May 2010

As most people know, the Texas Panhandle is an ideal place for wind energy. With Competitive Renewable Energy Zone (CREZ) high voltage transmission lines scheduled for completion by the end of 2012, wind power plants should begin to pepper the region. Texas Tech University is in the ideal location, boasting the only doctorate program in Wind Science and Engineering, and the only graduate certificate program in the wind energy industry. Here at the Wind Science and Engineering (WiSE) Research Center students are addressing “everything wind,” from tornados, hurricanes, and the damage they inflict to multifaceted wind energy research and education. Some current projects are titled Turbulent Far-Wake Development behind Wind Turbines, Innovative Gearbox Design for Reliability, Turbulence-Driven Gear and Bearing Test Systems, Quantifying Effects of Turbulence from Large-Scale Wind Energy Development on Local area Microclimate, and Firming Wind Energy with solar Photovoltaic (PV).

Independent system operators of various electrical grids have long been adept at handling the variability of the electric load. However, there is greater renewable energy penetration into the grids than ever before. Combining the variability of the electric load with the variability of intermittent generation sources such as solar and wind is creating a greater challenge for grid operators. By combining wind and solar PV on a utility scale, the uncertainty of generation is greatly reduced and a more reliable energy source is created.

On average wind energy is best at night, when the atmosphere is stable. While there are times when the wind does blow during daylight hours, wind energy tends to produce less during the day when the atmosphere is unstable due to solar heating. Solar PV energy is only available during the day, when the sun is shining and atmospheric mixing takes place. By combining these two inexhaustible and renewable resources, a single power plant can take advantage of Mother Nature and produce more-reliable and less-intermittent power.

Texas Tech operates one of the only 200-meter towers specifically utilized for atmospheric data collection as well as a 58-station MesoNet—learn more at www.mesonet.ttu.edu—that records a plethora of data above and below the surface at five-minute intervals for public use. By utilizing data from the 200-meter tower at heights equivalent to the blade-swept area of large wind turbines and the nearby Reese MesoNet station’s solar radiation data, times of intermittency can easily be identified. Currently in the early stages of research, analysis of the first six months of 2009—an average wind and solar year—has produced some interesting results.

For wind energy, times of intermittency were relatively small since the winter and spring months produce the best winds. A five-minute time scale is used since some electric grids will be converting to a “smart grid,” which utilizes increments of that same length. Over the first six months there were 640 occurrences of too-low or no wind which lasted from five minutes to one hour, 40 occurrences of no wind power production from one to two hours, 17 occurrences from two to three hours, 11 occurrences from three to four hours, and 19 occurrences of greater than four hours. A five-minute breakdown of nonproductive wind occurrences can be seen in (Figure 1

Over the same time period in 2009, times of no solar energy, sunset to sunrise, ranged from nine to 14 hours. By combining the two data sets, the synergy is quite obvious. Times of no energy production from either wind or solar dropped by 50 percent or more for each time. Periods of no energy production from five minutes to an hour dropped to only 300 occurrences, from one to two hours occurred 18 times, two to three hours occurred 10 times, and greater than three hours only occurred 13 times. Figure 2 shows a five-minute time scale of the combined data.

Wind and solar’s combined output also shows their collective advantage. Given a one-megawatt wind turbine and one megawatt of solar panels placed immediately south of the turbine, they can both utilize the same pad mount transformer and electrical collection system, reducing installed costs. While periods greater than one megawatt did occur, they were only for short periods of time. For prolonged times of overproduction, either the wind power or solar power production would need to be curtailed to protect the transformer equipment. To take advantage of this dual “bonus” production, a larger transformer could be utilized. However, there would be greater energy loss when production would be lower than 50 percent of the transformer’s capabilities. The losses during low production times could be greater than the added production by utilizing the larger transformer.

During the winter months when wind energy is greatest and solar energy is at its lowest maximum, the colder temperatures would allow the transformer to be slightly “overtaxed” to capture more than the allowed energy without the transformer overheating and losing efficiency. During windy summer days, this would not be the case.

Figure 3 shows the synergy that wind and solar produce together. Expecting a smooth bell curve centered on 1,000 kilowatts, the data does not support this with a smooth transition. Instead, there is an abrupt change shown by the tremendous difference between the 900 and 950 kilowatt values and the 1,000 and 1,050 kilowatt values. These abrupt changes negated the standard curve with longer transitions over time and exemplified the benefit of combining wind and solar power together. As both the wind and solar production values should be evenly distributed, the greatest concentration of values should be toward the middle, with smaller “tails” located at the maximum and minimum values. Together, however, because of the “clock-time” of the recorded values, they match precisely, creating a large pronounced peak at one megawatt. This demonstrates how decreasing wind energy values during the morning hours matches with increasing solar energy values, as well as full solar energy during the day coupled with full wind energy during the night. This is similar to two gas-turbine engines operating in tandem, as one is turned up and down and the other is adjusted opposite the first. By pairing the two individual one-megawatt systems that have nearly opposite production phases, a gross capacity factor of 0.608 was attained when not curtailing either wind or solar.

By combining wind and solar power plants, the whole is greater than the sum of its parts, and the intermittency of renewable energy is significantly reduced. The Texas Panhandle has considerable amounts of both inexhaustible resources. Once the CREZ lines are operational, there is no reason not to see more reliable renewable energy power plants being constructed. The only question remaining is: Why not take advantage of Mother Nature?

Note: The author is a fellow in the Integrative Graduate Education and Research Traineeship (IGERT) program of the National Science Foundation.

As the giants of the wind industry gather for WINDPOWER 2010, the American Wind Energy Association (AWEA) is projecting that this year’s show will be the biggest yet. Walk the show floor and you will no doubt be overwhelmed by the size of the booths, the scale of the equipment within them, and the huge interest shown by industry members. Given that the event takes place in Texas, it should surprise no one that BIG is a major theme, but what about small… as in the small wind industry?

Lurking among these literal giants is the small wind industry. Generally referred to as turbines with 100kW capacity and less, the small wind industry is enjoying incredible growth as homeowners and businesses around the country take advantage of an uncapped 30-percent federal tax credit and improved technologies that make harnessing your own clean energy easier than ever before. AWEA reported that the U.S. small wind market grew by 78 percent in 2008, and it is expected to release similar figures for 2010. How do these two dichotomies of the industry go together? Is it possible that small wind can actually help big wind?

Few members of the industry realize that AWEA was founded by a group of small wind enthusiasts in the seventies during a time of incredibly high oil prices. The first conference was timed to coincide with the World Energy Conference in Detroit, Michigan, and was attended by 20 people. AWEA’s first president, Allan O’Shea, reminisces about the early days when the industry focus was on small wind. “When we—me and 15 other founding fathers—started AWEA it was under the premise that small wind begets big wind,” he says. “We got together and put up a wind-powered billboard welcoming the World Energy Conference, because at the time they were only talking about oil.”

While big wind is generally sited far away from the urban and suburban centers of most communities in order to avoid noise pollution and height restrictions, with shorter towers and quieter systems small wind turbines are designed to be installed in the heart of communities. By having turbines on display in inhabited areas, more people within the general community gain firsthand exposure to wind power. Figure 1
“It makes it real to people,” according to Dan Loundy, vice president of Devon Bank in Chicago, Illinois. Loundy installed six 1.2kW vertical axis wind turbines at the bank’s newest retail branch to help with energy costs and to make a statement to the community about the bank’s commitment to being green. “When it’s out in a field it’s what some other guy is doing. Until you see it on a regular basis, it’s an abstraction,” he says, adding that he even installed benches below the turbines to encourage the community to interact with the technology. Figure 2

This accessibility of small wind turbines gives the public the opportunity to live with the technology on an everyday basis. This can lead to a better understanding of how wind power works, while at the same time addressing classic concerns such as noise, wildlife, and the visual impact that often hamper large wind installations. Acceptance of wind power on a small scale can lead to acceptance of wind power on a big scale. Think of it as traditional product sampling done by packaged good marketers. The concept is to give a taste test that is so satisfying the consumer will want more.

Spirit Lake, Iowa, is an example of a community that used small wind to “taste test” future wind power development. The community started with the installation of a 250 kW system at a local school in 1993. In 2001 the school district installed a 750 kW system based on the success and community support for the original turbine. The community eventually welcomed multiple large wind farm developments. Not surprisingly, Iowa now hosts 22 MW of locally owned and 814 MW of commercial-scale wind.

“You go into any township and put in a small wind system and that ends the whole discussion about bird kill, fall zone issues, tower height, and sound,” O’Shea says. “All of the things that big wind has to deal with.”

Small Wind, Big Conservation
Few will debate that larger turbines are a more-efficient way to generate energy. But installing small turbines at the site of energy consumption, as you can uniquely do with small-scaled turbines, can create benefits that make up for any loss in efficiency.

A welcome side effect of on-site installations is that having energy generation right outside an office or home can impact energy consumption within the building. Suddenly energy doesn’t come from a switch or outlet in a wall, it comes from the turbine spinning just outside. Once people start thinking about where their energy is coming from they tend to start thinking about their use of this energy, and ways to conserve.

In fact, building operators and energy providers are using visible small wind installations as a tool to encourage building occupants to conserve the energy they can see being produced outside their windows. Leading software company Adobe Systems of San Jose, California, recently installed 20 vertical axis wind turbines at its corporate headquarters. “Adobe has a state of the art campus with a multitude of green attributes that have earned us three USGBC LEED-Platinum certifications, so our employees are very conscientious about their environmental footprint,” says Randall H. Knox III, senior director of Global Workplace Solutions at Adobe. “We believe the wind turbines are a positive enhancement to our headquarters, and that their presence will spur people to talk even more about conservation.” Figure 3

DONG Energy of Denmark is a leading European energy provider, and a market leader in the development and construction of offshore wind farms. DONG sees the value in installing small wind turbines at the sites of its energy customers, recently announcing plans to market vertical axis turbines throughout Denmark.

Jan Darville, manager of electric installations at DONG, recently told the International Herald Tribune: “It’s about starting a chain reaction of thinking green. If people see wind turbines outside their office window, maybe they’ll start thinking about what happens if everyone at the office actually shuts down their computer when the day is over, and all of a sudden energy usage is down 2 percent.”

Conservation is an important piece of the energy puzzle for big wind and all energy providers. If we can reduce overall energy use we can better control the availability of energy and the cost to provide it to consumers.

Small Wind, Big Future
Small wind turbines are also an excellent tool for wind-power education. If the U.S. is going to be the leader of clean energy, younger citizens need to get involved now. It is nearly impossible for most schools to install their own 1 MW turbines, but schools all around the country are installing 1-3 kW systems right on school property. Teachers are able to incorporate the installation into a full wind energy curriculum, exposing the students to a firsthand wind experience while preparing them for a future in wind energy. Exposed to the power of wind, these youths are more apt to embrace a future with larger turbines spread across their local landscapes and the policies that will be required to make this vision a reality. Figure 4
Caitlin Wargo is the director of sustainability and energy management at Far Hills Country Day School in New Jersey. She has recently ordered four 1.2kW wind turbines for the campus to compliment other renewable energy systems already installed. “It is our hope that by involving our students in our energy initiative, they will get a foundation in the issues surrounding renewable energy, from science and engineering to socio/political and economic,” she says. “Coupled with the critical thinking and leadership skills that are the hallmarks of a FHCDS education, our students will be ready to take their place as the leaders of tomorrow in renewable energy.”

Getting Started
As an industry, small wind is ready to help big wind overcome current market adoption barriers that will lead to growth. The technologies that are currently available are efficient, silent, and attractive, and a wide array of rotors—from traditional horizontal axis to innovative vertical axis designs—are available to meet individual design needs. In addition, the newly announced small wind certification program will ensure that the turbines are safe and tested. It’s time for big wind to start using small wind as a tool to grow the entire industry.

Enclosures form an integral part of a wind energy circuit protection system in which circuit integrity has to be provided. Typically the enclosure is a container for electrical controls, acting as a safeguard against tampering and providing environmental protection of the electrical connection. It can be made from metal or non-metallic material, but it must serve its protective function for the life of the installation, so durability is key. There are three typical types of enclosure materials available: metal, plastic, and composite.

Metal: Common metal enclosure choices include carbon steel, stainless steel, and aluminum, with carbon steel being the most prominent choice based on its low initial cost. Carbon steel is typically galvanized or painted to prolong the service life. Premium metals such as stainless steel and aluminum are used where long life, corrosion resistance, and weatherability are critical, such as protecting controls for junction boxes for wind installations.

Plastic: Thermoplastics such as polycarbonate, polyester, and PVC offer a degree of corrosion protection beyond painted carbon steel. Thermoplastics, though, are more susceptible to UV and weathering degradation over time. Certain stabilizers can be added, but ultimately the nature of the thermoplastics will yield to extended weathering.

Composite: Thermoset materials such as a polyester resin combined with glass create a unique composite FRP (Fiberglass Reinforced Polyester) that is exceptionally durable and weather resistant (Figure 1). Like thermoplastics FRP provides a greater degree of corrosion than painted carbon steel, yet will perform better in more harsh environments.

There are risks associated with improper enclosure selection, including failure. Enclosure failures caused by environmental corrosion or impact damage resulting in a breach of proper sealing can cause a multitude of problems ranging from catastrophic and dangerous system collapses, production downtime, and increased maintenance costs, in addition to losses in customers and revenue. Proper product selection is therefore critical as it relates to both the design and the material of the enclosure.

When choosing an enclosure there are influential factors that will help reduce failure. Selection ultimately comes down to optimal performance and value. Often tradeoffs between performance, acquisition cost, and operating cost are made in the process to find the ultimate choice in a unique application. Consider three factors that influence the enclosure specification for wind applications and how an enclosure might stack up: environmental characteristics; physical characteristics; and material and material utility.

Environmental Characteristics
The foremost motivating characteristic influencing the enclosure choice for wind engineers and specifiers is the environment of the site. This consideration envelops temperature, chemical, moisture, and concern for the physical world of the permanent installation. Whether the environment is hostile or passive, an attempt is made to match the capabilities of the enclosure with the anticipated ambient environment. An over-specified enclosure will work effectively in a natural environment, but there are severe repercussions for using an under-specified enclosure in a hostile environment, thus making the environment the over-riding consideration.

There are environmental conditions that are specific to the wind industry. Corrosive environmental conditions can act as accelerants for corrosion, just as gasoline does for fire. The factors that determine the level of corrosion in an environment include extreme weather conditions such as dust, moisture, ultraviolet radiation, and temperature (spread between the daily high and low temperatures as it influences condensation and evaporation of moisture).

Dust: Dust particles can cling to surfaces and retain moisture. Typical sources of dust include soil/sand, smoke and soot particles, and salts. Depending on the chemical composition of the dust it may contribute to the corrosive attack, or it may act as a catalyst. Even if the dust is chemically inert a concentration cell can be set up under the dust particles due to differential aeration.

Moisture: The level of corrosion typically increases with moisture content. In fact, if there was no moisture there would be no electrolyte, and hence no corrosion. Common atmospheric sources of moisture are rain, dew, and condensation. Rain can have a beneficial effect in that it washes away contaminants from exposed surfaces. If rain collects in pockets or crevices, however, it can be very detrimental because it supplies a source of continued moisture. When relative humidity exceeds 70 percent a thin film of moisture will form on a metal surface, providing an electrolyte. This dew or condensation can become very corrosive if it is saturated with a contaminant like sea salt, or acid compounds from industrial sources.

Ultraviolet Radiation (UV): UV has been a concern with non-metallic manufacturers for many years. The rate at which the UV degradation occurs will vary depending on heat, humidity, and the altitude at which the product is installed (Figure 2). There are also differences in the way UV breaks down differing non-metallic materials. For instance, the effects of UV light become critical more quickly with thermoplastics than with thermosets of similar chemical structure. This happens because thermoplastic materials have a much lower mass (molecular weight) than thermosets, so breaking the bonds in thermoplastics cuts the polymers into much smaller fragments than does each breaking bonds in thermosets. In thermosets the cross-linking limits unzipping the polymer chain and requires more UV energy to break it down, thus giving increased UV resistance and weatherability.

Temperature: Increasing the temperature of a corrosive media will generally increase the rate of corrosion. Temperature gradients on the same piece of metal can create a basic corrosion cell. The part of metal with the higher temperature will become anodic to the area with a lower temperature.

Physical Characteristics
When considering the physical characteristics of an electrical enclosure, there are certain basics to take into consideration. The most notable are:

• Corrosion resistance
• Size
• Weight
• Mounting
• Flammability
• Safety
• Security
• Flexural strength
• Hardness
• Impact resistance
• Tensile strength
• Sunlight (UV) resistance
• Hardness
• Heat transfer
• Radiused edges
• Water absorption
• Available access
• Cabinet design
• Bending radius limitation

Three physical characteristics are particularly important for wind-industry professionals to consider: strength, ultraviolet radiation, and electrical.

Strength: Strength measures the resistance of a material to failure, given by the applied stress or load per unit area, tensile and compression. Strength is a measure of a material’s ability to withstand stretching or compressing under load. On the other hand the toughness of a material is its ability to withstand sudden impacts. Increasing strength, tensile or compression, usually decreases toughness and vice versa. Whereas steels often have high strength they exhibit low toughness, which means they dent easily and are difficult to drill or penetrate. Thermosets and thermoplastics, or composites, exhibit average strength but high toughness, meaning they can withstand sudden impacts and maintain their shape. Today’s composites have improved dramatically, in that they can now be designed for both high strength and toughness via additives and fiber reinforcements.

Ultraviolet Radiation (UV): Many professionals are still unaware of how non-metallic composite materials are able to effectively withstand damage from UV exposure. In enclosure applications a unique formulation is now available to provide superior molded-in UV resistance, requiring no field maintenance and at no additional product cost. It is a double protection formulation technology that significantly enhances the molecular bond strength and cross-linking that occur during the curing process in thermosetting polyester sheet molding compounds (SMC). This new formulation fights polymer degradation by making it much more difficult for UV light to attack molecular bonds of both primary chains and cross-links. This is accomplished by increasing the molecular density of the base resin system, thus increasing its resistance to UV degradation. An additional benefit of this formulation is its use of additives or antioxidants that help protect the polymer chain and resist photo induced oxidation from exposure to UV sunlight.

Electrical: Like the physical, there is concern regarding the protection offered for the installed components as well as protection of the enclosure itself. An enclosure that breaks down over time can no longer perform the duties for which it was specified. Therefore, the following characteristics are important: electrical conductivity, service temperature, thermal conductivity, grounding, and arc resistance.

Material Selection and Utility
If you have the opportunity to select the material, make sure that you have investigated the material and that it is the most appropriate for the type of environment. Every application has its unique demands and elements of this list, and do not follow a precise order. Indeed, many of the capabilities are considered to be inherent in certain material choices. An errant or over-estimated material choice, however, can have many repercussions in the lifecycle of a product. It makes good sense to specify a product that qualifies in almost every category, insuring satisfactory results without regard for the type of installation.

Material utility addresses the consideration for machining, cutting, sawing, drilling, and modifying the material of choice. User preference plays a significant part in this selection, and material familiarity overrides practicality in many instances. A few things to keep in mind for both non-metallic and steel modifications are as follows.

Modifications can be made to non-metallic enclosures that make them particularly useful for a wide variety of situations and applications (Figure 3). Non-metallic composites offer the benefits of part integration and minimization, along with substantial savings in weight. Along with that advantage is a reduction in the requirements for machining operations that need to be performed to complete an assembly. Drilling and cutting operations can’t be avoided completely in all cases, however. There are several types of machining operations that can easily be performed on composites, including turning, drilling, routing, trimming, sanding, and milling. Most of these operations are similar to metal removal techniques, but there are some differences that need to be addressed in order to make clean, high-quality holes and cutouts in composites.

Delaminating of the outer surface and glass fibers directly below the surface are the main failure modes noticed when holes or cutouts are drilled, or cut out improperly. Most of the time excessive edge chipping around the perimeter of the cutout or hole is due to improper tools used and methods applied. A little upfront planning and understanding of the proper methods to machining composites can make all the difference in the final outcome of the operation. Factory-option modification is also a highly desirable alternative for many end users, because it enables the manufacturer to use its skill in providing enclosure modifications and shipping to the customer ready to use.

When approaching stainless modifications, keep the type of tools used in mind. If modifications such as custom sizes or shapes are required, both mild and stainless steel are good candidates. Both are fabricated from a flat sheet of metal, making them easier to form to custom specifications in the fabrication process. Mild steel is a viable option, and with the correct metalworking tool holes can easily be added in the field. Stainless steel, due to its hardness, is very difficult to cut and far more challenging to modify on site. The elements of chrome and nickel provide stainless steel great corrosion resistance to oxidation, but stainless steel can be contaminated by using carbon steel power tools and other tooling equipment. Skilled shops with proper quality control programs and knowledgeable craftsman can prevent stainless steel contamination.

Conclusion
Specifiers for enclosures for wind applications must carefully evaluate all factors to ensure that an enclosure made of any material type will withstand its environment. The process for proper material and enclosure selection begins with a detailed consideration of the application. Each wind environment is unique, and all possible applications should be identified for the intended enclosure application. So start with a simple list of your needs and ask plenty questions, because failure of your enclosure can’t be an option when so many are relying on the wind industry to help reduce our dependence on fossil fuels and reduce CO2 emissions. 

Whether it is carbon based, nuclear, or alternative energy, production availability and equipment reliability are crucial issues for any power generation facility. For wind energy operations, managing multiple turbines in remote locations complicates how these goals are attained. Even with improved SCADA information, critical performance indicators are often recognized too late for any action beyond watching the generator fail.

This article will identify some of the actions owners or operators of wind generation systems can take to extend the life of their assets. Proper periodic maintenance and field data analysis—as well as winding design and other improvements while in a service facility—contribute to the safe, reliable performance of a wind generator throughout its lifecycle.

Field Lessons
The owners of a wind power system have invested millions of dollars in capital equipment. To keep their venture paying back they must maintain and service the equipment as required per the manufacturer’s recommendations—at a minimum. In saying that, there are additional “best practices” recommended in consensus industry standards.

Having a savvy and qualified field service group servicing your equipment is of the utmost importance, which will increase the life of generators and other related components. Because these machines can be complex, managing common service items as well as unforeseen issues on a daily basis means the difference between acceptable uptime and unprofitable downtime. So, what maintenance philosophy should be used to maximize the return on investment and increase reliability? Let’s begin with a discussion about the applications for preventive and predictive maintenance.

Preventive/Predictive
A combination of both preventive maintenance (PM) and predictive maintenance (PdM) go hand in hand when best practices are implemented within wind generator fleets. Preventive maintenance techniques would typically be performed on carbon brushes, slip ring assemblies and the cleaning of internal generator components, etc. Predictive maintenance techniques would be used to circumvent possible mechanical and electrical failures of the wind generators by performing tasks such as vibration analysis, infrared thermography and monitoring electrical conditions.

Driving a car to and from work every day with the expectation of reliability and many years of service requires maintenance on a continuous basis, both preventive and predictive. The same is true of a generator fleet. Critical testing and maintenance tasks collected from many years of experience in servicing wind generators are identified in Table 1 and Table 2. Following these basic guidelines will result in quicker return on investment by creating greater system reliability and longer service life.

It should be noted that the tasks outlined in Table 1 require the unit to be offline and de-energized. Because electricity is a highly toxic substance, only properly qualified and experienced personnel should be asked to perform these tasks. Technicians must be able to identify the hazards associated with their jobs (Figure 1) and receive the proper training and documented education to be considered qualified.

The second set of tasks outlined in Table 2 require even more experience and higher levels of qualification, since they involve energized and rotating components. Electric shock, arc flash, and mechanical hazards are all very real and prevalent during the process of performing these tasks. Don’t take shortcuts on personnel training or work practices, because the results may be disastrous. These requirements should extend to on-site personnel and all outside contractors. Both must be qualified and properly trained.

Preventive maintenance techniques can help prevent common failures such as shorted or faulted slip rings due to excessive carbon build up or insulation failure, and predictive maintenance techniques can help predict common failures such as the electrical fluting of bearings. Using these various techniques can help with decision-making related to the frequency of testing and planned maintenance outages (see note, Table 1), and also help prevent a catastrophic generator failure.

Speaking of testing frequency, many factors must be considered when scheduling the work associated with the servicing of machines in the field. The idea is to perform scheduled maintenance and servicing as needed, but not too often or too little. Factors that affect the decision-making process include items such as equipment criticality and device significance, current condition, lubrication life, maintenance history, operational history, industry experience, maintenance philosophy, operating environment, time allowed for maintenance,  and the manufacturer’s recommendations.

Service Center Lessons
Just like field services, utilizing a quality service center will help increase the life of your generator and help assure wind park productivity. When evaluating a service center there are many important criteria. At a minimum, the service center should be a member of the Electrical Apparatus Service Association (EASA) and perform repairs in accordance with the ANSI/EASA Standard AR-100, Recommended Practice for the Repair of Rotating Electrical Apparatus. Not only is being a member of EASA important, but having a Quality Management System (QMS) and being ISO certified are additional criteria to look for when evaluating a service center. Some of the highlighted requirements for service centers are shown in Table 3.

When generators find their way into a service center for repairs it is normally due to mechanical or electrical failures while in service. There are many lessons to be learned from the service center’s failure analysis procedures if they have a QMS program in place. Evaluating the failure data can provide valuable information for the balance of your fleet. It’s the “learn from your mistakes” effect that oftentimes pays back many times over. Improper lubrication, inadequate ventilation, brush grades and slip ring material choice, etc., can all contribute to failures, but if these are not identified at the initial inspection they are often missed.

Extending Generator Life
Wind generators come in all sizes and ratings, and the designs of these units certainly vary among manufacturers. They have one thing in common, however: they all eventually fail. A wind generator’s life can be influenced by ensuring that the best-possible processes and procedures are in place during a shop repair.

When a wind generator has failed much time, effort, planning and money are needed to remove the generator from the wind-farm site and ship it to a qualified repair facility. Once the repair facility receives the generator and inspects the unit after disassembly they can normally recognize what repairs are needed. However, what is crucial at this stage is understanding why the machine failed. This critical point of the inspection process—the root cause failure analysis—is often left out, or forgotten. Corrective actions must be taken at this point.

Examples of the root failure causes include improper grease in a given unit, the breakdown of wire strand insulation due to heat over a long period of time or even improperly sized cable. Poor craftsmanship from a manufacturer or repair shop could be a culprit, as well. There are many possibilities that can lead to failure and, again, this must be investigated so corrective action can be put into process on the front end of the repair cycle.

As mentioned earlier, qualifying a repair shop is an essential part of the repair process and is detailed in Table 3. One additional item to look for in particular is the working area where the repair of the windings will be performed. When installing the windings in a wind generator, care must be taken and the condition of the work area must be considered. A climate- and condition-controlled winding area, or winding bay, should be a high priority for any repair facility to ensure the best possible results can be attained for the rewind of a wind generator. Any dust or foreign debris within a stator or rotor during the rewind process can cause serious problems for the generator once placed back in service, so contaminants need to be avoided.

Insulation System
The most-costly components of a wind generator repair are the electrical windings of the rotor and stator. Manufacturers mass produce wind generators, and many use the minimum insulation required. For example, many manufacturers still use class F (155° C rating) material for the insulation system in the stator or rotor windings. Heat developed during the operation of a wind generator is detrimental to stator and rotor winding insulation life. It is generally accepted that every 10° C rise in temperature will result in a halved life expectancy of the insulation. A simple upgrade to class H system (180° C rating) will enable the repaired wind generator to operate at a higher temperature with little additional cost (Figure 2). The entire insulation system and methods of application should be equal or better to the original machine manufacturer. All components of the insulation system must be compatible with each other with respect to electrical, mechanical, and thermal characteristics. At the end of the day, the insulation system should withstand the normal operational requirements of the machine, as well as necessary over-potential tests performed at the service center.

Operating Speed
Many manufacturers originally designed their wind generators to operate at 50 Hertz. In the United States, the wind generators operate at 60 Hertz. It is imperative the fiberglass bands used to control rotational forces on rotors will accommodate the speed increase caused by 60-Hertz systems. A mathematical formula can be obtained to calculate the amount of banding material needed from organizations such as EASA. Another important aspect is to balance the rotor while at the service center (Figure 3).

Conductor Type
The conductor (or magnet wire) strand insulation is vital for the life of any repaired wind generator, as it is the primary insulator for the winding circuits. There are many different types of polymers and enamels for the wire insulation. Some materials may be very resistant to voltage spikes but susceptible to mechanical stress, while other types may be very tough mechanically but difficult to install. Additionally, some types of wire are very costly and are not beneficial to use. Choosing the proper wire strand insulation to be used can be difficult, but it is essential for the extended life of a repaired wind generator. It would benefit any end user of a repaired wind generator to be involved in deciding which type of wire insulation to use in this part of the process.

Processing
Assuring proper spacing and clearances, correctly impregnating resin and bracing winding end turns is often overlooked. Airflow can be blocked around end turns, causing unwanted higher temperatures. This can lead to premature failure on any repaired wind generator. Additionally, vibrations of wire strands not bundled as a solid mass or without proper resin impregnation can also lead to premature failures. Many times this common mistake is made when a failure analysis is not properly conducted. Care must be taken when adding additional components to stiffen the winding end turns, because additional insulation uses more space (Figure 4).

Insulating Resin
The resin treatment of winding used in wind turbine generators should be done utilizing a vacuum pressure impregnation (VPI) system. Many wind generators have very long bores compared to the inside diameter of the stator or rotor core. Properly processed, the appropriate resin will penetrate and fill the slots of a rotor or stator, and it will also create solid end turns that can dissipate heat more readily.

Summary
While this article highlights some of the key lessons from the field and the service center, the central message is to be involved with the service and repair process, pre-qualify service providers, and make sure all of the work is done safely. Uptime availability is important, and extending the life of generators will lead to higher performance values and greater financial returns. 

From blade manufacturers and wind farm designers to wind turbine operators and power providers, the companies involved in the production of wind energy have very different production applications. Yet both the personnel behind the blade production machinery and those responsible for wind turbine controls are tasked with similar goals: They must keep safety, reliability, and maintenance at the forefront to help their companies—and their customers—succeed. Key to accomplishing these goals is leveraging an integrated control solution.

The Rockwell Automation Integrated Architecture™ system provides that solution, integrating motion, sequential, safety, and power control on a single platform, all networked over EtherNet/IP for improved data communication and remote troubleshooting capabilities. Before discussing the system’s attributes and capabilities, allow us to provide a real-world example of how one company is benefiting from this technology.

Case History

Founded more than 13 years ago, Tecsis Tecnologia e Sistemas Avançados is dedicated to manufacturing blades for wind turbines. Tecsis is the only 100 percent Brazilian-owned company currently producing this type of blade. It exports all of its products, which are manufactured at its own production sites located in the city of Sorocaba, 96 km from São Paulo. Currently the company produces more than 12,000 blades, which have been installed in more than 10 countries in Europe (Spain, Ireland, and Greece) and North America (the United States and Canada), as well as in Australia and Japan, among other countries.

Tecsis ships its blades to clients from the port of Santos, some 70 km from São Paulo. Due to the length and weight of the blades, transporting them requires complex logistics. They are moved from Sorocaba to Santos in convoys of 25 to 30 trucks that pass through the capital and descend from the mountains during the nighttime hours. To prepare the blades for loading in ships, they are placed in special packing to protect the product to make it easier to transport and move them.

The Challenge

The length of the Tecsis blades varies from 37 to 50 m, and weigh from between 6 to 12 tons. However, before initiating the manufacturing process itself, Tecsis develops a specific design for the molds based on a detailed profile to meet the customer’s specific needs. The profile is determined by each customer’s specific design and depends on the location where the wind turbines will be erected. Each project is therefore unique. “This requires the manufacturing of specific equipment and tools, as well as carrying out the necessary tests to put the product into production, which can take months,” according to Aureo de Oliveira, electrical engineer coordinator at Tecsis. “Despite being highly ‘engineered’ in initial stages, the manufacturing of the blade is done completely by hand and requires about 24 hours.”

Each blade is manufactured in a die, also produced by Tecsis, in which layers of materials are applied. Modern wind turbines are made up of three blades, manufactured with reinforced plastic (polyester and epoxy) and fiberglass. Other materials are also used such as carbon fiber, steel, aluminum, textile fibers, wood, and wood epoxy. The blades must be lightweight, resistant, and rotate easily to withstand winds that can reach 180 km/h. Although the blades are able to stand up to these wind speeds, the wind turbines normally operate at winds up to 90 km/h. Any higher speed will shut down the system, and the generation of power will be cut off as a safety measure to protect the structure.

Two key factors are responsible for keeping the blades lightweight and resistant: the temperature of the material that is applied to the mold, and the amount of time that this material is exposed to that temperature.

The Solution

Automation of the heating system plays an important role in the final solution because it allows precise temperature control throughout the entire process, eliminating safety and quality deviations. However, the system used by Tecsis did not meet all of the company’s needs. Aureo explains how control was handled, and what the difficulties were. “Control and handling of the heating process data were limited because control was done individually by heat section, which made it difficult to balance the temperature between the controlled points,” he says. “In addition, the generated reports did not provide information to track the heating process variables in real time, or to create production history. It wasn’t possible to compare the temperature of the control zones and the different areas, which would require more homogeneous temperature stabilization in the mold.”

With the CompactLogix controllers and RSViewSE Stand-Alone software, Tecsis was able to reduce mold assembly time, increase panel installation reliability, and allow employees to dedicate more time to other applications, in addition to decreasing indirect project costs.

The Results

In practice, the system is simple: There are dozens of temperature measurement points (thermocouplers) for each mold. The signals emitted by the thermocouplers are transmitted to the CompactLogix PAC, which receives information about the temperature of each point and activates heating when needed. The need to heat each point at a lesser or greater degree is determined by the temperature profiles that control these variables throughout all production stages of the blade. Each customer has a unique design based on the process conditions of each blade, and the temperature curves change for each customer. In this regard Tecsis also obtained a degree of autonomy, because its own technicians defined the system parameters for the curves.

“We plan to apply Rockwell Automation controls to all systems where electric control of temperatures is required because the application is easy and configurable, and it reduces maintenance,” says Aureo, emphasizing “the excellent service, information security, and broad product knowledge” he enjoys while working with the Rockwell Automation team of engineers.

The Details

After companies like Tecsis manufacture blades and other turbine components, wind-farm designers and turbine OEMs can leverage the same scalable Integrated Architecture system for all of their control needs, including:

• Pitch control systems to position the blades and spill wind when the turbine is operating above the rated wind speed, while also maximizing power output;
• Yaw control systems including across-the-line starters, soft starters, or variable frequency drives to keep the turbine facing the wind;
• Main control systems to calculate the power optimization algorithms based on available wind, pitch angle, and blade geometry. This system then sends a set point to the generator or grid tie inverter and can provide for many startup and shutdown safety checks as well;
• Condition-monitoring systems to monitor vibration on the main drive shaft bearings, as well as each gearbox and generator bearing.

Standardized Safety

In each of these applications, as in the manufacture of the blades themselves, safety is critical to protect both people and the large capital investment that a turbine represents. Any diagnostic failure would typically force the wind turbine into a shutdown mode, which commands the blades to a shutdown position.

An extremely important safety consideration for wind turbine manufacturers is overspeed protection. The turbine control system should include a safety-rated circuit to monitor both the high-speed and low-speed ends of the gearbox and hub, and help avoid reaching a specified rotational speed. If the limit is reached, the circuit can initiate a shutdown sequence, protecting the turbine and any people on or near the equipment. In the past decade, safety-based control has made its way into manufacturing and assembly equipment controls designs. Similarly, wind turbine control designs are adopting many of the same safety design practices.

Improved Reliability

Wind turbines, particularly those located offshore, also require a control system with excellent reliablity that will stand up to the high-temperature and harsh, corrosive environments that exist out at sea. Controllers designed using hardened components suited for rugged environments can help provide OEMs and wind-power providers with excellent reliability and reduced panel costs. In addition, operational costs and carbon emissions are reduced because these controllers do not require the additional installation and energy costs associated with auxiliary heating and cooling systems.

Reliability also can be improved through predictive maintenance solutions. With the application of condition monitoring technologies such as vibration analysis, wind turbine operators can detect problems early, identify the cause, and take corrective action, all with minimal impact on energy production. This also helps optimize maintenance planning and costs.

Easier Maintenance

Unexpected shutdowns and maintenance of any kind can represent significant investments of time and capital. An integrated control system that is networked over EtherNet/IP can provide wind-power companies with remote-monitoring capabilities on the same platform as the standard control. This provides a smaller physical footprint with improved diagnostic and troubleshooting capabilities.

A commercially available, off-the-shelf solution like the Integrated Architecture system from Rockwell Automation helps reduce training needs and spare part requirements, offering global support no matter where the turbine is installed. In addition, with more utility companies using similar integrated control systems in their facilities, wind-power providers can more easily integrate with their systems to supply wind-generated power to customers around the globe.

From end to end, companies across the range of wind-system applications are taking advantage of integrated control solutions to help standardize safety, improve reliability, and reduce maintenance concerns. Learn more about Rockwell Automation’s solution by visiting www.rockwellautomation.com/solutions/integratedarchitecture/index.html.

Although wind’s power has been harnessed for thousands of years, the demand for green, alternative energy in the United States has spurred significant growth in the wind industry over the last decade, and particularly the last five years. The American Wind Energy Association (AWEA) reports that tower construction in 2009 surpassed all previous years with over 9,900 MW installed, bringing the total power contributed by wind in the U.S. to more than 35,000 MW.

The push for more-efficient towers with increased power generating capacity is driving tower dimensions to new heights. Turbine manufacturers have developed towers with more than 5 MW capacity and exceeding 125m in height. While the larger turbines provide more capacity, engineering and construction of cost-effective foundations for these towers becomes an increasing challenge.

Solid Support
Turbine foundation design considers traditional geotechnical engineering analyses for bearing capacity and settlement for static dead and live loading conditions much like conventional building foundation support. More importantly, though, the designs are often controlled by large transient pressures attributed to significant overturning moments from mean and critical wind characteristics applied to the turbine foundation. The geotechnical and foundation design must also provide stiffness characteristics of the foundation soils for acceptable tower performance under the wind gust loading conditions. Most manufacturers specify horizontal and dynamic (rotational) stiffness criteria specific to the particular of tower type that must be met.

Figure 1

Designers, contractors, and owners must weigh a variety of different foundation support solutions for their towers based on performance, cost, and speed of construction. Priding themselves on being part of the green movement, designers also seek to provide sustainable solutions for projects. These decisions are often simplified when the ground conditions allow for large turbines to be supported on shallow concrete mat foundations bearing on competent soils or rock. Shallow foundations—often hexagonal or octagonal in shape—provide significant economy and speed of construction in these cases. In addition to meeting performance requirements this approach is often a relatively sustainable construction method as well, because it avoids the increased carbon footprints and other detrimental environmental impacts often associated with more-elaborate foundation support solutions.

Unfortunately, many tower sites are characterized by weak or soft soils that do not provide sufficient support for the high applied pressures or meet the foundation settlement or stiffness requirements. Design teams must consider alternative support solutions including massive grading operations to remove and replace the unsuitable soil, installing deep foundations to bear on competent soils or improving the existing poor soils.

Deep Foundations
The process of massive removal (overexcavation) of the poor soil and replacement in thin, controlled lifts with high-quality engineered aggregate (similar to roadway or building construction) is a desirable and cost-effective solution when the depth of poor soils is limited to only a few feet below the foundation bearing elevation. Tower foundations may be designed as large mat foundations, the same as those supported on competent soils. The overexcavation process becomes more complicated when poor soils extend deeper below the foundations, or shallow groundwater results in the need for dewatering of the excavation. These conditions not only adversely affect the economics, but also the construction schedule. From a sustainability perspective, the earthwork operation uses locally occurring, natural materials to create the engineered bearing layer. The carbon footprint of the construction activities begins to add up, however, when considering transportation of both the engineered material and the poor soils removed, as well as the large earthmoving construction activity.

Deep foundation support is typically used when poor soils extend to considerable depths below foundations. Deep foundations—such as driven piles made from timber, concrete, or steel, drilled concrete shafts, or grouted auger-cast-in-place piles—are used to transfer tower foundation loads through soft, compressible soils to bear on competent soil or rock. It is common for deep foundations to be used to treat sites with soft soils extending more than 50 or 100 feet in depth.

Although the geotechnical and structural design of the piles and foundation may be more intensive, the deep foundation system will provide superior performance for settlement and bearing. The superior performance comes at a high cost, both financially and environmentally. Besides the initial high material cost, pile-supported tower foundations may result in longer construction schedules, often related to slow installation of the piles, delays in material fabrication, or delivery to remote sites. Steel pile foundations incorporate energy- and resource-depleting manufactured materials such as steel or concrete. Piles typically utilize material that is manufactured or fabricated at locations hundreds or even thousands of miles away. Although local, concrete production is also energy intensive and ozone depleting. The combination of the energy-intensive manufacturing process and the significant transportation efforts required for deep foundations to arrive at the site increase the carbon footprint of the foundation solution, adversely impacting the sustainability of the solution.

Reliable Reinforcements
A trend that has continued to grow with the wind industry is the use of soil reinforcement to improve the existing poor soils to support shallow turbine foundations. Soil reinforcement approaches attempt to balance the critical factors of cost, performance, ease of construction, and environmental sustainability that could control the success of the project. Figure 2
Soil reinforcement for tower support using Rammed Aggregate Pier® (RAP) systems designed by the Geopier Foundation Company have continued to gain momentum and provide value on wind projects. Intermediate foundation solutions using RAP systems have provided building foundation support in the commercial, industrial, manufacturing, and power markets for over two decades. The same soil reinforcement technology has been supporting turbines in Europe for over a decade and it is now commonly used to provide improved strength and stiffness of soft or compressible soils, eliminating the need for massive overexcavation and replacement and deep foundations for the support of wind turbine foundations in the U.S.

Installation of RAP elements (also known as Geopier® or Impact® elements) involves drilling a 24- to 36-inch diameter cavity or driving a specially designed hollow mandrel to design depths ranging from 10 to 40 feet, depending on design requirements. Thin lifts of aggregate are then placed within the cavity and vertically rammed using high-energy patented beveled impact tamping devices.

During construction the high-frequency energy delivered by the modified hydraulic hammer, combined with the beveled shape of the tamper, not only densifies the aggregate vertically to create a stiff aggregate pier but also forces aggregate laterally into the sidewall of the hole, resulting in lateral stress increase in surrounding soil. The lateral stress increase reduces the compressibility of the surrounding soil and promotes positive coupling of the RAP element and the soil to create an improved composite, reinforced soil zone.

RAP systems are designed to reinforce the poor foundation soils and provide adequate bearing support for the large foundation pressures. The soil reinforcement system is also specifically designed to deliver total and differential settlement control (angular distortion) of the foundations, and to improve the rotational and dynamic stiffness values to achieve the desired tower performance. The soil reinforcement designs are developed on a project-specific basis depending on the site conditions and the tower loading conditions.

The system utilizes locally available natural aggregate, or even recycled concrete for pier construction. In addition, the volume of material utilized for Geopier elements beneath the foundation is typically only 10 to 20 percent of the material required for massive overexcavation and replacement. These factors limit excessive fossil fuel required for material delivery and disposal, as compared to other solutions. The use of small, mobile excavators further limits the fossil fuel consumption and dramatically reduces the carbon footprint of the foundation construction activities, making the RAP solution a uniquely sustainable solution for the wind industry.

Team Approach
One such project that incorporated the Rammed Aggregate Pier solution was the Winnebago 1 Wind Farm in Winnebago County, Iowa. The project incorporated 78 meter Gamesa G83 2.0 MW wind turbines with static vertical loads of 2,936 kN (660 kips), horizontal base shear of 676 kN (152 kips), and overturning moments of 50,335 kN-m (37,120 ft-kips).

Soil conditions at the tower locations explored by geotechnical engineering consultant Terracon Consultants with supplemental borings provided by Barr Engineering Company showed variable-strength clay soils extending to depths of up to 18 feet, followed by competent soils. Combined with a groundwater level as high as 5 feet below the ground, the overexcavation alternative was expected to be costly and time consuming. Figure 3

At the recommendation of the project geotechnical engineer, the project team—led by Iberdrola Engineering and Construction—elected to incorporate a RAP foundation system to support the “inverted tee” octagonal foundations. The system was designed to improve the bearing support, to control settlement, and to provide acceptable levels of rotational and dynamic stiffness values. The 2.5-ft diameter piers ranged from 7 to 15 feet in length depending on the site conditions. To get the wind turbine towers up and running, Peterson Contractors installed 640 RAP elements in less than one month as they moved from site to site. The rapid installations allowed the sites to be quickly turned over for foundation construction with little time delay.

Performance of the RAP system was verified in the field using quality control observations and testing, including full-scale modulus load testing. The modulus load testing is performed to evaluate the stress-deflection behavior (stiffness) of the pier and verify the performance under the design stress levels. The system performed well, with less than 0.2 inches of pier deflection at a stress of over 18,000 psf; a pressure over four times the maximum applied pressure from the foundation.

Conclusion
Engineers, owners, and contractors continue to search for the most cost-effective and reliable support solutions for tower foundations. The balance of cost, speed of construction, quality, sustainability, and performance are paramount in the decision process. With over 1,000 MW of towers supported, the use of Rammed Aggregate Pier soil reinforcement solutions has helped maintain the proper balance for many project teams.