Non-Destructive Techniques for the Condition and Structural Health Monitoring of Wind Turbines: A Literature Review of the Last 20 Years

A complete surveillance strategy for wind turbines requires both the condition monitoring (CM) of their mechanical components and the structural health monitoring (SHM) of their load-bearing structural elements (foundations, tower, and blades). Therefore, it spans both the civil and mechanical engineering fields. Several traditional and advanced non-destructive techniques (NDTs) have been proposed for both areas of application throughout the last years. These include visual inspection (VI), acoustic emissions (AEs), ultrasonic testing (UT), infrared thermography (IRT), radiographic testing (RT), electromagnetic testing (ET), oil monitoring, and many other methods. These NDTs can be performed by human personnel, robots, or unmanned aerial vehicles (UAVs); they can also be applied both for isolated wind turbines or systematically for whole onshore or offshore wind farms. These non-destructive approaches have been extensively reviewed here; more than 300 scientific articles, technical reports, and other documents are included in this review, encompassing all the main aspects of these survey strategies. Particular attention was dedicated to the latest developments in the last two decades (2000–2021). Highly influential research works, which received major attention from the scientific community, are highlighted and commented upon. Furthermore, for each strategy, a selection of relevant applications is reported by way of example, including newer and less developed strategies as well

Development of Novel Ultrasonic Monitoring Techniques for Improving the Reliability of Wind Turbine Gearboxes

In this work, an ultrasonic method was developed to detect lubricant on rolling bearing raceways and also the load imparted by each of the rolling elements onto the raceways. It has been implemented both in the lab and an operating wind turbine. Sensors were mounted outside the raceway and sound waves reflected from the inner face. The ratio of reflected ultrasound to transmitted (Reflection Coefficient, R) was used to infer the raceway interface condition whilst the change in time of flight of ultrasonic waves was used to deduce the deflection of the raceways and subsequently roller load. Features observed in the field measurements include variation of roller load across different rollers within a complement, variation of roller load and lubrication along with turbine operation and various bearing lubrication condition

Wind turbine drivetrains:State-of-the-art technologies and future development trends

This paper presents the state-of-the-art technologies and development trends of wind turbine drivetrains – the system that converts kinetic energy of the wind to electrical energy – in different stages of their life cycle: design, manufacturing, installation, operation, lifetime extension, decommissioning and recycling. Offshore development and digitalization are also a focal point in this study. Drivetrain in this context includes the whole power conversion system: main bearing, shafts, gearbox, generator and power converter. The main aim of this article is to review the drivetrain technology development as well as to identify future challenges and research gaps. The main challenges in drivetrain research identified in this paper include drivetrain dynamic responses in large or floating turbines, aerodynamic and farm control effects, use of rare-earth material in generators, improving reliability through prognostics, and use of advances in digitalization. These challenges illustrate the multidisciplinary aspect of wind turbine drivetrains, which emphasizes the need for more interdisciplinary research and collaboration

Electrostatic Sensors – Their Principles and Applications

Over the past three decades electrostatic sensors have been proposed, developed and utilised for the continuous monitoring and measurement of a range of industrial processes, mechanical systems and clinical environments. Electrostatic sensors enjoy simplicity in structure, cost-effectiveness and suitability for a wide range of installation conditions. They either provide unique solutions to some measurement challenges or offer more cost-effective options to the more established sensors such as those based on acoustic, capacitive, optical and electromagnetic principles. The established or potential applications of electrostatic sensors appear wide ranging, but the underlining sensing principle and resultant system characteristics are very similar. This paper presents a comprehensive review of the electrostatic sensors and sensing systems that have been developed for the measurement and monitoring of a range of process variables and conditions. These include the flow measurement of pneumatically conveyed solids, measurement of particulate emissions, monitoring of fluidised beds, on-line particle sizing, burner flame monitoring, speed and radial vibration measurement of mechanical systems, and condition monitoring of power transmission belts, mechanical wear, and human activities. The fundamental sensing principles together with the advantages and limitations of electrostatic sensors for a given area of applications are also introduced. The technology readiness level for each area of applications is identified and commented. Trends and future development of electrostatic sensors, their signal conditioning electronics, signal processing methods as well as possible new applications are also discussed

Development of a Novel Bearing Concept for Improved Wind Turbine Gearbox Reliability

The wind industry has experienced rapid growth in recent years as a result of ever increasing concerns over fossil fuel power generation and the associated impact this has upon climate change. Wind turbine technology has however displayed relatively poor reliability from the outset, largely due to the highly variable nature of the wind, which subsequently places system components under extremely harsh loading conditions. Such reliability issues are becoming increasingly problematic for the wind turbine operator, as wind farms – and indeed the turbines themselves – continue to be up-scaled thus increasing the complexity and cost of maintenance, particularly in off-shore environments. Gearbox failures have been found to be responsible for a large proportion of wind turbine downtime, and this is very often linked to the failure of bearings at certain locations within the gearbox. One such critical location is the epicyclic stage, where planetary support bearings are often found to exhibit damage to a localised portion of their inner raceways, corresponding to the location of the applied load. A concept has been proposed to extend the life of such bearings by periodically rotating the normally static inner raceway so as to avoid the build of damage to one localised region. The concept has been termed the MultiLife(TM) mechanism and a key aim of this thesis was to establish proof-of-concept for this system. Initial analytical work was performed, through which it was identified that a five-fold enhancement to bearing life would be theoretically achievable. The concept was subsequently validated experimentally through testing on a bespoke test platform. In addition to this, the ultrasound technique has been explored as a means to provide early warning of bearing failure and thus support the operation of the MultiLife system. This technique has previously been proven as a method to measure the thickness of the oil films that form within bearing contacts. It was identified for this study that oil films would be too small to measure due to the low viscosity lubricants and high bearing loads utilised to accelerate bearing failures. Nonetheless, a general reduction in the amount of ultrasonic energy reflected from the rolling contact was observed during bearing failure, which was linked to a breakdown of the lubricating oil layer due to degradation of the rolling surface. In this case the ultrasound technique did not provide any advanced warning of bearing failure over more classical condition monitoring techniques; however, it was identified that the use of higher resolution ultrasound sensor arrays would enhance the capabilities considerably. Currently, very few monitoring techniques are applied to wind turbine gearbox bearings, and as a further part of this work a condition monitoring system has been installed within an operational 600kW wind turbine. A high speed shaft bearing has been instrumented with a variety of sensors, including ultrasound instrumentation, to assess the potential of such a system in providing early warning of impending gearbox failures

A review of physics-based models in prognostics: application to gears and bearings of rotating machinery

Health condition monitoring for rotating machinery has been developed for many years due to its potential to reduce the cost of the maintenance operations and increase availability. Covering aspects include sensors, signal processing, health assessment and decision-making. This article focuses on prognostics based on physics-based models. While the majority of the research in health condition monitoring focuses on data-driven techniques, physics-based techniques are particularly important if accuracy is a critical factor and testing is restricted. Moreover, the benefits of both approaches can be combined when data-driven and physics-based techniques are integrated. This article reviews the concept of physics-based models for prognostics. An overview of common failure modes of rotating machinery is provided along with the most relevant degradation mechanisms. The models available to represent these degradation mechanisms and their application for prognostics are discussed. Models that have not been applied to health condition monitoring, for example, wear due to metal–metal contact in hydrodynamic bearings, are also included due to its potential for health condition monitoring. The main contribution of this article is the identification of potential physics-based models for prognostics in rotating machinery

Gear wear process monitoring using a sideband estimator based on modulation signal bispectrum

As one of the most common gear failure modes, tooth wear can produce nonlinear modulation sidebands in the vibration frequency spectrum. However, limited research has been reported in monitoring the gear wear based on vibration due to the lack of tools which can effectively extract the small sidebands. In order to accurately monitor gear wear progression in a timely fashion, this paper presents a gear wear condition monitoring approach based on vibration signal analysis using the modulation signal bispectrum-based sideband estimator (MSB-SE) method. The vibration signals are collected using a run-to-failure test of gearbox under an accelerated test process. MSB analysis was performed on the vibration signals to extract the sideband information. Using a combination of the peak value of MSB-SE and the coherence of MSB-SE, the overall information of gear transmission system can be obtained. Based on the amplitude of MSB-SE peaks, a dimensionless indicator is proposed to assess the effects of gear tooth wear. The results demonstrated that the proposed indicator can be used to accurately and reliably monitor gear tooth wear and evaluate the wear severity