Current commercially available main bearing designs use rolling-element bearings . Spherical roller bearings are utilized in a significant proportion of currently operational main bearings, with usage of this technology likely to continue (to a greater or lesser extent) in sub-5 MW machines. Tapered roller bearing designs are now also common at these power levels. For larger wind turbines (≥ 5 MW), however, the industry has very much converged on tapered roller main bearing technology . Because of the large applied nontorque loads, the resulting bearing designs are likewise large in diameter, limited in size only by manufacturing and transportation restrictions. Continued use of rolling-element bearings is likely because it is a familiar technology, albeit with design trends moving outside the envelope of prior experience. A central driver behind the move to large-diameter rolling-element bearing arrangements is the need for cost-effective rotor support solutions. Other existing bearing technologies – such as hydrostatic, air and magnetic bearings – tend to require very rigid support structures or are limited to smaller diameters than would be required by modern wind turbines, although hybrid solutions combining different technologies have been proposed . Given the continued increases in main bearing diameter, understanding the effects of deflections in large-diameter rolling-element bearings is essential, and the current practice of assessing bearing design life through only the conventional calculation methods in the International Organization for Standardization (ISO) standards 76 and 281 and technical specification (TS) 16281 might be insufficient with respect to the resulting service life observed in operation.

Main bearings have been shown to experience repeating, large-scale fluctuations in load, even during normal operation . These fluctuations likely increase the risk of other damaging mechanisms (such as roller skidding, surface fatigue, wear and abrasion) not accounted for in fatigue-life calculations. As such, the analysis of the operating conditions of these components has further indicated that current life-assessment standards might be insufficient. Main bearing failure rates of up to 30 % during a 20-year design life have also been reported . Further work is therefore needed to identify principal drivers of main bearing failures, allowing for the development of appropriate design standards and best practice specific to this component, which, in turn, will lead to improvements in reliability. investigated one possible driver of main bearing failures, that of axial motion between rollers and raceways. It has previously been hypothesized that such motions may compromise the lubricant film which separates bearing internal surfaces, leading to increased levels of friction and wear. However, from analysis of both field measurements and analytical model outputs it was concluded that axial velocities are too small to have any significant influence on lubricant film formation .

Modern offshore wind plants are high-value assets, and there is an increasing interest in longer design life and lifetime extension. Turbine size and drivetrain arrangement can, however, result in main bearing replacement becoming more difficult and expensive, generally requiring removal of the rotor. Consequently, main bearings will increasingly be regarded as part of the load-carrying structure, with cost implications of failure more severe as a result. A further ramification of increased levels of integration is that main bearing operational requirements become linked to those of other components. For example, in addition to supporting the turbine rotor, some direct-drive configurations require the main bearing to also support the generator rotor while maintaining an appropriate generator air gap. Coupled approaches to the modeling and assessment of wind turbine drivetrain systems will therefore become increasingly important.

Novel main bearing design concepts are also being developed and tested. propose the use of asymmetric spherical roller bearings to improve main bearing internal load sharing during operation. Finite-element modeling was used to compare a standard and asymmetrical design, with results indicating significant improvements for the latter with respect to both main bearing fatigue life and isolation of the gearbox from transferred axial loads. Plain bearing (equivalently, journal bearing) technology is also being considered in this space. More specifically, a segmented plain bearing with conical sliding surfaces is being developed as a main bearing solution for wind turbines . Sliding segments are connected to the housing via doubly flexible supports to maintain their alignment with the tilting shaft and prevent edge loading. A major advantage of this design is that the sliding segments are individually replaceable uptower without requiring drivetrain disassembly. Other main bearing concepts utilizing this same technology have also been explored .

With respect to materials, chill-cast nodular cast iron (GJS) has been investigated as a possible alternative to forged steel for manufacture of the rotor-shaft and main bearing seat . Chill-cast GJS enables lightweight construction through freedom of design while also providing appropriately robust mechanical properties. Lighter and smaller hollow rotor-shaft designs may become feasible as a result. This, in turn, would have implications for main bearing design and selection. Cost reductions may therefore be possible through design optimization of GJS components, but resulting shaft–bearing systems can have reduced stiffnesses, which may lead to inner ring creep or fretting fatigue . Work in this area is ongoing.

Wind turbine gearboxes continue to increase in size (up to 3 m in diameter) and power (up to 15 MW) . With multistage gearboxes using four or more planets per stage, torque densities of 200 N m kg-1 and speed-increasing ratios up to 200 are now available . These increasingly flexible systems require detailed modeling to understand the effect of deformations and dynamics on internal loading . To achieve further cost reductions through economies of scale, modular gearbox designs have been introduced . Gearboxes are designed for a minimum of a 20-year life, as specified in the IEC 61400-4 and American Gear Manufacturers Association (AGMA) 6006 gearbox design standards. Provisions for uptower service or replacement of gearbox components are becoming more common and are required for components that have a design life less than the gearbox. The gearbox system comprises many elements (primarily the rotating shafts, gears and bearings), so the reliability of the gearbox is the product of the reliability of all the failure modes for which there exists a reliability calculation as described in Verband Deutscher Maschinen- und Anlagenbau 23904 and IEC Technical Specification 61400-4-1. But many, if not most, of the failure modes experienced in operation do not have a standardized reliability calculation; hence, as described earlier, there exists a difference between the apparent reliability observed in operation and the calculated design reference reliability. This is not unusual, and it occurs in other industries, although the O&M cost impact for wind turbines can be more severe. For gearboxes, the reliability calculation considers gear tooth surface durability (pitting) according to ISO 6336-2 and bending strength according to ISO 6336-3, rolling-element bearing rating life from subsurface-initiated fatigue (i.e., rolling contact fatigue) according to ISO 281 and ISO/TS 16281, and shaft fatigue fracture according to Deutsches Institut für Normung 743 and American National Standards Institute (ANSI)/AGMA 6001. In some cases, a safety factor for or percentage risk of these failure modes can at least be quantified in the gearbox design process, including gear tooth scuffing according to ISO/TS 6336-20, ANSI/AGMA 925 and ISO/TS 6336-21 and gear tooth micropitting according to ISO/TS 6336-22, or otherwise assessed for gear tooth flank fracture according to ISO/TS 6336-4. Safety factors for the static strength of gears and bearings are calculated according to ISO 6336 and ISO 76, respectively. Other bearing failure modes, such as surface-initiated fatigue (e.g., micropitting), adhesive wear, corrosion, electrical damage and white-etching cracks, can only be assessed qualitatively. Requirements for materials, processing and manufacture are part of these standards. Further in-depth contact and finite-element analysis are used to design the microgeometry of these rotating components, provide additional rating life calculations and analyze the supporting housing structures. In addition to classical reliability approaches, use of structural reliability methods for reliability analysis of gears has also been investigated . Design guidance for the use of plain bearings in the gearbox is under development because they offer advantages in terms of torque density and are life-limited only by wear rather than determined by load-dependent rolling contact fatigue, although they are already becoming common in new gearboxes . Surface engineering, lubricants and lubrication of the gearbox also play an essential role in gearbox design, operation and reliability .

As highlighted earlier, wind turbine drivetrains can be either geared or direct-drive generator systems . The geared generator system can be further divided into either a DFIG with a partial-power converter or a brushless generator with a full-power-converter (GFPC) system. The DFIG system has been the most popular topology for medium-sized turbines ranging from 3–6 MW. The GFPC system uses either a squirrel cage induction generator or a PMSG. Many manufacturers now provide commercial GFPC solutions at power levels up to 10 MW . In terms of direct-drive systems, rare-earth PMSGs are appealing for offshore applications. The mainstream power level is from 5–7 MW, but the top power level has kept increasing during the past 2 decades.

The stator elements of DFIGs and PMSGs are largely the same. The major difference in terms of the electrical machine hardware is the rotor design and the means – or lack thereof – of getting current onto and off the rotor. These differences can impact both efficiency and failure mechanisms and their rates. In terms of efficiency, induction generators use a set of currents on the rotor to produce the rotor magnetic field. This leads to joule rotor losses and hence a decrease in efficiency. In contrast, a PMSG uses rare-earth permanent magnets to produce the rotor magnetic field, hence avoiding further joule losses.

In terms of failure types, a DFIG uses carbon brushes and slip rings to conduct the currents between the rotor and the stator. The brushes typically wear out over time and need frequent inspection and replacement. The PMSG avoids those elements. There are also differences in reliability because of the presence of conductor and insulation systems (DFIG) and magnet materials (PMSG), but those are not yet clear. A comparative study of DFIGs and PMSGs showed that during the early life, a PMSG has a failure rate 40 % lower than that of a comparable DFIG .

Alongside this variation in the reliability of different electrical machine architectures, there is variation in the reliability caused by the torque rating of the generator. This was first shown by . For example, it is possible to conceive of two wind turbines that both use the same generator type, but one is in a geared configuration (with a gearbox ratio of 100), and the other is direct-drive. For the same wind turbine rotor (and subsequent power and rotational speed), the torque rating of the direct-drive generator will be 100 times more than that of the geared generator. Assuming that the same electromagnetic shear stress is produced by the two generators, the volume of the direct-drive generator will also be 100 times that of the higher-speed generator. If they have the same ratio of diameter to axial length, then the generator diameter will be ×1003 (i.e., ×4.64) that of the geared machine. The electromagnetic materials are approximately proportional to the surface area of the rotor and stator. In the case of the low-speed machine, this might be ×4.642 (i.e., 21.5) times that of the higher-speed machine. With more poles, more coils, longer conductors and insulation, it is likely that the failure rate is higher in direct-drive machines if there is no improvement in failure rate intensity. These variations in generator failure rate should be taken into consideration – along with the gearbox failures discussed in Sect. 2.2 – when assessing failure rates of different types of geared and non-geared drivetrains.

The reliability and availability of the wind generator system have a decisive impact on the cost of energy (COE), especially offshore . The generator design should consider the interactions with other components to improve the system reliability of the drivetrain . In large wind turbines, multiphase windings with modular converters can be used to improve the generator system availability .

Upscaling is still a continuing trend for both land-based and offshore wind turbines because a higher-power wind turbine system leads to a lower levelized cost of energy (LCOE) . For land-based wind, recent developments include the design of 6 and 8 MW turbines, which will be on the market in the near future, whereas offshore commercial applications now aim for 10–15 MW, and research is going beyond 15 MW . Upscaling brings many challenges, including large generator weight, high manufacturing and installation difficulties and cost, complicated electromechanical dynamics, and complexity of system monitoring. A systematic design approach will be required for the design of the generator, where cooling and efficiency will be among the challenges in higher-power systems. Depending on the type, nominal power, shaft speed, the specific electric loading and subsequently the armature thermal loading, three general solutions – namely, air–air, air–water and water jackets – are commercially available to implement the cooling system of a wind generator . Significant cost savings can be realized with the development of a more effective stator winding cooling system that further limits the current density to enable the development of higher-power PMSGs of substantially smaller diameters while not adversely affecting the electromagnetic performance of the generator.

Multiphase, modular designs are solutions to tackle some of the challenges, and they have been used in commercial systems . Concerns over the availability of rare-earth elements – such as neodymium, praseodymium and dysprosium – typically used in PMSGs have led the wind industry and other industries to develop innovative technologies to reduce, substitute for or entirely eliminate their need for generators . This also results in technologies that are lighter than PMSGs. Breakthroughs in superconducting materials could change the scenario of materials and upscaling completely  by eliminating rare earths and enabling further generator weight reductions, with several superconducting generators successfully tested  or in development .

The interactions between the generator and power electronics can bring issues including bearing currents, additional stress in insulation because of overvoltage in transients and high-voltage slew rates ; therefore, these interactions should be studied and modeled not only for the design but also for O&M. Proper filters and control methods should be integrated according to the generator types and power electronics topologies. For the upscaling of wind generators, various modular and multilevel power converter topologies feeding multiphase windings will be a promising solution. But attention should be paid to circulating current and potential asymmetric supplies to avoid risks .

Being between the generator and power grid, the power converter needs to fulfill the requirements for both sides . Wind turbine power converters used to have a topology as shown in Fig. a, where the generator-side converter is a diode rectifier cascaded with a boost converter to maintain a stable direct current (DC) link voltage; then a two-level inverter is employed on the grid side to ensure full control of the grid current injection (e.g., total harmonic distortion and power factor). This topology has a relatively low cost because of fewer power switches than newer configurations, so it is widely used for generators in small- to medium-sized wind turbines. For megawatt-scale generators, however, the low-frequency torque pulsation and high total harmonic distortion become very harmful to the generator. As a result, the design of the boost converter becomes very challenging, and therefore the front end is then replaced by a two-level, six-switch converter with power factor correction, which is shown in Fig. b and is called a back-to-back (BTB) converter. A DFIG with a partially loaded BTB converter is commonly used for generators less than 3 MW, whereas a PMSG with a fully loaded BTB converter is commonly used for generators greater than 3 MW.

When the power rating is 10 MW or more, a single two-level BTB converter is no longer suitable because the current stress of the power devices would be extremely high. For two-level BTB converters, the grid-side voltage is typically 690 V. To solve this, multiple two-level BTB converters can be connected in parallel to share the current, whereas the connection of the generator side can be slightly different depending on whether the generator has a multiwinding (see Fig. a). Another way to upscale the power rating is to increase it to medium voltage, for instance, by a neutral-point-clamped (NPC) converter (see Fig. b) or a modular multilevel converter.

Other crucial topics for power converters include thermal loading and grid integration. Intermittent winds create temperature swings in power converters, which are the main factor of power converter aging. The possibility of applying lithium-ion battery energy storage systems to smooth wind power has been investigated to reduce this aging . The approach is effective, and the stress mitigation performance is affected by the energy and power rating of the energy storage system; however, the energy storage system's high cost is still the main barrier preventing its widespread application. Variable switching frequencies can also somewhat reduce temperature swings , which is an attractive option as a control approach that does not add hardware cost. Nonetheless, the grid filter design to handle the variable switching frequency might be challenging. A promising approach can be the use of the kinetic energy in the wind turbine's rotor as energy storage by the rotating speed control to suppress the power fluctuation in the power converter and thereby reduce the temperature swings . Another topic that is trending for the wind power converter is grid integration. Grid-following mode control was applied in the grid-side converter, and it is still mainstream; however, as wind power penetration is increasing, the grid is becoming relatively weak (low short-circuit ratio, low inertia, etc.). Grid-supporting mode control can cause power quality issues and even grid failures in some severe cases . Energy storage systems or synchronous condensers can be associated with wind plants to provide inertia and to reduce the burden of the grid without grid reinforcement (which is very expensive), but these components are still expensive. Another promising approach is to apply grid-forming mode control to the grid-side wind power converter so the inertia in the wind turbines can be used to support the grid and enhance grid stability and reliability. Grid-forming control is a group of controllers with which the wind power converters not only get the grid current controlled, but also support the grid voltage, frequency or inertia. Typical grid-forming control includes droop control, in which the active and reactive power injected to the grid is reacting to the grid frequency and voltage, and virtual synchronous machine control, in which the inertia of the synchronous machine is mocked in the wind power converter.

There is a trend in power electronics to evolve from silicon-based power semiconductors to wide-bandgap devices (e.g., silicon-carbide devices). This will have a positive impact on wind energy systems because it can improve the power density and improve the efficiency of the power converters . In the meantime, it also brings challenges to wind generators because of the high-voltage slew rate as a result of fast switching. Proper filtering and oscillation-damping technologies should be used to mitigate the side effects, such as common-mode current and insulation degradation.

The type of cooling system chosen for the converter depends on the nominal power and voltage, power density and thermal design, and generator technology, which can be based on either air or direct or indirect liquid cooling . By choosing a liquid cooling system, the size of the converter for high-power applications (>5 MW) can be significantly reduced.

Condition monitoring is an umbrella term that spans various different ways of tracking the health state of a machine in which typically vibration, temperatures, oil contamination or electrical signatures from the generator are used as input signals . By using appropriate analysis methods, system changes caused by damaged components (e.g., flaking of bearing raceways) or faulty system states (e.g., water contamination) can be identified. A recent review of the condition monitoring of drivetrains is offered by . The following sections discuss the two most commonly used types of condition monitoring for wind turbine drivetrains along with the acoustic emissions approach.

The term “consumed life” of a component is a metric that can be calculated based on existing rating methods by using actual loads (whereas design loads are used in the development phase), while “remaining life” is the best estimation of how long a component will survive, which requires statistical prediction models. The consumed life calculation of drivetrain mechanical components, such as bearings and gears, requires the analysis of measurements from the field, such as gearbox temperatures, bearing vibration measurements and wind turbine operational measurements. Such measurements can be continuous online recordings or measurements taken during inspections such as oil quality sampling. Typical indicators for the consumption of the life of the gearbox are

the number of particles in the gearbox oil per time duration or the particulates in the grease for greased bearings ,

With advanced multibody simulation tools, it is also possible to model all elements of the drivetrain fully coupled to the aeroelastic interactions of the rotor and mounted on a flexible tower. In such a software tool, the drivetrain is subject to continuous wind-driven excitation and grid-driven events. If the response of the drivetrain in terms of the bearing displacements and shaft loads is validated with the physical turbine during different operating conditions, then the software tool can be used to track fatigue damage consumption in the drivetrain by supplying it with measured operating conditions from the physical turbine. This also requires specific drivetrain component failure modes to be tracked in the simulation, such as the occurrences of bearing roller sliding or the vibration excitation of different components. Monitoring fatigue damage growth also provides knowledge of the remaining useful life of the drivetrain if the design life of the components of the drivetrain are known . Monitoring systems and modeling tools are also increasingly being integrated with supply chain management systems to reduce downtime and O&M costs .

Moreover, the inverse methods can also be employed to estimate the loads on drivetrain components from SCADA and vibration measurements and can be used to estimate the component fatigue damage and RUL . Dynamic models of the drivetrain might be needed in some cases if not enough data or measurements are available that can be developed provided that basic information about the drivetrain (e.g., geometry, bearing types and gear teeth numbers) is known .

Once a turbine reaches its 20-year design life, there are, in principle, three options that can be considered by the operator: decommissioning, full repowering or lifetime extension . Full repowering and lifetime extension can accomplish the goal of extending the service life of existing wind turbines. The main benefit is to increase returns on investments and reduce the levelized cost of energy for wind power. Full repowering refers to the dismantling of old wind turbines and replacing them with new ones. Lifetime extension, on the other hand, refers to the assessment of the remaining useful life and possible turbine component upgrades while keeping the turbine hub height, size or plant layout unchanged. Partial repowering or refurbishment is another term used in the industry for replacing and upgrading the components; this can be considered an option in the lifetime extension process.

Whether and how to conduct lifetime extension is a complex decision-making process and depends on many factors, including technical, economical and legal factors . The prerequisite is that the turbine's structural integrity – e.g., foundation, tower, nacelle and hub – has been assessed and can be safely used throughout the expected turbine lifetime extension span. Sometimes these assessments also include blades, which, if not strong enough, could be upgraded as well. Specific to drivetrain components, typical practices are replacing them with newer products – e.g., gearboxes, main bearings or generators – which have improved performance and reliability. On the other hand, from the drivetrain research-and-development perspective, some opportunities lie in reliability assessment and driving event identification, which can benefit from fault diagnostics and RUL prediction research conducted to support wind plant O&M. The expected outputs are more accurate and reliable drivetrain component integrity assessment based on historical data or inspection as well as prediction throughout the planned lifetime extension period. Should the evaluation be positive with an acceptable confidence level, the drivetrain components might not be replaced, as is currently practiced now, and additional costs of the turbine lifetime extension can be reduced.

investigated the lifetime extension practices in other industries and proposed a methodology for wind turbine drivetrains. The industry's experience with lifetime extension is still limited. Also, different markets – e.g., Europe or the United States – might require different strategies. There are a few uncertainty concerns: (i) technically, how trustworthy the integrity assessment of the structural components is; (ii) economically, how the assumed future electricity prices might hold true; and (iii) legally, how related policies might change.

Wind turbines are typically decommissioned and recycled – at least partially – at the end of their service life because of both their salvage value and local legislative requirements, so decommissioning has increasingly become part of the planning process . Detailed discussions on the decommissioning process for land-based and offshore wind turbines, recycling analysis of different turbine subsystems, basic cost analysis, and the environmental impacts can be found in . The main challenges surrounding the decommissioning process of wind plants are the regulatory framework, the overall planning of the process, the transportation logistics and the environmental impacts .

Studies show that recycling after decommissioning can pay back some of the decommissioning costs . The main parts of the drivetrain from the recycling perspective are the bearings, gears, frames, shafts, couplings, windings, cores (generator stator and rotor), permanent magnets, hydraulic cooling systems and electronics. Windings are made of copper. Cores are made of electrical steel lamination, which is an iron alloy to which silicon is added. Bearings, gears, frames, shafts and couplings are made of different types of alloy steel. Permanent magnets used in wind turbine PMSGs are often NdFeB magnets. Hydraulics are considered less problematic because the recycling industry is accustomed to handling the related components. Reuse of lubricants is also being explored . Electronics are usually difficult to recycle because of their complex material composition. Nearly all waste electronics equipment is currently shredded, and further physical processes, including magnetic and electrostatic techniques, are applied to separate different metal fractions. Two main methods are used to recycle these components: shredding and disassembling. The method used depends on the recycled component, the size and the material. Recycling can be supported by robotics and using optimization models to optimize the process . Separate parts of disassembled components are reused if they do not have any damage. The rest are remelted as the same raw material or to a new alloy.

High-volume drivetrain recycling needs to minimize environmental impact (no harmful chemicals or emissions or energy-intensive processes) and have high recovery rates of rare and precious materials while still being economically viable. Recycling can also be in the design phase. For example, surface-mounted PMSGs compared to interior rotor magnets have different disassembling procedures. Modular design can also offer a clear advantage in recycling because healthy parts can be separately reused in other components of that type.

Further, the use of recycled material can lead to cheaper new products , which, apart from being more environmentally friendly, can be a good motivation for recycling. Recycling and reusing can also contribute to reducing the potential supply chain risk, especially for rare-earth materials .

Drivetrains in floating wind turbines are exposed to different dynamic loads than those on bottom-fixed or land-based ones. Apart from the wind loads, the wave-induced motions can affect the drivetrain load responses. The wave-induced motions can have a negative impact on the main bearing fatigue life – particularly on the one carrying axial loads – as highlighted by . investigated the effects of the floating wind turbine motion on direct-drive generator air-gap integrity and showed that the air-gap stability of the generator is more sensitive to magnetic forces if the supporting frame is relatively rigid. They also highlighted the need for air-gap management for direct-drive generators on floating platforms.

A recent full-scale experimental field study of a 6 MW drivetrain on a spar floating substructure indicates that the effect of wave-induced motions might not be as significant as the wind loading on the drivetrain responses, particularly in larger turbines. A set of strain measurements on the main bearing show that the effect of wave motions is negligible compared with the tower shadow excitation .

The study by investigated the lessons learned in the last 10 years with regard to drivetrains on floating wind turbines. Among other things, the study highlighted that the maximum tower-top axial acceleration might not be a reasonable limiting factor for the fatigue life of main bearings in floating wind turbines, at least for a spar-type floating turbine . It also emphasized that a flexible bedplate influences the main bearing and components inside the gearbox, and therefore the coupling effects between the structure and the drivetrain need to be considered in floating wind turbines, particularly in compact design concepts. Given the limited experience with floating wind turbines, however, more research is needed.

Wakes induced by other turbines in the plant cause increased turbulence and thus can affect the loading on the drivetrain . How the wakes are controlled at the plant level will therefore influence the drivetrain design life . Several approaches which have recently been proposed to reduce overall plant losses by reducing wake effects employ either some form of static or dynamic induction control or wake steering by yaw control . Static induction control aims to reduce the strength of the wake of an upstream machine by changing the pitch of the blades or the rotational speed of the rotor in such a way as to reduce the thrust at the expense of some efficiency but to allow downstream machines to see an increased wind speed so that the combined output of the turbines is increased. However, in practice, little gain is seen using this approach . Dynamic induction control seeks to increase wake mixing and thus reduce plant losses by periodically pitching the blades during each rotor rotation . Wake steering takes a different approach, whereby an upstream turbine is deliberately yawed out of the prevailing wind direction (again slightly reducing turbine efficiency) with the aim of ensuring that downstream machines are out of its wake, thus increasing their output and increasing overall plant efficiency but also affecting shaft torque and blade moments . Wake steering typically does not have large effects on the drivetrain of the upstream turbine, whereas an increase in the yaw angle would lead to an increase in the drivetrain load variation in the downstream turbine . In contrast, increases in blade pitch angle associated with static induction control would reduce the standard deviations of the drivetrain dynamic response of the upstream wind turbine, whereas it would not significantly affect the standard deviations in the downstream turbine .

Based on the study performed by , the wake impact on the downstream turbine can reduce the lifetime of the first main bearing of the drivetrain by 17 %, and it can reduce the turbine power intake by 30 %. The mitigation of loads on the drivetrain of the wind turbine and an increase in power capture at the turbine level are addressed in the literature on turbine control by optimizing the generator torque, blade pitch and yaw steering controls (as shown in, for example, , and ). Optimized wind power plant management by considering the influence of wakes was recently studied by .

The optimized plant control design to maximize the wind plant power intake and simultaneously minimize the degradation of the drivetrain components influenced by wake loads is still an open research problem. The latter calls for high-fidelity models of wake flow and wake loads on the drivetrain of downstream turbines and the drivetrain system to study the drivetrain load effects and responses and, finally, the derivation of sufficient drivetrain-lifetime-related constraints that can be integrated into the plant stochastic model predictive control design framework. From this perspective, the additional role of wind plant control is to distribute accumulated fatigue evenly over the drivetrains of different turbines to improve the reliability of the plant.

The use of digital technologies and digitized data can support wind turbine drivetrain analyses at both the system and component levels, for instance by receiving and transmitting real-time data through the data acquisition and transmission layers; by storing and processing data through the platform layer; and, finally, by decision support through the application layer . For this purpose, digitalization targets the sensors and actuators installed on the drivetrain and the other turbine systems – such as the site network, servers and even smart phones – that are connected to the turbine's and plant’s control and monitoring systems to improve reliability, availability, quality of service and user experience.

Digital twin models can support the drivetrain's design and operation. Digital twin in this context includes sensors (data collection), models (dynamic and degradation models) and decision support platform (e.g., estimation of remaining useful life) . Computationally inexpensive digital twin models can support the predictive maintenance of drivetrain system components by monitoring the remaining useful lifetime of the critical components (e.g., the gears of the gearbox) in real time . Using the existing physical models, integrating them in a unified framework, applying signal-processing techniques to estimate these models and model inputs in real time from available measurements, optimizing the data streaming between models, using continuous processing architectures, and using statistical approaches and stochastic modeling techniques to model and mitigate the impact of uncertainties are the tasks under the umbrella of a digital twin. The risk associated with digital twin implementation, specially in terms of wrong decision support, should also be considered . In general, challenges of digital twin models mainly arise from the following :

finding the minimum or appropriate model fidelity required to capture the dynamics of the component for different operation and failure modes for a wide range of drivetrain components

To overcome these challenges, merging edge computing with the Internet of Things (IoT) can play a significant role. Using distributed computing algorithms supported by edge computing data-handling architectures (e.g., fog computing) has been recently adapted from computer science for wind plant control and monitoring systems to significantly reduce the computational burden of implementing digital twin models' complex algorithms to monitor drivetrain components in future wind turbine O&M analyses . By using fog computing, it is possible to break the digital twin into simple subproblems with less computational complexity, and each one can be executed on a fog. The IoT can provide real-time access to data in each network node with the possibility of using the processing and storage capacities of the nodes for control and monitoring purposes, and not all data in fogs need to be shared with other fogs or even the cloud. Cybersecurity frameworks should be deployed at the communication network and the computing modules to guarantee the integrity, authenticity, confidentiality and availability of the data while they are being transmitted over the network nodes. Because future wind plants' control and monitoring systems will need to handle more voluminous, heterogeneous data and distributed features, but storage and processing capacities are limited, the IoT can significantly improve turbine control and monitoring.