June 2013

Coming off a year when wind energy experienced the highest amount of growth in its history, industry players wasted little time celebrating as they convened in Chicago on May 5-8 for the American Wind Energy Association’s annual WINDPOWER Conference and Exhibition.

In 2012, driven largely by the looming December 31 expiration of the Production Tax Credit, wind energy developers installed more than 13GW of new wind energy capacity—exceeding 60GW of total installed wind energy capacity and making wind the leader among all power generation sources in newly installed capacity.

But the driving force behind that rapid growth is now providing uncertainty and instability, as Congress granted only a short-term extension of the PTC as part of the last-minute “fiscal cliff” negotiations early this year.

Overcoming that uncertainty and instability was a central theme throughout the conference, as evidenced by the event’s “Solutions for Success” slogan.

American wind power must develop a vision for predictable policies that create a stable business environment and allow the industry to grow to its full potential, industry leaders said during the first day of the conference on May 6.

“That’s good news, but it’s just another chapter in a story we keep writing every day,” incoming AWEA board chairman and EDP Renewables North America CEO Gabriel Alonso said regarding record installations in 2012 and future growth.

Alonso went on to say that the industry could still be “vibrant and sustainable” if it deployed significantly less than that—to the tune of about 8GW annually. However, a boom-bust cycle brought on by uncertain policies “does not make for a sustainable industry,” he said.

The incoming chairman’s comments came during and immediately following the conference’s opening session, at which incoming AWEA CEO Tom Kiernan, Iowa Gov. Terry Branstad, Chicago Mayor Rahm Emanuel also spoke.

In spite of the challenges wind energy currently faces, Alonso applauded the industry’s recent accomplishments. Aside from the continuous growth, “The technology evolution has been phenomenal over the last few years,” Alonso said, with capacity factors surging right along with the size of blades and towers.

In outlining the association’s vision for achieving stability and sustainability within the industry, Alonso described a five-pillar strategy:
First, AWEA must be strengthened so it can better advocate, provide essential data for members’ success, develop standards, powerfully convey wind power’s compelling message, and do more for the industry.

Second, the industry must strengthen its brand against competitors and unify its message in order for America to truly understand how wind power is good for America.

Third, the industry must develop a long-term plan and strategy that includes clear near-term and mid-term action steps.

Fourth, members of the industry must become united by tapping the power of AWEA, its regional partners, and other vehicles to speak for wind power with one voice.

Finally, “We need YOU,” Alonso said. The number of people speaking to their government representatives, whether in Washington, D.C., or in their communities, must grow manifold, he said. If 100 people today are participating in trips to the nation’s capital to visit with their representatives, that number must become 1,000. “You are powerful,” said Alonso. “You have a message that matters.”

Tom Kiernan, who would officially take on his role as AWEA CEO later in May, was officially introduced to his new industry during the opening session. Kiernan,who was previously president of the National Parks Conservation Association since 1998, and a senior official at the Environmental Protection Agency under President George H.W. Bush, said his passion for wind energy is personal.

“Wind power is clean, affordable, and homegrown,” Kiernan said. “The country needs us to succeed. The natural world needs us to succeed. And frankly, my children and your children need us to succeed.”

Kiernan also echoed Alonso’s call for a new vision. “We need to craft a long-term plan for both the industry and for AWEA,” he said.

Nevertheless, near-term issues will need to be addressed, and Kiernan made that clear. Following the opening session, he made sure that members of the media understood the immediate matter at hand. “To be clear, it is AWEA’s top priority to extend the PTC and ITC.”

Meanwhile, at the afternoon’s Power Session, high-level representatives of AWEA, the National Wildlife Federation, the American Wind Wildlife Institute, and the National Audubon Society joined Jose Zayas, director of the Wind and Water Power Technologies Office at the U.S. Department of Energy (DOE), to launch discussion of a new vision for the wind energy’s future.

According to Zayas, DOE’s new effort will succeed the “20 Percent by 2030” technical report produced by DOE in 2008, and will take into account changes in wind technology, energy markets, and competing forms of energy in the intervening years. “My job,” Zayas said, “is to make sure wind energy is at the table and has a key part to play in the energy mix of the future.”

The process for developing the new strategy, he said, will be a collaborative one much like that employed in developing the 20 Percent report, in which environmental groups, utilities, energy experts and others will be brought together to look at the potential for wind energy in 2020, 2030, and 2050. He said DoE’s goal is to reduce carbon emissions by 80 percent by 2050.

The general session on day two of the conference featured two separate panels of wind industry leaders and prominent wind energy users, who discussed how U.S. wind energy is strongly positioned to build on its growth and status as a mainstream energy source, particularly by capitalizing on its many benefits.

During the panel discussions, industry leaders spoke of how they share the good news that the clean, affordable energy source is a win-win choice for America. The second panel of corporate purchasers of wind power – including some of the best-known brand names in American business – told how wind energy is saving their businesses money, giving them protection against energy price spikes, and making their communities cleaner.

“The business case for wind is very compelling,” said Paul Gaynor, CEO of First Wind. He satated that wind power is now saving consumers large sums in Massachusetts and Connecticut—to the tune of $1 billion and $800 million, respectively.

Oklahoma has embraced wind energy because of the “realization that they have this great wind resource, and the price is very attractive,” said Kevin Walsh, managing director for renewable energy at GE Energy Financial Services.

As the president and CEO of a company in wind energy’s extensive supply chain, Shermco Industries’ Ron Widup offered his own perspective on building political clout for wind power: “We have a three-pronged approach,” he said. “Jobs, jobs, jobs.”

Panelists spoke of the importance of wind, with its long-term power contracts, as fixed-cost insurance against volatile fossil fuel prices. Wind energy, Walsh said, is “a form of hedge because the fuel is free.”

“It’s about long-term stability,” echoed Tim Rosenzweig, CEO of Goldwind USA. “It’s a great way to lock in rates.”

Panelists were bullish on continued improvements in the industry, driving down costs in all facets from technology to finance and improving turbine performance via more output. Turbines are already lasting longer than their 15-20 year expected lifespan, and that durability “will continue to get better and better,” said Widup. That impacts everything from a project owner’s bottom line to the cost of capital for new projects, panelists noted.

The result is that the industry is advancing much more rapidly than anyone might have predicted. Susan Reilly, CEO of RES Americas, highlighted the initiative announced yesterday to expand on the DOE’s 2007 report showing the feasibility of 20 percent wind energy by 2030. “I think that’s incredibly exciting for all of us because we’ve achieved more than what we predicted when the study was first done,” she said.

After industry leaders shared their product’s benefits, corporate purchasers shared their firsthand experience with the product from the user perspective, and provided hard numbers to back up the benefits.

Lee Balin, Sustainability Manager for the Bloomberg’s Global Sustainability Group, reported that his company has saved $48 million since 2008, thanks to its renewable energy purchases and efficiency programs.

“We believe [in renewable energy] as a hedge against rising fuel prices in the future,” said fellow panelist Greg Butler, Global Supply Chain Stewardship Director in the Office of Global Sustainability at medical technology company BD.

For Walmart, energy is its second greatest controllable expense, said Greg Pool, senior manager for Renewable Energy and Emissions. Saving on renewable energy, therefore, helps the company’s bottom line as well as helping the company in its goal to be a good global citizen, and fostering clean air in communities where it operates—which in itself has economic benefits for the company, he noted. Walmart has set the bold goal of running on 100 percent renewable energy worldwide.

“Walmart is open for business to do wind deals,” Pool announced. He said the industry should look beyond utilities to diversify its customer base.

Direct purchases are also in the playbook of fellow panelist Google, which must power its electron-hungry data centers as cost-effectively as possible. The tech giant has entered into PPAs for wind and even invested in projects. It purposely seeks to locate its data centers in close proximity to renewable energy and renewable resources, said Gary Demasi, director of operations for data center location strategy and energy. The company has over 260MW of wind secured through long-term contracts and utility agreements. Available wind power and utilities willing to provide it, he said, “is becoming an increasingly important part of our consideration of where we go and where we grow.”

The conference also hosted more than 40 individualized sessions, including specialized technical training courses, presentation sessions, and panel discussions on a wide range of wind energy-related topics. A poster presentation and display drew submissions from more than 200 wind energy personnel.

In total, nearly 600 exhibitors and close to 10,000 attendees filled the WINDPOWER 2013’s classrooms and exhibition floor, a sign that despite current challenges, the wind energy industry and its personnel are committed upholding the industry’s new pledge toward sustainability, stability, and growth.

Next year’s event will mark AWEA’s 40th anniversary, and for the first time ever, the site for WINDPOWER 2014 will be in Las Vegas, next May 5-8. Booth reservations are now open and were selling briskly in Chicago. For details, see www.windpowerexpo.org/2014.

Airgenesis LLC’s Twin-Rotor Turbine Design Secures New Power Curve Validation
Airgenesis LLC, designers of a unique twin-rotor utility-scale wind turbine, has announced that its turbine design has received additional power curve validation and its fourth U.S. patent. Power curve data was created and validated by DARcorp. of Lawrence, Kansas.

Findings show that the double-blade configuration offset at 30 degrees allows the wake of the front rotor miss the aft rotor, producing a 92 percent yield. Increasing rotor speed achieved higher electrical production, at the same time reducing the torque on the drive train.

The Airgenesis double-blade configuration (described at length in the April 2012 issue of Wind Systems; https://windsystemsmag.com/article/detail/357/) is capable of producing an unprecedented 11 net MW per tower at 16.5m/s with a 48.8 meter blade length, eliminating any need for government subsidy.

In addition, the structure can begin to produce electrical energy at wind velocities of 3m/s.  Designed for maximum efficiency and ease of maintenance, the Airgenesis design results in a lighter and significantly less complicated nacelle.

Once installed, routine maintenance on Airgenesis’ towers can be performed at ground level including swapping out generators and clutch systems. The drive system features five 50-foot drive shafts maintaining vibration free operation due to its low rpms, expansion couplers, and thrust bearings.  All 12 of the turbine’s generators are located at ground level.

The design allows for the relocation of a large portion of the nacelle weight to the base of the tower. A sophisticated mechanical clutch system and integrated ratcheting generator mechanism have nearly tripled the energy production of a single tower.

“This is going to be a game changer for the wind industry,” said inventor Danny J. “Skip” Smith of Wheatland, Wyo., who  has invested nine years in perfecting Airgenesis’ technology. “It may very well make other wind energy turbines obsolete in comparison.”

Airgenesis LLC currently has more than 250 patents and patents pending encompassing 144 countries, and final engineering and CAD files are nearing completion.  Airgenesis LLC carries no debt and Smith maintains 87 percent ownership.

With engineering development reaching successful conclusion, Airgenesis LLC is currently seeking experienced industry investors/manufacturers to purchase part or all intellectual property including patents, patents pending, engineering drawings, and CAD files for unlimited U.S. and worldwide potentials.

For more information, visit www.airgenesiswind.com.

UpWind Solutions Awarded O&M Contract for BP Wind Farms in Texas and Kansas
UpWind Solutions, Inc., a third party provider of asset management operations and maintenance services for the wind energy, announced they have signed O&M agreements for the Trinity Hills, Sherbino 2, and Silver Star 1 wind farms in Texas and the Flat Ridge 1 wind farm in Kansas, representing a combined 485MW of production. The four wind farms are owned by wholly-owned subsidiaries of BP Wind Energy. The contract is for UpWind technicians to perform O&M services on a total of 194 Clipper C-93 and C-96 turbines.

“The geographic location of our regional hub in Sweetwater, Texas and five years of experience working with Clipper technology is what established UpWind Solutions as the strongest independent service provider candidate to take on the operations and maintenance of the four BP Wind Energy sites” UpWind Solutions’ CEO Peter Wells said.

The four wind farms provide the equivalent electricity needed to supply over 145,000 average American homes annually.

As part of the company’s overall approach, UpWind Solutions’ technicians not only complete rigorous and comprehensive technical training with a focus on maintaining the highest safety standards in the industry, they are also encouraged to find solutions that optimize wind turbine availability and performance for wind project owners, ensuring the highest return on investment.

For more information, visit www.upwindsolutions.com.

Romax launches InSight Health Management Platform predictive maintenance suite
Recently launched at WINDPOWER 2013, Romax Technology seeks to provide the wind energy industry with a complete health monitoring service suite with the InSight Health Management Platform predictive maintenance suite. Following on the heels of its April 2012 software launch, the company now offers comprehensive health monitoring services for a full lifecycle management approach.

InSight leverages Romax’s work over the last two decades in drivetrain design, analysis, monitoring and operations and maintenance technical services for onshore and offshore wind farms.

At present, there are many data sources and analytical capabilities, available to monitor different aspects of the drivetrain such as the gearbox and bearings for conditions such as vibration, oil analysis, wind direction and temperature. Romax’s world-leading software and services allow a combination of multi-source data to be unified on a single intuitive software platform. When combined with engineering consultancy and turbine maintenance services to ensure optimum turbine reliability and health, Romax provides maximum flexibility and helps mitigates risk.

This launch signifies the continuous development of InSight in response to an increasing industry need to monitor more than just the drivetrain. This means that failures will be detected up to six months in advance, allowing for preventative correction, rather than a costly, complete deterioration of the components.

Based on over two decades of operating in the industry, Romax has recognized a trend in the phases of wind farm health management maturity and has loosely segmented these phases into three groups – “fire-fighting”, “managed” and “predictive.” The InSight solution enables wind farm owners and operators to evolve from ‘fire-fighting’ through to managed and ultimately to an optimum state of predictive maintenance. The benefits of this are that operating costs are significantly reduced, maintenance and investment planning improved and revenue and energy generation is increased.

“This latest version of the InSight software combined with our engineering service team will change the way you view your turbine’s health” Romax CEO Dr. Peter Poon said. “We are expanding the value and breadth of knowledge we can provide to our customers through an advanced and integrated system approach that works openly with your existing systems. With our expert engineering knowledge and visibility of turbine health, we can recognize warning signs early enough to reduce downtime, and costs to optimize reliability.”

For more information, visit www.romaxtech.com.

GL Renewables Certification Publishes New CMS Certification Guideline
GL Renewables Certification has published its new “Guideline for the Certification of Condition Monitoring Systems for Wind Turbines.” This guideline was compiled in cooperation with its Wind and Marine Energy Committee. The new guideline, presented at WINDPOWER 2013, covers the most important requirements for the development, installation and operation of condition monitoring systems.

Obtaining economical wind turbine operations is vital in light of initial wind farm development costs. Operators and manufacturers aim to achieve wind turbine availability of more than 97 percent. In order to reduce unplanned downtime, increase availability and to reduce maintenance costs, continuous condition monitoring of wind turbines is indispensable.

The updated guideline reflects the latest developments of condition monitoring systems for wind turbines and future requirements.

“Currently almost all condition monitoring systems are operating independently from the control system and almost exclusively the drive train is monitored,” GL RC CMS expert Dr. Karl Steingroever said. “Future condition monitoring systems will be partly or fully integrated into the control system and include the monitoring of the entire wind turbine. In tsxhis context the definition of interfaces between the systems is playing a major role.”

GL RC’s guidelines and technical notes are discussed in its Wind and Marine Energy Committee before publication. The Wind and Marine Energy Committee represents the relevant groups in the field of wind energy. The members of the committee are drawn from public authorities, wind turbine and component manufacturers, engineering consultants, institutes, universities, technical associations and insurance companies.

For more information, visit www.gl-group.com/en/certification/renewables/.

Women of Wind Energy Annual Luncheon Recognizes Honorees
The winners of the Women of Wind Energy (WoWE) Awards were announced May 8 at the annual luncheon held at WINDPOWER 2013. The awards spotlight both prominent industry professionals as well as up-and-coming next generation of leaders.

WoWE continued its tradition of bestowing the “Women of Wind Energy Champion” award on a recipient who has repeatedly gone above and beyond to encourage and support the career development of women in the wind energy sector. All three winners provide examples of achievement, creativity and courage.

“Highlighting and recognizing the stories of incredible women and men like this year’s WoWE Annual Award winners is critical not only to recognizing and appreciating their successes but also to help other women in the sector see role models and new career pathways,” said Kristen Graf, WoWE Executive Director.
2013 WoWE Award Recipients:

• Champion Award—Ian Baring-Gould, Senior Mechanical Engineer at the National Renewable Energy Laboratory, received WoWE’s Champion Award. This newest award seeks to honor an individual who has significantly contributed to the advancement of women in the field of renewable energy through mentoring and professional development that goes beyond expectations as well as advocating for work place practices that support women in a manner that fosters a path forward. Since the inception of WoWE, Ian has been an active participant in the organization and its membership. Through his involvement with the Wind for Schools program, he has consistently encouraged the involvement of young women. Ian has a deep understanding of the importance of engaging kids at a younger age, particularly girls. Ian is at the forefront of educating the next generation of wind consumers and professionals. As a result of his passion, creativity and commitment, countless women have been inspired and involved in this exciting industry.
• Woman of the Year—Lucille Olszewski was honored with the Woman of the Year Award. Each year WoWE seeks out a woman who stands out for her leadership, inspiration, and significant contributions to the expansion and improvement of renewable energy completed at the highest level. Lucille first started in the wind industry in the mid-1980s as a young meteorologist with one of the largest wind manufacturers and developers in the world. During her 30 years in the industry Lucille has developed a keen eye on all fronts having assessed over 9,000MW of planned capacity and sited over 14,000 wind turbines. She has pioneered radio telemetry systems for meteorological data collection and was the first to use cell phone technology for data collection. Lucille is highly respected among her peers and known for her integrity.
• Rising Star—Dr. Julie Lundquist, Assistant Professor at the University of Colorado at Boulder with a joint appointment at the National Renewable Energy Laboratory, was honored with the Rising Star Award. This award is given to a woman who is relatively new to wind energy and is already making significant professional contributions by demonstrating unusual talent, dedication and innovation in her renewable energy work. During her time at Lawrence Livermore National Lab (LLNL), Julie’s research moved ahead the frontier of wind energy forecasting. She has studied the effects of atmospheric stability and turbulence on wind energy forecasting and wind turbine performance. This research has revealed relationships in the atmospheric boundary layer that affect wind resource assessment and wind energy forecasting. Her work extends beyond traditional academia into how the industry can revolutionize wind resource assessment. 

For more information, please visit www.womenofwindenergy.org. 

Cost effective condition management is one of the key ways to optimize the overall drivetrain performance and successfully achieve the estimated energy production in wind turbine generators. Wind turbine gears are exposed to extreme changes, both in terms of load and environmental circumstances. A drivetrain failure can mean prolonged down-time and very expensive major component repairs or replacement work. Therefore, it is essential to develop condition monitoring systems and analyzing methods that are truly comprehensive and relevant to the operating system.

Few condition monitoring systems have been developed specifically for wind turbine drivetrains. It is critical that these systems incorporate precise measuring instruments and highly automated analyzing tools. One single service engineer must be able to accurately provide information about each possible upcoming failure based on the available instruments and analyzing tools while closely monitoring hundreds of wind turbines. The most challenging components to monitor are the slow rotating machine parts in the drivetrain.

Traditional condition monitoring systems focus mainly on measuring vibrations from rotating components. For proactive results it is no longer sufficient to simply perform vibration analysis. This needs to be done in combination with oil condition and particle contamination analysis.

Proactive Versus Reactive Maintenance Strategies in Mixed Generation Grids
Electrical power generation is a critical resource and is managed and regulated by many federal organizations like NERC, FERC, and independent grid-system operators to ensure supply and demand management, optimal power quality, and uninterrupted supply. With many new sources of power generation being added to grids coming from wind and other renewable plants, effective management of generation forecasting by asset owners is critical. Nuclear asset operators are required to maintain detailed plans for asset monitoring, predictive maintenance, and long term maintenance plans. First quartile performing thermal asset owners also have well developed strategies to analyze data and plan maintenance activities into future planned maintenance outages. Thermal asset operators do have an advantage over renewable asset operators: location of generating assets. There is a large disparity in physical layout and original equipment instrumentation between traditional thermal power generation assets and renewable assets. Thermal assets are housed most commonly in a cluster of buildings and structures located more conveniently in a well-defined area and sub-systems are normally enclosed from the elements. In many cases, wind plants can stretch over thousands of acres/hectares and have hundreds of individual generating assets in one location. Given this more challenging plant layout and associated terrain based and seasonal access challenges, it is critical for owners of wind power generating assets to secure instrumentation to monitor “critical to failure” operational performance data and create proactive or predictive maintenance strategies to control costs of operation, optimize capacity factor, and generation revenue and provide accurate day-ahead generation bids to their grid operators. Wind generating asset operators seeking first quartile cost per megawatt and revenue optimization must incorporate not only hardware and software to collect data and stream it to a central location, but they must also have the necessary resources to trend and analyze this data. More importantly, the data must be converted from strictly technical form to more digestible presentation quality to facilitate dialogue and quick action when required.

The Challenge
More often than not, the level of accuracy and cost effectiveness comes in to play when condition management of wind turbine gears is considered. The continuous surveillance of a wind turbine gear fleet must be automated and optimized because one analyst must be able to monitor hundreds of systems and, at the same time, every failure has to be found. A wind turbine drivetrain can face several different failure modes, which require extensive and reliable instrumentation.

The combination of oil metal contamination and vibration analysis has been previously studied on machines with high rotation speeds. However, there are many challenges in monitoring machines with low rotational frequencies and especially those with intermediate speed gears. Low rotation frequency demands a relatively lengthy measuring time, which can be very challenging due to limited memory of the measuring device. It is difficult to achieve a proper time domain signal from a wind turbine where the rotation frequency varies during the measurement. Varying load and rotation frequency is problematic when calculating the Fourier transform as regular impacts do not show up clearly in the spectrum. With low rotation speeds the energy of impacts is low and therefore difficult to differentiate from the background noise of the vibration spectrum.

Oil contamination can provide valuable information about the existence of a failure but it does not say anything about the root cause. This is why we need a combination of oil contamination and vibration analysis. With low rotation frequencies, vibration levels do not rise above alarm limits but do provide valuable information about the root cause of a failure when that failure is initially detected with oil contamination surveillance.

Hardware
In the following case, the condition of the various gears within the gearbox is monitored using the Moventas condition management system (CMaS). This is a remote monitoring system specifically designed for wind turbine gears. CMaS is developed to monitor a large number of wind turbine gears very effectively by using sophisticated analyzing methods, algorithms, and intelligent sensors. These sensors are used to recognize possible damage to gears and other mechanical components in advance, before they start to affect and interfere with the turbine operations. When the sensor has processed and analyzed the data, it is then transferred to the CMaS main unit. Data transfer is conducted over a field bus. This means that in a normal operation mode, only key parameters are transferred. Spectrum and time domain data is also transferred regularly, but daily analyses are based on monitoring key parameters. When something outside the norm occurs, more-detailed data and raw signals are transferred. It is very important to successfully transfer key figures without losing any information. When daily monitoring is based on the analysis of key figures, monitoring is extremely cost effective.

In this particular case, the monitoring system has three Moventas intelligent vibration sensors (IVS-20), which all measure the spectrums, time domains, and key values in all three directions. Also included is a GasTOPS MetalSCAN full flow oil particle counter and other sensors for measuring temperatures and oil quality. Because wind turbine nacelles are a dynamic environment and have limited access for maintenance and troubleshooting, the connections between sensors and signal processing units must be durable and reliable to ensure maximum equipment up availability. Every nacelle and drive train has a specifically engineered configuration of sensor locations, cable routings, and processing units. Heavy duty, signal grade cabling is used to connect all devices. Connections at the processing units for oil and vibration data are heavy duty mil-spec components. Uninterrupted transmission of data is achieved through the use of mobile broadband technology. Mobile broadband SIM cards are employed to transmit data to surveillance centers. The use of mobile broadband also avoids any potential conflicts with access to turbine controller fiber communication access, and prevents the possibility of compliance issues with regulations pertaining to power generation cyber security. The flexibility of the CMaS unit does allow users the option to between broadband connectivity and their own turbine level fiber connection to fit their surveillance and data collection needs.

Analysis Methods
Daily monitoring routines (Figure 1) consist of checking each instance where the alarm has exceeding the alarm limit in every gear that is monitored. If abnormal behavior is found, a closer inspection is performed to check for patterns and trends of those measurements. Spectrum and time domain analysis are used for finding the root cause of a possible failure.

Intermediate Speed Gear
This example will concentrate on remote monitoring of a medium speed gearbox (Figure 2), having one planetary gear and one helical stage gear. The ratio of this gearbox is 1:28 and the nominal rotation speed of the output shaft is 464 RPM.

Case Study: Broken Tooth in Intermediate Speed Gear
At the beginning of December 2011, an unexpected rise in oil particles was detected with alarm limit monitoring (Figure 3). At the same time, there was no change in overall vibration levels or bearing temperatures (Figure 4).

Closer analysis of vibration spectrums indicated that there was a possible failure in the intermediate speed gear. This can be seen in the velocity spectrum (Figure 5), where there are side bands on both sides of the rotation frequency of the medium stage.

This provided the first clue that there was a failure in the intermediate stage, which was verified by analyzing the time domain signal where there were clear hits on the spectrum (Figure 6).

As soon as the root cause of this failure was identified, the client was immediately informed and a Moventas’ Field Service Engineer was dispatched to make a visual inspection of the gear. During the visual inspection, a fractured tooth was found on the intermediate speed gear. The tooth was not yet broken, but it was cracked (Figure 7).

Conclusions
In this case, a failure would not have been found if vibration analysis was the only option in the condition monitoring system because the vibration levels remained steady despite the failure. Nevertheless, the root cause of the failure would be impossible to find without vibration measurements and spectrum and time domain analysis.

For more information, visit www.moventas.com. 

Abstract
This computational work investigates planetary gear load sharing of three-mount suspension wind turbine gearboxes. A three-dimensional multibody dynamic model is established, addressing gravity, bending moments, fluctuating mesh stiffness, nonlinear tooth contact, and bearing clearance. A flexible main shaft, planetary carrier, housing, and gear shafts are modeled using reduced degrees-of-freedom through modal condensation. This drivetrain model is validated against the experimental data of the Gearbox Reliability Collaborative for gearbox internal loads.

Planet load sharing is a combined effect of gravity, bending moment, bearing clearance, and input torque. Influences of each of these parameters and their combined effects on the resulting planet load sharing are investigated. Bending moments and gravity induce fundamental excitations in the rotating carrier frame, which can increase gearbox internal loads and disturb load sharing. Clearance in carrier bearings reduces the bearing stiffness, and thus the bending moment from the rotor can be transmitted into gear meshes. With bearing clearance, the bending moment can cause tooth micropitting and can induce planet bearing fatigue, leading to reduced gearbox life. At low input torque, planet bearings are susceptible to skidding. At rated torque and beyond, planet bearings are at risk of fatigue.

Introduction
Wind turbines have traditionally experienced premature gearbox failures [1]. The cost of gearbox rebuilds, as well as the down time associated with these failures, has elevated the cost of wind energy. The National Renewable Energy Laboratory Gearbox Reliability Collaborative (GRC) was established by the U.S. Department of Energy in 2006; its key goal is to understand the root causes of premature gearbox failures and improve their reliability using a combined approach of dynamometer testing, field testing, and modeling [2]. A major modeling activity of the GRC is to evaluate assumptions and uncertainties in current design practices that could affect gearbox reliability. As a part of the GRC program, this paper investigates planetary gear load-sharing in three-mount suspension wind turbine drivetrains that affects the load path and gearbox component life.    

Compared to parallel axis gears, planetary gear systems provide high power density by splitting the input torque into multiple, parallel sun-planet and ring-planet load paths. Planetary gear systems are commonly used in wind turbine drivetrains and they use nearly or exactly equally-spaced planet gears, theoretically leading to equally shared loads at each planet. However, in reality planet gear loads are not always equally shared among planets [3,4,5,6,7,8,9,10,11]. With unequally shared loads, planet bearing forces increase, leading to reduced bearing life and potential premature failure. Planetary gear load sharing is an important design parameter for drivetrain reliability. The degree of unequal load sharing has implications for tolerance schemes and gearbox loads.

Early studies by Hidaka and Terauchi [3] investigated disturbed load sharing by manufacturing and assembly errors on a Stoekicht planetary gear. This unequal load sharing was not significant when the mesh frequency was lower than 1000 Hz. Hayashi et al. [5] developed a method to measure the planet gear shearing stress and studied the influences of gear tooth profile error and eccentricity on load sharing. Ligata el al. [6] investigated the effect of pin position error on planet load sharing and a method for computing the planet load sharing from root strain-time histories was proposed. Singh [12] found that the tangential pin position error has a greater effect on the load sharing compared to the radial error. It was also shown that the sensitivity to pin position error increases as the number of pinions in the planetary gear set increases [6,12]. Singh [13,14] developed a formulation to estimate load sharing considering the unequal planet spacing for three to seven planetary gears. The analytically predicted load sharing factor was compared against a finite element analysis using the program developed by Vijayakar [15].

Various techniques have been investigated to improve load sharing. Studies show using a flexible ring gear improved load sharing when a number of manufacturing and assembly errors were present [4,16]. Kahraman et al. investigated the effects of ring gear flexibility on planetary gear loads using a finite element model and experiments and found contradictory results [10,17]. They found that adding flexibility to the ring gear was not as effective as a floating sun in improving load sharing. Kahraman [9] developed a two-dimensional lumped-parameter model to calculate tooth and bearing loads of planetary gears. The study also investigated the effect of a floating sun on load sharing and found it did not improve disturbed load sha

This aforementioned research on load sharing is limited to unequally spaced planets due to manufacturing errors and eccentricity. Input torque is considered as the only applied load to these planetary gears in prior studies. Nearly all horizontal-axis wind turbine gearboxes carry various combinations of input torque and non-torque loads. The non-torque loads include bending moments caused by the rotor weight and tower shadow, wind induced moments, moments caused by the controller, thrust, etc. Three-mount suspension drivetrains studied by NREL GRC show significant bending moment on the main shaft, which is mainly caused by the rotor weight and aerodynamic forces [2]. This bending moment has the same order of magnitude as input torque [2,18]. Other wind turbine designs adopt a two main bearing configuration to reduce the transmission of these non-torque loads into gearboxes. These turbines still have a small amount of bending moment present on the main shaft.

The measured load spectrum of the GRC turbine considering various wind and weather conditions is shown in Figure 1. Over 90% of the time, the GRC wind turbine operates below rated torque. Within the entire load spectrum, the turbine operates below 10% of rated torque over 50% of testing period. Gearbox reliability at low input torque has received little attention in the past because prior research has mainly focused on the low bearing and tooth loads with low input torque. Influences of bending moment are not considered for the low load conditions.

With the increasing size of wind turbines, gravity induced excitation in the rotating carrier frame becomes an important vibration source. Gravity led to tooth wedging, a potential source for premature planet bearing failure, in a wind turbine planetary gear investigated by Guo and Parker [19]. Clearance is introduced in rolling element bearings to account for thermal expansion, interference fit, and surface roughness during operation. Clearances in wind turbine bearings are large and can cause gear tooth misalignment, leading to

This study investigates the combined effects of gravity, bending moment, bearing clearance, and input torque on planetary load sharing of three-mount suspension wind turbine gearboxes. Gearbox internal loads at low input torque–a wind turbine operating condition that is rarely considered–are also studied under effects of bending moment.

GRC Wind Turbine Drivetrain Description
This study investigates the 750 kW turbine drivetrain used by the NREL GRC. This drivetrain has a spherical roller main bearing that supports the main shaft and rotor weight, and two trunnion mounts that support the gearbox. The gearbox includes a helical planetary stage with three equally-spaced planets and two parallel stages, with stage ratios of 5.71, 3.57, and 4.00, respectively. The rated input speed is 22.2 rpm. A cut-away view of the gearbox is shown in Figure 2. There are two parallel carrier bearings supporting the carrier. Each planet gear is supported by two identical cylindrical bearings. The sun shaft is connected to the intermediate shaft through a spline connection with 300 μm backlash to partially float the sun. The ring is bolted to the gearbox housing. This drivetrain configuration represents the majority of the existing three-mount suspension wind turbine drivetrains. Key parameters of the planetary section are listed in Table 1 and Table 2 (Appendix). Additional details of this gearbox are described in the literature [22].
 
Experimental Setup and Instrumentation
The GRC project instrumented two identical 750 kW wind turbine gearboxes for dynamometer (Figure 3) and field testing. Internal measurements include gear tooth loads, main shaft torque and bending, internal component deflections and misalignments, and planet bearing loads. The full description of instrumentation is detailed in [2].

Main shaft torque and bending were measured using three sets of strain gauges in full bridge arrangements. These measurements are taken near the center of the main shaft between the main bearing and gearbox [2]. The shaft torque and bending measurement serves as a reference for the input load being applied to the gearbox from the rotor side, and can also be used as the time series input for dynamic simulations. The bending moments caused by the blades and hub weight and aerodynamic forces were measured in the field tests, and are 29.2% to 64.6% of rated input torque [2].

For each planet bearing, three axial slots were machined into the inner diameter of the inner ring and instrumented with strain and temperature gauges [2]. Two of the slots were located at different locations in the bearing load zone for each planet, and the third slot for every bearing was oriented 90° from the sun-planet axis, referred to here as top dead center (TDC). Two gauge sets in each TDC slot and two bearings on each planet provided an axial distribution of four radial loads along each planet pin. These gauges were calibrated to loads applied to the fully assembled planet pins and bearing pairs in a bench top test setup [23].

Dynamic Drivetrain Modeling
The three-dimensional dynamic model of the wind turbine drivetrain is established in SIMPACK [24]. This model includes the main shaft, low speed planetary gear, planetary carrier, and housing as shown in Figure 4(a) and Figure 4(b). The parallel stages are not included in this study.

Gears are modeled as rigid bodies with six degrees-of-freedom x, y, z, θx, θy and θz. The gear contact analysis considers the time varying tooth meshing when the gears rotate by modeling the fluctuating mesh stiffness according to the AGMA 6006 standard [25]. The gears are modeled using a slicing approach to determine the load distribution across the gear tooth facewidth, which accounts for tooth profile and lead modification, tooth contact loss, and fluctuating mesh stiffness. The tooth load distribution of the sun gear teeth with tooth modifications is shin Figure 5. This model also considers shuttling contact and sliding friction forces [26].own

 Bearings are modeled using diagonal stiffness matrices. Radial bearing clearance is included in the model as shown in Figure 6. Radial forces develop only when the relative displacement between the connected bodies exceeds a specified clearance. For instance, the radial forces of the carrier bearings are

(1)

where  are the bearing stiffnesses.  are the displacements of the carrier and housing. The variable   tracks if the bearings are in contact according to

(2) 

The model includes the flexibilities of the main shaft, planet carrier, shafts, and gearbox housing, which are important structural components in the drivetrain that could affect gear dynamics. The gearbox front and rear housings and planet carrier are included as reduced finite element flexible bodies. Finite element models are created in Abaqus and reduced using the Craig-Bampton method [27]. Master degrees-of-freedom, referred to here as “nodes”, are selected at each interface point. For the planet carrier, nodes at the main shaft connection, upwind and downwind carrier bearing connections, the planet-pin upwind and downwind bores, and planet pins are retained for the drivetrain model. Housing nodes were retained at each bearing location, the ring gear interface center, and the yoke mount centers. All the eigenfrequencies below 1,000 Hz of the housing and carrier are included for the super element creation in Abaqus. This cut-off frequency (1,000 Hz) is selected to include higher than 25th harmonic of the planetary stage mesh frequency at rated speed. After creating the super elements and performing modal analyses on the reduced matrices, the flexible bodies are imported into the multibody drivetrain model. The ring is modeled as a rigid body due to software limitations. Shafts are modeled using beam elements.

The sun spline connection is modeled using its diagonal stiffness matrix obtained through a finite element analysis in RomaxWIND [28] (Table 4 in the Appendix). Spline facewidth, crowning, applied load, and surface lubrication affect these stiffnesses. Trunnion mounts are modeled using diagonal stiffness matrices extracted from the measured load-displacement curve [2].

Model Validation by Experiments
A steady state dynamic analysis is carried out on the established model using the Newmark method at the rated speed. A step size of 1 ms is selected to include the first 10 harmonics of planetary gear mesh frequency. All components are allowed to vibrate. An average of 5% modal damping is used. Applied torque to the main shaft ranges from 35 kNm to 350 kNm. The measured bending moment is applied to the carrier.

The load sharing factor often refers to the ratio between the maximum and the average bearing force among planets [29] .

(3) 

where  is the planet i bearing force that includes the upwind and downwind rows and N is the number of planets.
 
This load sharing factor defined in Eq. (3) is a function of time. This study uses the maximum value of   over time, defined as

(4) 
   
The computational results are compared to the measured planetary load sharing in Figure 7(a) and Figure 7(b). Results show the load sharing factor exceeds 1.1 with disrupted load sharing. The calculated load sharing among planets agrees with the measured data in general. The fluctuating amplitude of load sharing factor is slightly different between the computed results and measurements. The measured shaft moment is applied to the carrier in the drivetrain model. Due to the main shaft weight, the shaft moment measured at the carrier during the experiment is about 15% lower than that at the measurement location. Different excitation sources during the measurement and simulation also contribute to this difference. Figure 8 shows the frequency spectrum of the measured planet bearing force. Measurement data is filtered with low sampling frequency that did not capture mesh frequency 36.7 Hz and its higher harmonics. The drivetrain model does not include the bearing excitation due to its stiffness variation with roller kinematic rotation that was observed during the experiments as shown in Figure 8.

The maximum load of planet 2 over a carrier revolution is higher than the other planets due to the asymmetry of planet spacing during operation, which is seen in both measured and calculated results (Figure 7). This planet asymmetry is caused by planet bearing clearance and tangential pin position error.

Tooth loads at the sun-planet and ring-planet meshes create significant tangential bearing forces under operating torques, which have the same order of magnitude as tooth loads. Planet bearings are in contact when gears are loaded under most operating conditions except at vibration resonances with tooth contact loss [21]. Planet bearing clearance, however, disturbs the planet symmetry by introducing tangential position error, leading to asymmetric shared loads among planets over time as shown in Figure 7 (a) and Figure 7 (b). With identical planet bearing clearance, planet bearing forces have identical shapes with only phase differences. The measurements in Figure 7 (a) are achieved by introducing planet bearing clearances in the drivetrain model (Figure 4). Bearing clearances in planets 1, 2, and 3 are 179 μm;  80 μm; and 179 μm, respectively that are chosen based on the measured clearances by the GRC project [30]. Figure 7 (b) can be regenerated by disturbing the mounting angles of planets by, where  is the center distance and  is planet bearing clearance. Having a planet bearing clearance different from other planet bearings has the same effects on load sharing as tangential pin position error.

• Results and Discussion Excitations Caused by Gravity and Bending Moment

With a rotating planetary carrier, the gravity force on a planetary gear is periodic in the rotating carrier frame. The gravity force vector is
    
(5) 

where  denote the gravity forces of the carrier, ring, sun, and planet, respectively.  is the carrier rotation frequency and  is the bedplate tilt angle.
The bending moment vector in the rotating carrier frame is

(6) 

The effects of gravity and bending moment on tooth loads are shown in Figure 9. Without considering gravity, tooth loads at different sun-planet meshes are nearly identical and time-invariant. Gravity causes the fluctuation of tooth loads by disturbing the system symmetry. The tooth load at the  sun-planet mesh has a phase difference of  compared to that at the first sun-planet mesh. When the bending moment is considered, the tooth load fluctuation becomes significant and the maximum tooth load increases 11% compared to its nominal value. The high frequency content (99× and 198×) of the tooth load with bending moment in Figure 9 is caused by fluctuating mesh stiffness that is the dominant internal excitation.

Figure 10 shows the effects of gravity and bending moment on tooth load distribution. Without gravity and bending moment, tooth contact has a time-invariant, parabolic shape as shown in Figure 7(a). When gravity is included, the gear teeth carry the carrier weight, leading to a disturbed tooth load distribution. This tooth contact pattern changes periodically about carrier frequency. With bending moment, the load distribution is poor and the gear teeth have edge loading by carrying the bending moment and carrier weight. Excitations caused by gravity and bending moment in the rotating carrier frame are unavoidable for wind turbine gearboxes mounted in the horizontal axis. Bending moment plays an essential role in gearbox internal loads compared to gravity and the excitation from gear meshing.

Effect of Carrier Bearing Clearance on Load Transfer Path
In three-mount suspension drivetrains, carrier bearings are designed with a high load capacity to reduce the gearbox load sensitivity to non-torque loads such that gears only carry torque loads. Carrier bearing forces are one order of magnitude smaller than planet bearing forces because they are nearly balanced by almost self-equilibrating tooth loads at the sun-planet and ring-planet meshes. When the bearing clearance is larger than the carrier motion, the carrier bearings are not in contact during operation. Consequently, the gear teeth carry the non-torque loads and the carrier weight instead of the carrier bearings, resulting in cyclic tooth loads similar to Figure 9, Figure 10(b), and Figure 10(c). Clearance in carrier bearings plays an important role in this load path.

The variation of the load sharing factor (defined in Eq. (3))  with carrier bearing clearance is shown in Figure 11 . The bearing clearance is nondimensionalized by dividing the motion amplitude of the carrier with infinite bearing clearance. When the bearing clearance is larger than the carrier motion, the load sharing factor converges to a constant, which indicates a limiting system with infinite bearing clearance. The load sharing factor increases 2% when carrier bearing clearance increases from 0 to 1 with gravity. With bending moment and gravity, the load sharing factor increases from 1.02 to 1.13 when bearing clearance increases. Without bearing clearance, the load sharing is nearly ideal. Reducing or eliminating this bearing clearance reduces gearbox load sensitivity to non-torque loads and improves load sharing.

The recommended bearing clearance within C1-C4 range according to AGMA 6006 standard [31] is larger than the carrier motion for the pure torque case. With the recommended bearing clearance, carrier bearings are lightly loaded or can be entirely out of contact during operation, leading to a reduced load carrying capacity. Calculating the nondimensional bearing clearance helps design the appropriate radial clearance for increasing the load capacity during operation.
 
Effects of Input Torque on Gearbox Internal Loading
The highest torque measured during a braking event in the field testing of the GRC gearbox is 189% of input torque [18]. Considering bending moment, results suggest that planet bearing loads at high input torque exceed the fatigue limit, which could lead to premature bearing failure.

When input torque is low, bending moment plays a dominant role in gearbox internal loads. Planet bearing loads of upwind and downwind rows at 5% torque are shown in Figure 12. The upwind planet bearing is heavily loaded compared to the downwind bearing over 50% time within a carrier cycle. The upwind bearing load reaches a maximum of 9.7 times the average load of the downwind planet bearing when the planet gear aligns with –y direction (Figure 12(a)). At this instant, the upwind planet bearing carries the majority of the bending moments while the downwind planet bearing carries much less. When this planet is moving away from the –y axis and the next adjacent planet is moving towards -y, the load sharing of the prior planet at the upwind and downwind rows is nearly equal. This unequal load sharing between upwind and downwind rows of planet bearings was observed during the testing. Figure 12(b) shows the measured planet bearing loads and tilting angle (initially located at 210 degree counter-clockwise from -y) at rated torque. The upwind bearing load reaches the maximum when this planet aligns with -y, which agrees with the model prediction on the locations with highest upwind bearing loads as shown in Figure 12(a). This agreement qualitatively validates the model for capturing the load sharing between the upwind and downwind row bearings.

As shown in Figure 12(a), planet bearing loads are lower than the required minimum load for preventing rolling element sliding. Planet bearings are scontactusceptible to skidding that compromises the rolling integrity of the bearing, leading to a reduced bearing life. Typical planet bearing damage caused by skidding motion is shown in Figure 13. There is no skidding damage observed in the GRC gearbox because the gearbox was removed from service (due to other failure modes) long before skidding damage could develop.

Under the same loading condition of Figure 12(a), the tooth load distribution of the first ring-planet mesh is shown in Figure 14 When the upwind planet bearing carries majority of bending moments (carrier cycle from 7.5 to 8 in Figure 12 (a)), the gear tooth has edge  towards upwind side and partial contact loss towards downwind side as shown in Figure 14. Unequal planet loads between upwind and downwind rows cause tooth edge loading.

Combined Effects on Load Sharing     
Planetary gear load sharing relies on gravity, bending moment, bearing clearance, and input torque. Among them, bending moment and input torque are decisive parameters. A nondimensional quantity M=M_y/T_in introduced here to consider the combined loading condition, where My is bending moment and Tin is input torque. Figure 15 shows the planetary gear load sharing with various values of M. Gravity and bearing clearances are included in the results. Load sharing is computed at 10% and rated torques. The load sharing-bending curve at 10% torque is higher than that with rated torque because the influence of gravity on load sharing factor is relatively higher at low input torque. Bending moment increases load sharing factor nearly linearly. A large bending moment induces nonlinear tooth contact, particularly at low input torque. At 10% torque, gear tooth load contact changes from slightly disturbed load distribution when the nondimensional bending moment is less than 0.02, to edge loading when it is larger than 0.02 and less than 2, and eventually reversing contact when it is larger than 2. With edge loading and reversing contact, gear teeth lose contact and reengage repeatedly over time. The time-varying tooth contact causes dynamic forces on planet bearings that could result in acceleration and deceleration of roller elements in entering and leaving the contact zone, thereby reducing bearing life.

Gear tooth edge loading has been observed during the GRC experipwments as shown in Figure 16. Micropitting induced by the edge contact was evident at the uind side of on ther ring gea teeth, which agrees with the model predictions in Figure 14 and Figure 15. As found in both simulation and experiment, it is clear that the shaft bending moment has been transmitted from the rotor to the gears, which suggests the design assumption that non-torque loads are uncoupled from gearboxes is not valid.

Conclusions
A multibody dynamic drivetrain model was established to investigate planetary gear loads in wind turbine drivetrains. This model includes gravity, bending moments, tooth modification, fluctuating mesh stiffness, tooth contact loss, and bearing clearance. Unequal load sharing among planets and between upwind and downwind rows of each pair of planet bearings predicted by the established model agrees with the GRC dynamometer measurements. Planetary gear load sharing in three-mount suspension drivetrains depends on gravity, bending moment, bearing clearance, and input torque:

• Bending moments and gravity are fundamental external excitations for wind turbine planetary gears mounted in the horizontal-axis, leading to cyclical loading on gear teeth and planet bearings. They have stronger influences on planetary gear loads than fluctuating mesh stiffness. Bending moments cause unequal load sharing between upwind and downwind planet bearings, which can cause abnormal tooth contact consisting of tooth edge loading, partial contact loss, and reversing contact, resulting in tooth micropitting.
• Clearance in carrier bearings affects the gearbox load sensitivity to non-torque loads. With carrier bearing clearance, bending moment can transmit into gear meshes and planet bearings, leading to a reduced gearbox life. Planet bearing clearance has a similar effect on load sharing as tangential pin position error. It disturbs the symmetry of planets, leading to asymmetric bearing loads.
• At low input torque, the effects of bending moment on gearbox internal loads are dominant. Upwind planet bearing loads can be an order of magnitude higher than the downwind bearing loads. Planet bearings, in particular the downwind row, are at risk for skidding.

Acknowledgements
The authors thank Mark McDade, Brian McNiff, and Jeroen Van Dam for providing the pictures; the GRC team for valuable discussions throughout this study; Robert Solomon from SIMPACK, Inc. and Dr. Ashley Crowther from RomaxWIND Inc. for providing the software. The Gearbox Reliability Collaborative (GRC) project at the National Renewable Energy Laboratory is funded by the Wind and Water Power Program of the United States Department of Energy.
 

References
[1] W. Musial, S. Butterfield and B. McNiff, “Improving wind turbine gearbox reliability,” Milan, Italy, 2007.
[2] H. Link, W. LaCava, J. V. Dam, B. McNiff, S. Sheng, R. Wallen, M. McDade, S. Lambert, S. Butterfield and F. Oyague, “Gearbox reliability collaborative project report: findings from phase 1 and phase 2 testing,” NREL technical report: NREL/TP-5000-51885, 2011.
[3] T. Hidaka, Y. Terauchi and K. Nagamura, “Dynamic behavior of planetary gear (1st report, load distribution in planetary gear),” in Bulletin of JSME, 1976.
[4] A. Kahraman and G. W. Blankenship, “Gear dynamics experiments, part ii: effect of involute contact ratio.,” in ASME Power Transmission and Gearing Conference, San Diego, 1996.
[5] T. Hayashi, Y. X. Li, I. Hayashi, K. Endou and W. Watanabe, “Measurement and some discussions on dynamic load sharing in planetary gears.,” in Bulletin of JSME, 1986.
[6] H. Ligata, A. Kahraman and A. Singh, “An experimental study of the influence of manufacturing errors on the planetary gear stresses and planet load sharing,” Journal of Mechanical Design, vol. 130, pp. 041701-1, 2008.
[7] H. W. Muller, W. G. Mannhardt and J. H. Glover, Epicyclic Drive Trains: Analysis, Systhesis, Synthesis, and Applications, Detroit: Wayne State University Press, 1982.
[8] D. L. Seager, “Load sharing among planet gears,” in SAE Technical Paper, 1970.
[9] A. Kahraman, “Load sharing characteristics of planetary transmissions,” Mechanism and Machine Theory, vol. 29, no. 8, pp. 1151-1165, 1994.
[10] A. Kahraman and S. M. Vijayakar, “Effect of internal gear flexibility on the quasi-static behavior of a planetary gear set,” Journal of Mechanical Design, vol. 123, no. 3, pp. 408-415, 2001.
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**2011.Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314.  Statements presented in this paper are those of the authors and may not represent the position or opinion of the AMERICAN GEAR MANUFACTURERS ASSOCIATION.

Modern wind turbines can provide large amounts of electricity, cleanly and reliably. Nearly eight percent of electricity in the state of Texas comes from wind energy, and Germany, Spain, Portugal, Denmark, and Ireland now obtain more than 10 percent of their electricity from wind energy, according to the American Wind Energy Association (AWEA)[1]. The cost of producing wind energy is as competitive as any other new power source.

Because the Global Wind Energy Council (GWEC) predicts that off-shore wind farms can be installed and operational in less than two years[2], wind energy is also proving to be a quicker return on investment compared with other “green” energy sources. As a result, estimates from the GWEC indicate that investment in wind energy will reach $196.7 billion by 2020.[3]

Wind power’s unique features—including large turbines and remote locations where wind farms are built—both contribute to its success and pose challenges for maintenance and repair. However, with the right planning and an experienced partner, these obstacles can be overcome to quickly maximize the benefits and ROI.

Unique Maintenance Challenges Can Lead to Potential Problems
Wind turbines are large and immense equipment—five times larger than they were 10 years ago.[4]
The largest wind turbines have blades that are 415 feet long and weigh 25.5 tons.[5] These massive turbines have the capability to produce 15 times as much electricity as ones that were built a decade ago.[4] Even though turbines in offshore wind farms require service only once or twice per year[6], their sheer size and remote locations pose unique challenges for routine maintenance. However, if the equipment is not checked or monitored regularly, reliability issues may arise that can result in excessive downtime and lost revenue. Figure 1

Storms and other natural events can lead to gusting wind, which can cause unanticipated component damage in wind turbines such as cracks and damage between layers of composite materials on the surface of the blades. Without regular maintenance, the cracks can widen and deepen, causing the blades to become imbalanced, placing more stress on the gearbox and other components. Large gears in traditional gearboxes can become misaligned due to the uneven loads that are generated by damaged blades.

Although most blades are coated with lightning-proof epoxy, they are susceptible to lightning strikes when moisture seeps into developing cracks. When struck by lightning, the water that has accumulated in the blades instantly turns to steam. Depending on its intensity, a lightning strike could cause cracks to deepen, or spark a fire from the oil or lubricants in the gearbox, possibly destroying the entire turbine.

In addition, turbines located on offshore wind farms are particularly at risk for cracks and other damage that occurs from driving ice in the ocean, as well as from sea spray and precipitation that can freeze on the blades or on other components. Ice can cause minor cracks to expand, and create excess loads for the blades, leading to damaging vibrations.

The remote locations of wind farms can present challenges when it comes time to inspect the nacelles for repair or maintenance. Inclement weather and the turbines’ location can make access difficult, often limiting service to just one or two times a year. Additionally, workers might need to repair multiple turbines that are spread out in the same wind farm, which may result in higher labor costs and a shorter time spent at each nacelle.

In addition, advanced logistics are required to coordinate the maintenance of the turbines and the repair of faulty components, including the shipment and installation of new parts. Because the components are so massive, their transfer to a remote location usually involves coordinating among various logistics providers, including trucking companies, barge and vessel owners, among others.

Optimized Components and Service Plans Provide Ideal Solution
These potential problems can be avoided with optimized components and software solutions, along with a proactive service and preventive maintenance program that boosts reliability and minimizes downtime and financial risk.

Because of the varied sizes and harsh conditions, wind turbines need efficient components that have been optimized to ensure maximum performance. Proven solutions include a specialized gearbox design and a robust remote condition-monitoring system that helps lengthen the life of the wind turbine and eliminate downtime. For example, the appropriate gearing design can harness gusts of wind and minimize its potentially uneven impact on rotors and other internal components. Alternatives to planetary gears include direct drives, magnetic bearings, torque-splitting systems, continuously-variable transmissions, and hybrid gearboxes.

Direct drives eliminate the need for a gearbox by increasing the number of magnetic pole pairs in a generator from the four or six conventional generators to 100 or more. In addition, a smaller number of moving parts boosts efficiency and reliability. However, turbines with direct drives cost up to three times as much as those with traditional gearboxes. The components themselves are costly and the systems are expensive to manufacture.

An active magnetic bearing system includes a magnetic shaft, controller and multiple electromagnetic coils attached to a stator shaft location. Magnetic bearings reduce the amount of wear, are more efficient than traditional gears and do not require lubrication. However, when exposed to higher loads, they need to be replaced more frequently. In addition, they require a great amount of power to generate the current needed to create enough of a magnetic force to handle those higher loads.

Torque-splitting systems utilize a gearbox design featuring external double helical gears that spread the torque among various generators that operate simultaneously. Like magnetic bearings, torque-splitting systems are unable to handle high loads, but one generator can be replaced without having to remove the entire gearbox. In addition, a turbine with one faulty generator can still operate at reduced capacity until it can be fixed. Figure 2

Output shafts in gearboxes with continuously-variable transmissions, or CVTs, have the ability to maintain a constant speed of rotation, despite the fluctuations of wind gusts and changes to the rotor speed. Although CVTs are more efficient than traditional gearboxes, it’s unknown whether this technology can be scaled for larger, off-shore wind farms.[7]

Besides the gears themselves, other components in the nacelle can be optimized to maximize the benefits of wind power. Designing the turbine with modular components, including multiple generators and gear units, can simplify access for staff that need to replace faulty equipment. Other components, like asynchronous motors in electromechanical pitch adjustment systems, eliminate the need for regular maintenance.

Hybrid gearboxes, like the Rexroth REDULUS GPV main gearbox, weigh 15 percent less than traditional gearboxes, making installation and replacement easier and faster. Serviceability is also enhanced by the lighter weight as rigging equipment can be installed in the nacelle so repairs could be made without heavy cranes and lifts.

Control systems in wind turbines can be designed to maximize efficiency, as well. A control system that includes redundant monitoring and data exchange between and among the master control communication layer and axis drives allows the wind turbine to continue to operate and produce electricity in the event of a fault. Other areas of redundancy in wind turbines include backup systems for cooling, controls, and fluids.

Besides efficient components, a variety of monitoring systems can reduce downtime by detecting problems before they occur. A lubrication and oil condition monitoring system can detect the effects of wear and friction in cooling systems. A robust remote online monitoring system continually inspects the status of the rotor blades and alerts off-site operators to any conditions that deviate from the normal operation of the turbine. It can detect minor cracks and ice that can affect the performance of rotor blades. In addition, a remote online condition-monitoring system can extend the life of the equipment by detecting problems at an early stage, so components can easily be repaired instead of replaced.

In addition to monitoring systems, a failure analysis program can help simplify maintenance and repair by determining why a fault occurred. The program should include a list of potential failures and the corresponding steps needed to repair or replace components. The plan also serves as a guideline for inventory and other equipment that will be required in case of an emergency.

Simply monitoring for problems is not enough. It is imperative to partner with a service and support organization that has technical knowledge and engineering expertise regarding wind turbine repair and maintenance. Some companies offer localized service centers that can handle preventive care, including the ability to perform measurements and tests. The service partner also should be able to provide a number of repair services—including overhauling, remanufacturing and regrinding of gear teeth—as well as OEM-quality spare equipment and upgrade kits for outdated parts.

After years of successful worldwide development and support, wind energy has proven to be a worthwhile investment. To maximize ROI and reduce high costs that result from unplanned downtime, ensure that the turbines are designed with control systems and optimized components specifically for the unique nature of a wind power application. A comprehensive preventive maintenance program and conditioning monitoring system helps lengthen the life of those components. Finally, partnering with an organization that specializes in the service and maintenance of off-shore wind farms can help overcome any potential obstacles that may occur. 

Sources
[1] American Wind Energy Association, http://www.awea.org/learnabout/publications/upload/Wind-Reliability-Fact-Sheet_WP11.pdf
[2] Global Wind Energy Council, http://www.gwec.net/index.php?id=138
[3] Global Wind Energy Council, http://www.gwec.net/index.php?id=137
[4] American Wind Energy Association, http://www.awea.org/learnabout/publications/factsheets/upload/Market-Update-Factsheet-Final_April-2011.pdf
[5] European Wind Energy Association, http://www.wind-energy-the-facts.org
[6] American Wind Energy Association, http://www.awea.org/learnabout/publications/upload/O-M-PPR_1-pager-3.pdf
[7] Ragheb, A.M. and Ragheb, M. Wind Turbine Gearbox Technologies, University of Illinois at Urbana-Champaign, http://cdn.intechopen.com/pdfs/16248/InTech-Wind_turbine_gearbox_technologies.pdf