A Roadmap To Western Wind Energy

A Roadmap To Western Wind Energy

An Academic White Paper December 2010

Mark W. Jessup


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Contents

Executive Summary 2

A Short History of U.S. Wind Energy 2

Finding the Real Customers 3

United States Department of Energy 4

States, Utilities and the Northwest Power Pool 4

Independent Power Producers – On the Front Line 5

Understanding the Problem Areas 6

Problem One – Turbines Are Reaching Middle Age 6

Problem Two – Creating a Reliable Fleet 6

The Gearbox—Where All the Forces Meet 7

The Bearing Killer – Electric Discharge Machining 8

Location Induced Contamination 9

Providing Solutions 9

Adding and Improving Filtration 9

Oil Condition Monitoring 10

Additional Opportunities 10

Partnering With Existing Service Companies 10

Partnering With Vocational Education 10

Charting a Course 11

Facility Table 12 - 14

References 15

© Mark W. Jessup, Kirkland, WA. This white paper was created as a student project as part of the Bellevue College Technical Communications Program, Bellevue, WA. 98005.


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Executive Summary

After twenty-five years of development marked by recurring boom-bust cycles, wind power just concluded five years of stable growth. New turbines are under construction, with a majority coming online before 2012.

After 2012, future Federal policy and funding for renewable energy development is uncertain. The most recent version of the proposed budget contains funding for future turbine construction.1 The financial realities of the times will determine if they become a reality.

On the technical front, data analysis unmasked two key causes of turbine failures - internally and externally generated contamination. Wind turbine operators have a fighting chance of extending turbine life, while reducing operation and maintenance costs.

The State of Texas has the largest concentration of wind turbines in the United States followed by seven western states. Construction of the vast majority of those turbines has occurred in the past ten years. Consequently, the warranty coverage on many aging turbines has expired, or soon will be expiring. This is an area of significant concern for system operators. While under warranty, operators look to the manufacturer for all support and service. Once the warranty period is over, that burden reverts to the operator. Many operators are unprepared for that moment.

General solutions currently exist to ensure aging turbines operate reliably. Upgrading turbine systems with modifications will extend turbine service life. Presently, turbine manufacturers are offering different service options, however operators want competitive alternatives to improve their bottom line. They will select the specific components, systems, and suppliers.

System operators, service companies, and the vocational training industry all represent potential customers for fluid power solutions. This paper covers the key players, the newly defined problems, and target opportunities with possible solutions.

A Short History of U.S. Wind Energy

The U.S. Wind Energy Market has been a roller-coaster ride lasting over a quarter century. The story of wind turbine development reads as if scripted from the 120-year-old history of electricity’s introduction. Both sagas contain stories of painful product development, bankruptcy, patent litigation, corporate piracy, corporate consolidation, and even accusations of international espionage.2

There is one significant difference. America was at the end of the 19th century when electrical power was developed. The country eagerly welcomed this new development. Electric lighting was replacing candles and gaslights. Electric motors replaced steam engines. Daily life, (and work), would no longer be limited to the hours of daylight.


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Wind energy has not elicited the same excitement or demand. It is an alternative to the status quo. There is still considerable debate about the size and effect of our carbon footprint, and America expects instant-on power. Consumers expect it to be available whenever they need it. Industry relies on it. Neither wants to pay more for it.

As a result, wind power suffers from a cyclical rise and fall in public interest that closely parallels the cost and availability of energy. As energy costs rise, so does interest in wind power. As availability falls, it spurs the demand for alternative sources. The path of turbine development tracks along a similar path, somewhat acerbated by variations in tax incentives. An unintended consequence of that history has been a peppering of turbines across the country, widely varied by manufacturer, age, design, and stage of retrofit.

Outside of Texas, the Western U.S. comprises the next largest concentration of wind turbines. Many were manufactured after 2001, a high water mark in turbine design. It also means that many turbines have, or soon will be exiting warranty coverage. This is an area of significant concern for system operators. During the turbine warranty, operators look to the manufacturer for all support and service. Once that warranty period is over, that burden reverts to the operator. Many operators are unprepared for that moment.

There are a number of reasons behind that state of unpreparedness. The dependence upon factory-supplied support is understandable. A majority of that support comes under warranty, a result of the rocky learning curve the industry has endured.

Many wind turbine manufacturers, (and operators), entered the industry with conventional power generation experience. That industry experience comes primarily from oil/gas fired steam and hydroelectric power generation. They share the attribute of being able to be ramped up to increase output. Speeds and loads are generally stable, while changes occur gradually. Wind, with its endless variability, is the exact opposite.

Availability is a matter of chance, there is no stored pool of wind to tap when electrical demand is high. Wind generated power is often at peak availability when regional energy demand is low. This lowers the price that the utility pays to the operator for the wholesale power. As a result, system operators have not realized much of a return on their investment.

Finding the Real Customers

To get to the actual decision-makers requires transiting through a bureaucratic maze. A number of government agencies at all levels are involved in supervising and regulating wind energy. Public utilities and community cooperatives also have a say in when, where, and how the generated power is used. A wind project will only begin after someone commits to purchasing the generated electricity. Profit for the power generator is not part of that commitment.

Being aware of how all these entities interact is an important step to understanding the service needs of the ultimate customer.


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United States Department of Energy

The creation of the Department of Energy (DOE) in 1977 was a result of the oil crisis. It is the government agency responsible for nuclear weapons and energy. It is also in charge of renewable energy research, production, and conservation. DOE is a major resource of wind energy information and research. For instance, the National Wind Technology Center conducts wind turbine testing. It is part of the National Renewable Energy Laboratory (NREL) in Colorado. A significant portion of the information presented in this paper comes from studies conducted there. That laboratory is one of many funded and operated by the DOE.

In 2009, DOE published a report that analyzed the technical and economic feasibility of wind energy providing 20% of the nation’s electrical needs by 2030. The report laid out an aggressive expansion path towards achieving that goal.3

Economic events since publication of that report cast real doubt about achieving that goal. It would require the immediate ramp up to an annual installation rate of 16 gigawatts (GW) per year. It would require immediate Wall Street financial investment and federally guaranteed backing. Maintaining that rate for a decade requires a major public and political commitment. The pace set in 2009 occurred primarily to meet a deadline set by expiration of the Production Tax Credit (PTC). In the present economic climate, prediction of the federal government’s attitude towards wind power after 2012 is purely speculative.

States, Utilities and the Northwest Power Pool

Even with a change in national policy, individual states still have legislative renewable energy goals. Public support seems secure, but all states face serious budget shortfalls. Current legislative support exists in the form of Renewable Portfolio Standards (RPS), already adopted by twenty-nine states and the District of Columbia. These require that a specific share of the electricity sold in the state, come from renewable sources by a specific date. Five of the seven member states of the Northwest Power Pool (NWPP) have an RPS in place. This is important. One industry analyst recently commented that “wind and solar may not be the lowest cost generation, but state-mandated renewable standards often mean these plants are built anyway.”4 Looking at individual states further illustrates the connection.

Idaho – has 164 megawatts (MW) of installed capacity, no RPS in place.

Montana – has 386 MW of installed capacity and an RPS in place. A 15% share of all electricity sales must be from renewable sources by 2015.

Nevada – has no significant wind installations. The state’s RPS scales up incrementally, to 20% of total electricity sales from renewable sources by 2015. Nevada offers bonus credits for Photovoltaic (PV) systems.

Oregon – has 2095 MW of installed capacity. The RPS requires 25% percent of energy sales come from renewable sources by 2025. This target only applies to large utilities. Reduced targets exist for smaller utilities.

NV 0


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Utah – has 223 MW of installed capacity. Utah does not have a true RPS. It does have renewable energy legislation in place; however, utilities are only required to pursue renewable energy to the extent that it is "cost-effective" to do so. The goal is for 20% of all retail electricity sales to come from renewable resources by 2025.

Washington – has 1964 MW of installed capacity. The state’s RPS requires that 15% of sales from the State’s largest generators must come from renewable sources by 2020.

Wyoming – has 1101 MW of installed capacity, but does not have a renewable energy standard in place.

Independent Power Producers – On the Front Line

Private independent power producers (IPPs) own 83% of the cumulative wind power capacity in the United States. Utilities own 15%; community power projects own the remaining 2%.5 These IPP’s constitute the core group of wind turbine operators. They are the people challenged with keeping the turbine fleet running.

Independent power producers, (also referred to as wholesale generators), were the founding fathers of electric power generation. One of the largest, oldest, and most visible examples of that history is the hydroelectric facility at Niagara Falls, built and operated by a private company.6

Public utilities came into being primarily to ensure that the benefits of electricity became available to the (less profitable) rural regions of the country. As electricity became a critical and necessary part of daily life, the government assumed more control over maintaining the supply.

Public utilities supplied most of the country’s electricity up until the energy crisis of the 1970’s. As the country exited that crisis, the government enacted the Public Utilities Regulatory Policy Act. This law gave independent generators, (a group consisting primarily of large industrial facilities with onsite power generation), an opportunity to sell power back to the utilities.

The law left actual creation and regulation of this market to the states. The net result was a patchwork implementation across the country. Some states created an open power market, treating electricity as a commodity. Many states of the southwest opted to do nothing, bending to political pressure from regional utilities. Twenty years later, additional legislation finally created a real opportunity for independent power producers to compete in the electricity market.7

There is a crucial difference between utilities and independent power producers though. Utilities have a profit guarantee, while IPP’s are at the mercy of the market.

Robert Kahn is the executive director of the Northwest & Intermountain Power Producers Coalition. He recently outlined the reality quite bluntly. “We are paid to perform," he stresses. "We only make money when we're generating electricity, while a utility is paid based on their capital assets invested. We live and die by competition; they don't."8


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The customer base for regional IPP’s consists primarily of public utilities. The power is sold at a wholesale price dictated by when the energy is needed and how much. Given the variable nature of supply, wind energy must compete with the more stable forms of generation. System operators need to reduce operating costs, improve system reliability, and increase profit margins.

Understanding the Problem Areas

In the Northwest region, independent power producers are the largest group of potential customers. However all regional turbine operators have aging turbines, the problems associated with them will exist regardless of changes in national policy.

Problem One – Turbines Are Reaching Middle Age

As of September 2010, there were 3747 wind turbines operating in the NWPP. The original manufacturers still exist; some are currently supplying an additional 923 turbines for new regional facilities.9

What is important to recognize is that 42% of the existing fleet either is out of warranty, or will be going out of warranty over the next three years.

From that point forward, approximately 20% of the remaining units will annually lose warranty coverage. By the end of the decade, all of the units presently under construction will follow suit. System operators are principally “hands off” during warranty periods, the burden of keeping these systems becomes theirs once the warranty expires. While manufacturers do offer service contracts that will kick in when that happens, the cost of that service is extremely high. Even if the system operator negotiates an event-based service agreement, involving the manufacturer in routine maintenance or system retrofits is an expensive proposition. This is why many operators have decided to take on the service function themselves, or to deal with local independent service providers.

Problem Two – Creating a Reliable Fleet

In 2007, the National Renewal Energy Laboratory (NREL) and the American Wind Energy Association (AWEA) began an industry-wide survey of turbine operations and problems. The survey results released in 2009 affirmed what everyone already knew. The greatest challenge facing US wind farm operators is keeping existing wind turbines running.10

Wind turbines designed to have a 20-year life, continue to experience failures after three to five years of operation. The financial impact of a turbine failure is very significant. NextEra Energy, one of the larger US independent power producers, has calculated that “gearbox replacement and lubrication account for 38% of the turbine’s parts cost. Taking into account the cost of the gearbox, crane rental, labor and lost revenue, replacing a gearbox for a 1.5-mw turbine can run more than $250,000.”11


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As long as the life cycle remains below expectations, and the failure rates remain high, catastrophic, and costly, wind based energy will not be able to compete with other generation methods.

The Gearbox—Where All the Forces Meet

The historic weak link in wind turbine design is the turbine gearbox. The function of the gearbox is to increase the low input rotation speed of the turbine blades up to the necessary input speed of the turbine generator. Regardless of adjustment for changes in wind speed, the turbine gearing will be subject to cyclical acceleration—deceleration under high torque loads. This cyclical bending of the gear teeth can eventually lead to failure. The general industry assumption was that even though gearbox design had changed significantly to address these issues, it was still a primary problem.

However, the NREL - AWEA analysis of the study data indicated that gear tooth failure was rarely the initiating cause of the gearbox failures. The failures did not follow a pattern. They were generic in nature and occurred across all gear manufacturers and turbine models.

The recognition occurred that the gear failures were actually a telltale indicator of another type of component failure. Gearbox bearings were breaking down, actually disintegrating as the result of an unknown process. Whatever that process was, it did create one clear indicator of impending failure, which was an eruption of hard particle contamination in the lubricating fluid.

System generated contamination, as a function of normal wear, is not unique to wind turbines. All machinery utilizing bearings and gears in the power train generates contamination over time. However, the contamination levels found in wind turbines are higher than the levels found in traditional steam turbine and hydroelectric generation systems.

Some of that increased contamination is to be expected. The significant difference between these methods of power generation is that the gearing is subject to different forces. Traditional power generation operates at fixed speeds and relatively constant loads. Wind, on the other hand, is constantly varying in speed and force. Gear manufacturers know that gearing can withstand much higher torque loads when speed, load, and direction are constant. Wind turbine gearboxes are sources of tremendous heat generation, along with extremely high gear stresses. This intensifies the load on gear teeth and bearing elements, which results in metal fatigue and micro pitting. The resulting weak spots in the gears degrade over time. This in turn generates hard particle contamination.

Over the last decade, gear drive manufacturers redesigned turbine gearboxes to use different types of gearing. Turbine and generator manufacturers adjusted drive ratios. They expected these changes to result in a significant reduction in gear wear and fatigue. In addition, newly developed synthetic gear lubricants became available which also dramatically reduced gear wear. Designers expected to see a reduction in both contamination and failure rates. That expectation did not translate into a reality; instead, the problem simply seemed to move to another area.


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The Bearing Killer – Electric Discharge Machining

It is beyond the scope of this paper to go into an extensive discussion of Electric Discharge Machining (EDM). It is, however, important to have a basic understanding of the concept, primarily due to the impact it has on turbine life.

Readers may be familiar with the term as it applies to a method of cutting and shaping hard metals. The basic principle was first discovered in the 1770’s, the first practical application as a machining process occurred in 1943. An industry website states, “it is possible to cut small odd-shaped angles, detailed contours or cavities in hardened steel as well as exotic metals like titanium, hastelloy, kovar, inconel, and carbide.” There is one caveat, “An important point to remember with EDM Machining is that it will only work with materials that are electrically conductive.”12 The basic principle behind the process involves the use of very high voltages to create an arc, which melts the metal. Incorporating a thin wire into the process facilitates cutting.

Consideration of EDM as a cause of bearing failures occurred in the emerging VFD industry in the mid 1990’s.13 The application of vector duty motors to higher horsepower applications resulted in decreased bearing life. As the calendar rolled over into the 21st century, motor manufacturers began to consider a possible connection between the frequencies used in VFD operation, and motor bearing failure.14

While field study of VFD has continued, the NREL - AWEA study of wind turbine operations began. The technical committee focusing on gearbox failures could not discern an obvious root cause. Examination of the fluids data revealed the dramatic increases in contamination levels prior to failure. This redirected attention to the bearings.

Wind turbine generators are close cousins of the vector duty motors required to operate with VFD’s. They share common operating characteristics, so it was a logical question about sharing similar problems. Testing confirmed what many suspected. A case study conducted at an Oregon wind farm describes EDM as follows: “High-frequency currents induced on the shafts of wind turbine generators through parasitic capacitive coupling can reach levels of 60 amps and 1,200 volts or greater. If not diverted, these currents discharge through the generator’s bearings, causing pitting, and fluting that result in premature bearing failure and turbine failure.”15 In short, electrical “leakage” is eroding the bearings and causing them to break down.

The cause is an unintended consequence of the design. Electromagnetic fields created by the operation of the wind turbine generator induce voltage potentials on electrically isolated components. The isolation of those components may not be intentional. Lube oil can cause it to occur.

One component especially prone to this effect is the generator shaft. When the voltage potential induced on the shaft rises to a point sufficient to overcome the path of least resistance, it forces current through that path. Unfortunately, that path often goes through the bearings. This weakens them and creates contamination as well. Since newer turbine designs frequently incorporate forced lubrication channeled directly to the bearing races, the contamination migrates out into the rest of the system. Bearing breakdown compromises shaft alignment,


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bearing remnants become jammed between gear teeth, the gearbox failure becomes inevitable. EDM is clearly a significant source for internally created contaminants.

Although specialized discharge devices are being introduced that will significantly reduce this problem, EDM induced contamination will continue to occur, albeit at reduced levels. Capturing this contamination is a major objective for system operators.

Location Induced Contamination

The NREL – AWEA studies also found that the turbine’s physical location and surrounding environment are significant contaminant sources. The formation of GE Energy in 2002 involved the purchase of Enron’s Wind Energy Division. Enron had entered the market primarily through the acquisition of Zond and Kenetech. The net result for GE Energy was the inheritance of a large fleet of twenty-year-old wind turbines. Most were located in desert areas of California and Texas. GE technicians quickly learned that turbines located in open desert areas, are subject to significant quantities of airborne dust and sand during the hot season. It is very difficult to keep it out of the turbine nacelle and lube systems. Most of this contamination comes in during air exchange between the nacelle and the outside world, as the turbine heats up and cools off between periods of operation. In addition, the units also collect considerable moisture inside the nacelle during the rainy season. This results in the formation of rust and black iron oxides inside the nacelle and tower. In time, this also finds its way into the system.16

These two contamination sources jointly add to the volume of particles in the fluid, which triggers a cascading series of failures if not captured. The important takeaway is that the generation of this contamination is an unintended, but inevitable consequence of wind turbine operation. Proper monitoring and control prevents it from becoming a catastrophic consequence.

Providing Solutions

There is a significant advantage gained by focusing on wind turbines that are coming out of warranty. The system improvements outlined below will not violate warranty conditions, since the warranty has already expired. Identifying those systems in need of retrofit requires direct consultation with the operators, but turbine age does provide some general direction.

Adding and Improving Filtration

The historic viewpoint of operators is that the replacement of lubrication fluids occurs after failures, or on a set timetable. Recently the wind turbine industry refocused its attention onto keeping the lube oil as clean as possible. The industry is investigating active and passive filtration, coupled with fluid condition monitoring, with a renewed interest. As mentioned previously, the manufacturing date often dictates what filtration, if any, exists on a particular turbine.

Turbines manufactured prior to 2005, are unlikely to have filtration installed with sufficient capacity to address these problems. An industry-wide filtration standard was in the beginning


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stages of consideration. The true causes behind the majority of turbine failures were not on the radar at that time. Additionally, these turbines form the largest group poised on the edge of warranty expiration. It is unlikely that manufacturers have updated them in the last 24 months.

Turbines manufactured between 2005 and 2009 are likely to have some level of filtration incorporated into the lube system to capture contamination. Some turbines will have monitoring systems. However, the underlying technology behind both may insufficient to detect contamination accurately or capture it effectively.

Although the industry did eventually adopt an industry standard for filtration systems, it has not been widely put into place.17

Oil Condition Monitoring

Oil condition monitoring has increased in use because turbine lube systems are difficult to access. Everything service personnel need to get to is in the nacelle, which sits on top of a 200+ foot tall tower. GE Energy stated in an AWEA presentation that their best crews, when performing routine service averaged a service rate of two to three towers a day.18 This is for routine service. Having the ability to determine filter and fluid condition easily from the ground significantly reduces that labor time. It also allows service time to be scheduled based on need. Operators recognize that value, but the monitoring systems offered by turbine manufacturers are expensive and complex.

A significant opportunity exists in adding and augmenting both filtration and monitoring on these existing systems. There are other opportunities as well.

Additional Opportunities

Partnering With Existing Service Companies

Along with the system operators, another potential customer base exists with firms that service wind turbines. The region is home to a number, including H&N Electric, Rev1 Wind, UpWind Solutions, and Wazee Wind. These companies focus on construction, repair, and maintenance of all ages and types of turbines. They also possess tools not usually available in fluid power shops. Collaborating with these companies would be another means to penetrate the market. Many are members of the North American Wind Service Alliance (NAWSA).

Other NAWSA members include gearbox manufacturers and repair centers in the region. They are also potential customers for modular filtration and monitoring systems that they can incorporate into their reworked drive packages.

Partnering With Vocational Education

Another opportunity exists in vocational education. A few larger system operators maintain employee vocational training centers, but few independent training programs currently exist.


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Existing programs also focus principally on safety and tower climbing. Offering specific fluid power training to operators and service companies has revenue potential. The real value comes from being able to introduce trainees to topics such as filtration and condition monitoring, using your brand and equipment. This develops product loyalty, as well as invaluable long-term relationships.

Charting a Course

This paper identifies the key players and system operators who face the challenge of keeping wind turbines running.

Forty-three percent of the aging turbines in the region are approaching the end of their warranty periods. This opens up the opportunity for fluid power companies to offer system improvements without violating warranty conditions.

The data gleaned from the joint NREL – AWEA study clearly shows that turbine gearing failures have declined dramatically, but electrically and environmentally induced breakdown of the bearings has increased. Bearing lubricating oil fills with hard particle contamination.

There are two principal causes of that contamination:

o The bearings become the discharge path for induced shaft currents, causing them to disintegrate. This creates more contamination, which accelerates the failure.

o Location induced contamination migrates into the lubrication fluids through air exchange, also adding to the contamination load.

Many possible system solutions can address these issues:

o High efficiency filtration systems can capture this contamination before it reaches destructive levels.

o The use of precision particle counters within the circuit provides operators with early warning of impending contamination based failures. This can prevent a maintenance issue from turning into an expensive catastrophic failure.

Fluid power companies are ideally suited to provide support in this area. In addition, opportunities exist in collaborating with independent service companies, as well as gearbox manufacturers and repair centers. There is also opportunity in vocational education, through programs aimed at the education of service personnel.

By following this roadmap, fluid power companies seeking to take advantage of this opportunity should achieve a high level of success.


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References

1 Final editing of this document occurred on December 16, 2010.

2 Jonnes, J. (2004). Empires of light: Edison, Tesla, Westinghouse, and the race to electrify the world. New York: Random House

3 Energy Information Administration. (2009). Annual Energy Outlook 2009 (DOE/EIA-0383(2009)). Retrieved from URL http://www.eia.doe.gov/oiaf/aeo/pdf/0383(2009).pdf

4 Tudor, Pickering, Holt & Co. (2009). Electric Power Industry Primer [PowerPoint slides]. Retrieved from URL http://www.tudorpickering.com/pdfs/Electric.Power%20Industry.Primer.04.02.09.pdf

5 Wiser, R., & Bolinger, M., Lawrence Berkeley National Laboratory. (2010). 2009 Wind Technologies Market Report. Retrieved from URL http://www1.eere.energy.gov/windandhydro/pdfs/2009_wind_technologies_market_report.pdf

6 Jonnes, J. (2004). "Yoked to the Cataract!” Empires of light: Edison, Tesla, Westinghouse, and the race to electrify the world. (pp. 301-333). New York: Random House

7 Gibson, T. (2010). Calpine - Independent Power Generation With an Environmental Twist [Online exclusive]. Progressive Engineer. Retrieved December 9, 2010, from URL http://www.progressiveengineer.com/company_profiles/calpine.htm 8 Sickinger, T. (2010, September 22). PGE asks to sidestep public bidding to build 68,000 acre Oregon wind farm. The Oregonian. Retrieved from URL http://www.oregonlive.com 9 The Windpower - Wind turbines and windfarms database. USA Wind Farms [Data file]. Retrieved from http://www.thewindpower.net/index_en.php 10 Butterfield, S., Musial, W., McNiff, B. (2007). Improving Wind Turbine Gearbox Reliability (NREL/CP-500-41548). Retrieved from URL http://www.nrel.gov/wind/pdfs/41548.pdf 11 Van Rensselar, J. (2010). The elephant in the wind turbine [Online exclusive]. Tribology & Lubrication Technology. Retrieved December 6, 2010, from URL http://www.stle.org/assets/news/document/Cover_Story_06-10.pdf 12 IDS Machining. (2010). EDM Machining Process & Techniques [Online exclusive]. All about Electrical Discharge Machining. Retrieved December 14, 2010 from URL http://www.edmmachining.com/ 13 Macdonald, D. (2002). A PRACTICAL GUIDE TO UNDERSTANDING BEARING DAMAGE RELATED TO PWM DRIVES. Retrieved from URL http://www.groupedelom.ca/pages/doc/pdf/EDMCurrentBearingDamageIEEErpt.pdf 14 Busse, D., & Erdman, J., & Kerkman, R.J., & Schlegel, D., & Skibinski, G. (1995). Bearing Currents and Their Relationship to PWM Drives. Retrieved from URL http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.70.7017&rep=rep1&type=pdf 15 Oh, W. (2008). Preventing Bearing Failure At An Oregon Wind Farm. (2008) North American Windpower. Retrieved from URL http://www.est-aegis.com/press/nawind_PreventingBearingFailure.pdf 16 Giammarise, A., & Sirak, M., GE Energy. (2009). Challenges of Large MMW Gearboxes in Wind Turbine Applications. [PowerPoint slides]. Retrieved from URL http://www.sandia.gov/wind/2009Reliability/PDFs/Day2-03-AnthonyGiammarise.pdf 17 Stover, J. (2009). Two Keys of Reliability Initiatives – Clean Oil and Oil Condition. [PowerPoint slides]. Retrieved from URL http://www.sandia.gov/wind/2009Reliability/PDFs/Day2-07-JustinStover.pdf 18 Giammarise, A., & Sirak, M., GE Energy. (2009). Challenges of Large MMW Gearboxes in Wind Turbine Applications. [PowerPoint slides]. Retrieved from URL http://www.sandia.gov/wind/2009Reliability/PDFs/Day2-03-AnthonyGiammarise.pdf

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A Roadmap To Western Wind Energy

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