertainty in the vibration signal detection of wind turbine blade damage under different environments, and they developed three specific methods to improve the accuracy of damage detection. Furthermore, Wang et al.
[92][50] proposed a finite element method combined with dynamic analysis (modal analysis and response analysis) to obtain modal shape difference curvature. The numerical results show that the method can detect the blade damage location and improve detection accuracy.