Condition monitoring of wind turbines: Techniques and methods

Infrared thermography for condition monitoring – A review

As the demand for wind energy continues to grow at exponential rates, reducing operation and maintenance (OM) costs and improving reliability have become top priorities in wind turbine (WT) maintenance strategies. In addition to the development of more highly evolved WT designs intended to improve availability, the application of reliable and cost-effective condition-monitoring (CM) techniques offers an efficient approach to achieve this goal. This paper provides a general review and classification of wind turbine condition monitoring (WTCM) methods and techniques with a focus on trends and future challenges. After highlighting the relevant CM, diagnosis, and maintenance analysis, this work outlines the relationship between these concepts and related theories, and examines new trends and future challenges in the WTCM industry. Interesting insights from this research are used to point out strengths and weaknesses in today’s WTCM industry and define research priorities needed for the industry to meet the challenges in wind industry technological evolution and market growth.

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Wind Turbine Condition Monitoring: State-of-the-Art Review, New Trends, and Future Challenges

Deployment of larger scale wind turbine systems, particularly offshore, requires more organized operation and maintenance strategies to ensure systems are safe, profitable and cost-effective. Among existing maintenance strategies, reliability centred maintenance is regarded as best for offshore wind turbines, delivering corrective and proactive (i.e. preventive and predictive) maintenance techniques enabling wind turbines to achieve high availability and low cost of energy. Reliability centred maintenance analysis may demonstrate that an accurate and reliable condition monitoring system is one method to increase availability and decrease the cost of energy from wind. In recent years, efforts have been made to develop efficient and cost-effective condition monitoring techniques for wind turbines. A number of commercial wind turbine monitoring systems are available in the market, most based on existing techniques from other rotating machine industries. Other wind turbine condition monitoring reviews have been published but have not addressed the technical and commercial challenges, in particular, reliability and value for money. The purpose of this paper is to fill this gap and present the wind industry with a detailed analysis of the current practical challenges with existing wind turbine condition monitoring technology.

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Wind turbine condition monitoring: technical and commercial challenges

Clinical decision analysis

Ultrasonography is a valuable diagnostic tool widely used in medicine. During the last three decades, this non-invasive skin imaging method has been extended to dermatology. High-frequency ultrasonography with higher than 20MHz scanners is well-established for measuring tumour thickness and skin thickness when treating inflammatory skin diseases such as scleroderma or psoriasis. High-frequency ultrasonography has become extremely helpful for the preoperative assessment of skin melanoma. The correlation between ultrasonic and histological measurements of melanomas thickness is significantly similarly good using transducers of 20, 75 or 100MHz frequency (r range from 0.895 to 0.99) and better compared with transducers of 7.5MHz frequency (r=0.76). The preoperative sonographically estimated thickness of skin melanoma is sometimes overestimated, because of an underlying inflammatory infiltrate and other reasons. Assessment of skin melanoma thickness using transducers of 100MHz frequency has better agreement with histology, compared with ultrasonography with 20MHz transducers. However, the ultrasonic penetration depth is limited to 1.5mm in case of 100MHz. The newer ultrasonic techniques such as high-frequency ultrasonography and colour Doppler sonography could be used for assessment of the tumour vascularization and its metastatic potential. The wide variety of diagnostic information provided by high-frequency ultrasonography undoubtedly improves the management of oncological and inflammatory skin conditions and underlines its essential position in dermatological practice.

Principles of high-frequency ultrasonography for investigation of skin pathology

Wind power is a promising source of environmentally safe and renewable energy with a high potential. However, in order to fully exploit energy of wind power the construction elements of the wind turbine should be inspected periodically. Wind turbine blades are complicated objects for inspection because they have an arbitrary curved surface, are multi-layered, have variable thickness and are made from anisotropic materials.The presented study covers an overview of the techniques which are used or could be used for on site condition monitoring and effective NDT of wind turbine blades. Inspection methods based on vibration analysis, thermography, X-ray imaging, acoustic emission and ultrasound are reviewed. The objective of this study is review of different NDT techniques, which are used, or could be used for non-destructive testing of wind turbine blades, taking into account the complicated structure of the wind turbine blades as well as possibility to make non-destructive testing in harsh on-site conditions.

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The review of non-destructive testing techniques suitable for inspection of the wind turbine blades

The objective of this study was to adapt ultrasonic and radiographic techniques for the inspection of wind turbine blades and to compare the obtainedresults The measurements performed show that radiographic techniques are capable of reliably detecting a number of structural defects within the blade The adapted air-coupled ultrasonic technique, using Lamb waves, proved to be the most promising in terms of implementation as it only requires access to one side However, the novel combination of contact pulse-echo and immersion techniques using a moving container of water identified the shape and size of defects better Through comparisons of images obtained using both radiographic and ultrasonic techniques, different defect properties can be identified Hence, the best results are achieved when both techniques are combined together

NDT of wind turbine blades using adapted ultrasonic and radiographic techniques

In aerospace industry, one of the most important parts of gliders and motor-gliders is a lightweight longeron reinforcement made of carbon fibre reinforced plastics (CFRP) rods, known as Graphlite SM315 composite. During manufacturing, the rods as constructional elements are glued together in epoxy-filled matrix in order to build the arbitrary spar profile. The defects present in single rods such as breakage of fibres, multiple delaminations due to the lack of bonding between fibres and reduction in density affect essentially the strength and the fail-safety of the overall construction. The aim of the present work is to investigate the effects (transmission, reflection, scattering and mode conversion) of guided waves propagation along a square-shape CFRP rod in the case of contactless excitation/reception and interaction with region of multiple delaminations applying the numerical model and performing experiments. The square-shape CFRP composite rods possessing internal artificial delamination type defects have been investigated by numerical modelling and experiments employing the developed air-coupled technique (pitch-catch set-up) for cases of conventional transmission and advanced back-scattering configurations. Numerical predictions of guided wave interaction with a multiple delamination type defect in a CFRP composite rod have been made and the interaction mechanism explained. It is possible to conclude, that the actual sizes of the internal defects have been clearly detected using reception of the back-scattered waves over the edges of the defective regions only.

Ultrasonic air-coupled testing of square-shape CFRP composite rods by means of guided waves

One of the most important parts of gliders is a lightweight longeron reinforcement made of carbon fibre reinforced plastic (CFRP) rods. During manufacturing, in order to build the arbitrary spar profile, these small diameter (few millimetres) rods are glued together into epoxy filled matrix. Still, defects present in the rods, such as break of fibres, multiple delaminations due to lack of bonding and reduction in density affect construction strength markedly and are extremely complicated to eliminate. Therefore, appropriate non-destructive testing techniques intended for carbon fibre rods should be applied prior to gluing them together. The aim of the present paper is to analyse development possibilities of NDT technique based on application of ultrasonic guided waves and intended for CFRP rods that are used for aerospace applications and are defective with multiple delaminations. The regularities of ultrasonic guided wave propagation in a defective CFRP circular-shape rod with multiple delaminations have been investigated using 3D numerical simulations, finite difference and finite element models. The corresponding experiments have been conducted as well. Based on leaky wave suppression over a defective region due to the weak vertical component of particle displacements, the mechanism of guided wave interaction with the region of multiple delaminations is explained from the ultrasonic NDT viewpoint.

Renaldas Raišutis

Papers

Principles of high-frequency ultrasonography for investigation of skin pathology

The review of non-destructive testing techniques suitable for inspection of the wind turbine blades

NDT of wind turbine blades using adapted ultrasonic and radiographic techniques

Ultrasonic air-coupled testing of square-shape CFRP composite rods by means of guided waves

Application of ultrasonic guided waves for non-destructive testing of defective CFRP rods with multiple delaminations