Jonathan Allen Keller

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National Renewable Energy Laboratory | NREL · National Wind Technology Center

Previous computational and field experiments identify three characteristic time scales in the aerodynamic responses of utility-scale wind turbine loads to atmospheric boundary layer (ABL) turbulence: a 30-90 second time scale for the passage of high/low speed "streaks" through the rotor plane, the blade and rotor rotation time scales (approximately...

This design guideline describes the design criteria, calculation methods, and applicable standards recommended for use in performance and life analyses of ball and roller (rolling) bearings for yaw and pitch motion support in wind turbine applications. The formulae presented here for rolling bearing analytical methods and bearing life ratings are c...

The mesh load factor, K γ , describes how loads are shared between planet gears and has become one of the key design challenges in modern wind turbine gearboxes. Planet load sharing directly impacts tooth root stresses, a critical driver of torque density and gearbox reliability. Experimental evaluation of K γ is typically performed from sun gear t...

The United States is currently targeting 30GW of offshore wind to be installed by 2030, and 150GW by 2050. Even considering future turbine sizes, this represents thousands of new turbines installed in a diverse set of environments, each with their unique design, installation, and maintenance challenges. While much can be learned from European and A...

Recently, there has been increased interest in the reliability of pitch systems and pitch bearings. Pitch bearings, shown in Figure 1, are used in wind turbines to connect the blade root to the hub. They support simultaneous radial, axial, and overturning moment loads while allowing relative rotation of the blade with respect to the hub. Significan...

This paper studies the rating lives of wind turbine main bearings, as determined by the IEC 61400‐1 and ISO 281 standards. A critical review of relevant bearing life theory and turbine design requirements is provided, including discussion on possible shortcomings such as the existence (or not) of the bearing fatigue load limit and the validity of a...

Lubrication is a key aspect of maintaining a wind turbine in operational condition. In this study, we quantify the potential for increasing energy production and reducing maintenance costs across the current and future U.S. fleet of wind turbines as a result of improvements in lubrication performance. The modeled improvements reduce the median leve...

Leading wind turbine manufacturers are racing to build larger and more powerful offshore machines. Drivetrain configurations often use a permanent-magnet synchronous generator (PMSG), in either a direct-drive configuration or coupled to a gearbox. With increasing demand for critical rare-earth magnets, new generator technologies are emerging to ens...

Wind energy is foundational for achieving 100 % renewable electricity production, and significant innovation is required as the grid expands and accommodates hybrid plant systems, energy-intensive products such as fuels, and a transitioning transportation sector. The sizable investments required for wind power plant development and integration make...

This study seeks to establish a comprehensive baseline of knowledge for the replacement and damage of main bearings in wind turbines. The purpose of this report is to provide a high-level summary of the data set, methodology, and results of this work. Full technical details and an extended analysis will be made available in a future publication. We...

Gearbox and wind turbine design and application standards have contributed significantly to improvements in reliability over the past two decades. The International Electrotechnical Commission (IEC) 61400-4 standard for wind turbine gearbox design is currently being revised by a joint working group of experts in IEC Technical Committee (TC) 88 (win...

This paper presents a study on pitch bearing basic rating life affected by wind field characteristics at both onshore and offshore wind sites. The National Renewable Energy Laboratory 5 MW reference wind turbine is selected for the study. Wind field characteristics including reference hub height mean wind speed, wind speed distribution, wind shear,...

This paper presents a Digital Twin for virtual sensing of wind turbine aerodynamic hub loads, as well as monitoring the accumulated fatigue damage and remaining useful life in drivetrain bearings based on measurements of the Supervisory Control and Data Acquisition (SCADA) and the drivetrain condition monitoring system (CMS). The aerodynamic load e...

This paper considers the problem of the dynamic modelling of macro slip in spherical roller bearings. By revisiting the fundamental physics which drive these systems, potential issues in existing models have been identified. Furthermore, in pure rolling conditions it was found that governing differential equations become “stiff”, requiring the use...

Slew ring or "pitch" bearings are used in wind turbines to connect the blade root to the hub. They support the resulting simultaneous radial, axial, and overturning moment loads while allowing relative rotation of the blade with respect to the hub. NREL has recently begun a research program related to pitch bearing reliability, recognizing its grow...

ISO/TS 6336-22 (Calculation of load capacity of spur and helical gears — Part 22: Calculation of micropitting load capacity) is the ISO technical specification containing a proposal for calculations of the risk of micropitting in gear sets. Micropitting is a Hertzian fatigue phenomenon that appears as ultra-fine cracking and pitting on the flanks o...

This report described acoustic emissions and temperature characteristics of a commercial main bearing in a wind turbine drivetrain. These characteristics were measured on the outer ring of the bearing by SKF DVST nodes, which are mounted on both upwind and downwind rows of the main bearing at four equally spaced circumferential locations. The measu...

Wind Cybersecurity Workshop Edison Electric Institute, Washington, DC 28-29 September 2022

Recently, micropitting that occurred in a wind turbine gearbox has been used as a case study of the International Organization for Standardization technical specification ISO/TS 6336-22. This technical specification contains a proposed calculation of risk of micropitting in gear sets, in terms of a safety factor based on the ratio of the minimum to...

This work considers the characteristics and drivers of the loads experienced by wind turbine main bearings. Simplified load response models of two different hub and main-bearing configurations are presented, representative of both inverting direct-drive and four-point-mounted geared drivetrains. The influences of deterministic wind field characteri...

Wind energy is foundational for achieving 100 % renewable electricity production and significant innovation is required as the grid expands and accommodates hybrid plant systems, energy-intensive products such as fuels, and a transitioning transportation sector. The sizable investments required for wind power plant development and integration make...

The power take-off (PTO) is a key component for wave energy converters. In this paper, a compact PTO with mechanical motion rectification rated at 10 kW is designed and prototyped, and characterized. A piecewise nonlinear dynamic model is established to describe its unique dynamic property. A linear hydraulic actuator is adopted in lab testing to d...

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 dig...

This work considers the characteristics and drivers of the loads experienced by wind turbine main-bearings. Simplified load response models of two different hub and main-bearing configurations are presented, representative of both inverting direct-drive and four-point mounted geared drivetrains. The influences of deterministic wind field characteri...

Premature main bearing failures can be a significant operation and maintenance cost, although failure rates can vary between populations (e.g., land-based or offshore, direct-drive or geared, site-to-site, drivetrain mounting style). Unlike most gearboxes, main bearings typically cannot be repaired uptower and often require crane removal, which res...

A WEC is a particularly aggressive, unpredictable and wide spread rolling element bearing failure mode that is common for large multi-megawatt (MW) wind turbines. WEC is considered the single most expensive failure mode for all wind turbine components, and there is currently no commercial solution. The Technical University of Denmark (DTU) is leadi...

Summary of CRD-16-608

Summary of CRD-17-702

The information exchanged and new R&D opportunities identified at the DRC workshops and meetings are valuable for prioritization of future R&D plans. The DRC workshops and meetings provide a venue for exchange of information in an open and transparent manner, with the common goals of improvement in wind turbine drivetrain reliability and reduction...

This paper presents the state-of-the-art technologies and development trends of wind turbine drivetrains – the energy conversion systems transferring the kinetic energy of the wind to electrical energy – in different stages of their life cycle: design, manufacturing, installation, operation, lifetime extension, decommissioning, and recycling. Offsh...

Direct-drive wind turbine generators are increasing in popularity, thanks to recent project developments—especially offshore, where reliability and efficiency are major cost drivers. Yet, high capital costs are forcing many original equipment manufacturers to consider lightweight, high-torque density generators for next-generation multi-megawatt tu...

Wear-related failures of spherical roller bearings in the main bearing position of three-point mount wind turbines have been higher than expected and can contribute to higher than anticipated operation-and-maintenance costs. In this paper, the operational conditions of such a main bearing—including measured axial displacement and velocity subject t...

In this article, the type of roller slip behavior that may result in the formation of white-etching cracks (WECs) in wind turbine gearbox bearings is identified. A new hypothesis based on the inner raceway normal contact load magnitude at the time of roller slip is proposed as the probable cause of WECs. For this purpose, the maximum normal contact...

ISO/TS 6336-22 (Calculation of load capacity of spur and helical gears — Part 22: Calculation of micropitting load capacity) is the ISO technical specification containing a proposal for a method to calculate the risk of micropitting in gear sets. It assesses micropitting risk through a safety factor which is calculated as the minimum specific film...

In an ideal epicyclic gear set every parallel gear path transmits the same amount of torque. However, it is well known that certain manufacturing variations result in unequal load sharing between the parallel gear paths. Previous works have developed and validated a general closed-form analytical model of this phenomenon that describes the load sha...

ISO/TS 6336-22 (Calculation of load capacity of spur and helical gears — Part 22: Calculation of micropitting load capacity) is the ISO technical specification containing a proposal for a method to calculate the risk of micropitting in gear sets. It assesses micropitting risk through a safety factor which is calculated as the minimum specific film...

Li et al. present a worldwide analysis of rare-earth elements used in wind turbine generators. It focuses on the most extreme deployment scenario using rare-earth-intensive generator technology to demonstrate demand quickly outpacing supply, thus suggesting an existential threat to rapid wind deployment. However, alternative generator technologies...

In an ideal epicyclic gear set, every parallel gear path transmits the same amount of torque. However, it is well known that certain manufacturing variations result in unequal load sharing between the parallel gear paths. Previous works have developed and validated a general closed-form analytical model of this phenomenon that describes the load-sh...

This article describes an interdisciplinary methodology to calculate the probability of failure for bearing axial cracking, the dominant failure mode in the intermediate and high-speed stages of many wind turbine gearboxes. This approach is mainly a physics-domain method with needed inputs from the data domain. The gearbox and bearing design along...

With growing interest in electrification from clean energy technologies, such as wind power, and use of pure electric powertrains in various applications, the demand for next-generation, high-performance magnetic materials has risen significantly. Electrical machine design for these applications is facing challenges in terms of meeting very demandi...

This paper presents a combination of models that together calculate the cage and roller speeds of a cylindrical roller bearing. The models consider elastohydrodynamic lubrication and contact elasticity between the roller and raceway, roller centrifugal forces, hydrodynamic lubrication at the cage pocket, and frictional forces. Using these models, t...

Wind power plant operations and maintenance (O&M) costs remain an appreciable contributor to the overall cost of wind energy. Premature of wind turbine gearboxes failures are often a result of abrasive wear, micropitting, scuffing, white-etch cracks, and macropitting issues. Micropitting is a fatigue phenomenon that occurs in Hertzian contacts in b...

Failures in gearbox bearings have been a primary source of reliability issues for wind turbine drivetrains, leading to costly downtime and unplanned maintenance. The most common failure mode is attributed to so-called white-etching cracks (WECs). These cracks tend to propagate to spalls or lead to a complete splitting of the inner ring. This mode o...

A multipronged research program supported by DOE at Argonne National Laboratory and NREL is examining the causes of main bearing micropitting wear and white-etching cracks in wind turbine gearbox bearings. This purpose of this report is to summarize full-scale wind turbine tests to determine the operations conducive to these failures and fully desc...

Wind farms are an indispensable driver toward renewable and nonpolluting energy resources. However, as ideal sites are limited, placement in remote and challenging locations results in higher logistics costs and lower average wind speeds. Therefore, it is critical to increase the reliability of the turbines to reduce maintenance costs. Robust imple...

This presentation describes modeling and experiments that examine the extent of sliding of the rollers of a cylindrical roller bearing in a wind turbine gearbox and the relationship of that sliding to the formation of white-etching cracks.

In this paper, the planetary load-sharing behavior and fatigue life of different wind turbine gearboxes when subjected to rotor moments are examined. Two planetary bearing designs are compared – one design using cylindrical roller bearings with clearance and the other design using preloaded tapered roller bearings to support both the carrier and pl...

The goal of this paper is to investigate the behaviour of the high‐speed stage of a wind turbine gearbox during transient grid‐loss events triggered at different power levels in a full‐scale nacelle dynamometer test. The gear mesh behaviour and the bearing roller slip during these events are characterized by strain measurements placed in the gear t...

In this paper, the planetary load-sharing behaviour and fatigue life of different wind turbine gearboxes when subjected to rotor moments are examined. Two planetary bearing designs are compared – one design using cylindrical roller bearings with clearance and the other design using preloaded tapered roller bearings to support both the carrier and p...

A status update on the gearbox and main bearing instrumentation and tests on the DOE-owned GE 1.5 SLE turbine at the National Wind Technology Center to investigate bearing reliability

Failures in gearbox bearings have been the primary source of reliability issues for wind turbine drivetrains, leading to costly downtime and unplanned maintenance. The most common failure mode is attributed to so-called axial cracks or white-etching cracks, which primarily affect the intermediate and high-speed-stage bearings. These cracks tend to...

Many wind turbine gearboxes require repair or replacement well before reaching the end of their design life. The most common failure is bearing axial cracks, commonly called white etching cracks (WECs), which typically occur in the inner raceways of the high-speed parallel-stage rolling element bearings. Although the root causes of WECs are debated...

The most common failure mode in wind turbine gearboxes is axial cracking in intermediate and high-speed-stage bearings, also commonly called white-etching cracks (WECs). Although these types of cracks have been reported for over a decade, the conditions leading to WECs, the process by which this failure culminates, and the reasons for their apparen...

Wind turbine gearboxes do not achieve their expected design life. The Gearbox Reliability Collaborative (GRC) objective is to understand the root causes of premature gearbox failures and improve their reliability. To date, the GRC has focused on a 750-kW drivetrain with a three-stage, three-point-mounted gearbox. A nonproprietary version of the gea...

This work compares the planetary load-sharing characteristics of wind turbine gearboxes supported by cylindrical roller bearings (CRBs) and preloaded tapered roller bearings (TRBs) when subjected to rotor moments. Planetary bearing loads were measured in field-representative dynamometer tests and compared to loads predicted by finite-element models...

The planetary load sharing characteristics of wind turbine gearboxes supported by cylindrical roller bearings (CRBs) and preloaded tapered roller bearings (TRBs) when subjected to rotor moments are compared in this work. Planetary bearing loads were measured in field-representative dynamometer tests and compared to loads predicted by finite-element...

The GRC uses a combined gearbox testing, modeling, and analysis approach disseminating data and results to the industry and facilitating improvement of gearbox reliability. This test data describes the tests of GRC gearbox 3 in the National Wind Technology Center dynamometer and documents any modifications to the original test plan. It serves as a...

In 2007, the U.S. Department of Energy established the National Renewable Energy Laboratory Gearbox Reliability Collaborative (GRC). Its goals are to understand the root causes of premature gearbox failures and to improve gearbox reliability. The GRC uses a combined gearbox testing, modeling, and analysis approach focused on a 750-kilowatt drivetra...

Many gearboxes in wind turbines do not achieve their expected design life; they do, however, commonly meet or exceed the design criteria specified in current standards in the gear, bearing, and wind turbine industry as well as third-party certification criteria. The cost of gearbox replacements and rebuilds, as well as the downtime associated with...

This presentation summarizes the Drivetrain Reliability Collaborative Workshop and report and current and near-term testing activities.

This document describes the test plan for the Gearbox Reliability Collaborative (GRC) gearbox #3. The primary test objective is to measure the planetary load-sharing characteristics in the same conditions as the original GRC gearbox design. If the measured load-sharing characteristics are close to the design model, the projected improvement in plan...

This paper reports on the design and testing results from the U.S. Department of Energy Next-Generation Wind Turbine Drivetrain Project. The drivetrain design reduces the cost of energy by increasing energy capture through drivetrain efficiency improvements; by reducing operation and maintenance costs through reducing gearbox failures; and by lower...

A key task within Next Generation Drivetrain effort is the power converter fault control algorithm design and associated computer simulations using an integrated electromechanical model of the drivetrain. A list of issues to be addressed with these algorithms was developed by review of the grid interconnection requirements of various North American...

The Wind Turbine Drivetrain Reliability Collaborative Workshop was convened by the National Renewable Energy Laboratory (NREL), Argonne National Laboratory, and the U.S. Department of Energy to explore the state of the art in wind turbine drivetrain mechanical system reliability as well as research and development (R&D) challenges that if solved co...

This report documents the torsional drivetrain dynamics of the NWTC’s 2.5-MW dynamometer as identified experimentally and as calculated using lumped parameter models using known inertia and stiffness parameters. The report is presented in two parts beginning with the identification of the primary torsional modes followed by the investigation of app...