February 2011

At Avanti Wind Systems we work to design and manufacture the best work cages and fall protection systems currently available on the market, but technicians still must be trained and the systems inspected and maintained at least once each year. This is not only a legal requirement, but also a necessary safety precaution for technicians working in towers. Although there are some variations between states, U.S. law requires owners to perform annual inspections and to keep a certification journal including dates and detailed descriptions of the testing methods utilized. This journal must be present whenever the authorities perform a check. A description of these safety devices will not only make their utility clear, but will aid the owner in conducting these inspections.

Designing for Wind
An Avanti work cage allows technicians to avoid climbing up and down the ladder mounted inside of the tower. The cage will carry employees and their tools quickly and safely to the turbine nacelle. The system has also been designed so that it can be used during the construction phase, which means that equipment can be transported within the tower and to a platform close to the nacelle inside of the cage. It can be stopped at any point along its path, and when the doors are opened the drive mechanism is disabled and the technicians can work safely from the cage or climb out onto the ladder. Most Avanti work cages can carry two technicians and their equipment, and they can be used in most types of turbine towers that have been prepared for them. Avanti engineers are constantly developing new work cage systems to meet all industry needs. Figure 1

Avanti Wind Systems has also developed a ladder system with a pre-mounted fall protection system. It is a statutory requirement that all ladders in wind turbine towers should be equipped with a fall protection system that prevents service technicians falling from the ladders if, for instance, they lose their footing or suddenly become ill. The fall protection system stops the service technician after falling a few centimeters, and it remains locked until either the technician can continue alone or can be helped by colleagues. The system is based on a rail in the middle of the ladder. The service engineer’s harness is fitted with a click-on runner that then attaches to the rail. The runner is fastened to the engineer’s harness with a carabineer of the same type as used by rock climbers. The runner can be clicked onto the fall protection system anywhere on the ladder.

As the Avanti fall protection system is designed with a fixed rail on the ladder, several technicians can use the system simultaneously. When using wire-based systems, a technician must wait until his colleague has reached the nacelle before he can use the system himself. The Avanti system has been approved so that one technician can click onto the system every six meters. In other words, as soon as one technician is six meters up the ladder, the next one can start climbing. Figure 2

Avanti has also designed a system that makes it easier for technicians to climb towers. In simple terms, Avanti’s climb assistance is an endless rope that runs from the bottom to the top of towers. It is connected to a motor at the base of the tower, and here the engineer can set the system to a pull force between 30 and 40 kilos. When the technician links to climb assistance with a rope grab and gently pulls the rope, the system takes up the selected bodyweight in kilos during the entire ascent and descent. If the technician stops climbing the system stops as well, and then starts again after a gentle pull of the rope. The system reduces the risk of injuries to engineers’ knees, arms, shoulders, and feet.

Avanti’s climb assistance can be installed in both new and existing towers. In fact, Avanti has installed many climb assistance systems in older towers throughout the world. Older turbines must be serviced more frequently, and this means many hard climbs for technicians on ladders. This is why many owners of older towers decide to install the new system. Climb assistance is not an anti-fall protection system, however. It is merely an aid for engineers, and it must always be used together with an approved anti-fall system. The independent, international test institute Force Technology has approved the simultaneous use of the two systems, as they function perfectly together. This is an additional guarantee of safety.

Passing the Test
Our warranty for work cages, ladders, and climb assistance also requires test and inspection at least once each year by Avanti or personnel that we have trained. Turbine owners naturally accept that all the equipment in towers has to be correctly installed by authorized personnel. But a wind turbine tower is also a place of work, and in contrast to most other workplaces owners sometimes forget that safety equipment has to be tested regularly.

As important as this testing is, proper training is also critical. Every year Avanti Wind Systems trains several thousand wind turbine industry employees throughout the world, either at the Avanti training centers or onsite at wind farms where the employees are working. It also provides closed courses for employees of the same company, or open courses in which anyone can participate. Not many people doubt the importance of having effective safety equipment, but it is equally important that technicians working with the equipment are trained to work with it, and to deal with both dangerous situations and accidents. Figure 3

During the courses, which can run over several days, technicians learn everything about safety in relation to fire, evacuation, and rescue from towers. The courses include theory, but also rely heavily on practical exercises in towers. For example, one compulsory part of the course consists of a course member acting as if he is unconscious while his colleagues fasten him to the safety equipment from the rescue and evacuation unit, which must be in the nacelle, and then lowering first the unconscious subject and then themselves outside of the nacelle.

This is a situation we hope will never occur in the real world, of course, but one that requires special training for any employee who may encounter such a challenge. It’s also important for owner’s to keep in mind that they are responsible for providing this training to anyone who will be relying on this safety equipment, and that the training must be kept current as well.

The Avanti Advantage
Founded in 1885 as the Avanti Ladder Factory in Denmark, scaling heights safely has been the company’s primary concern for more than a century. Avanti Wind Systems has designed work cages, ladder systems, and fall-protection devices specifically for wind technicians for the past 26 years. Today it is the world’s leading producer of work cages and other personal safety systems for use in wind turbines, having supplied more than 12,000 such devices all around the world, totaling more than half of all those installed.

Avanti Wind Systems has recently begun production of ladder systems for wind turbines in the United States. Today the manufacturing of aluminium ladders is taking place near the company’s North American headquarters in New Berlin, which is close to Milwaukee, Wisconsin. The factory is intended to supply the North American wind market. Until now Avanti has manufactured its ladder systems in China and Germany, and these factories will continue manufacturing for the rest of the world.

Avanti Wind Systems has also become a strategic distributor and service partner for Cresto ResQ products in the wind energy industry. Cresto—previously named ResQ—has sold more than 25,000 sets of evacuation and rescue equipment for wind turbine towers around the world. The agreement between Avanti Wind Systems and Cresto means that Avanti has established training and service centers for Cresto equipment in the United States, Germany, Great Britain, Spain, India, Australia, and China, where Avanti already has offices and factories.

As with everything related to wind turbine tower safety, the evacuation equipment must be inspected and tested at least once each year. But Cresto has developed approved packaging that makes it possible to test the safety equipment just once in 10 years. The safety equipment is vacuum packed—that is, put into a foil bag from which all the air is pumped out. When the rescue device, ropes, and other accessories are not in contact with oxygen and humidity they last longer, since the materials cannot wear down. The vacuum packing also safeguards the equipment against the wind turbine tower vibrations. What must still be tested each year in this instance is the integrity of the vacuum seal, in order to make sure that no equipment or mateiral degradation has occurred. If the packing is soft, it’s a sign of a hole in the bag, at which point everything will have to be inspected, tested, and vacuum-packed again.

The Avanti-Cresto partnership also means that the two companies will jointly launch new series of harnesses and other personal protection equipment for technicians working on wind turbine towers. Safety has always been the top priority for Avanti Wind Systems, and the goal is clear: Safe work in wind turbines, anywhere in the world. 

When it comes to getting a job done right, having the right tools at hand can make all the difference. Whether that job is performing routine maintenance at a nearby jobsite or an emergency repair on a wind tower 250 feet in the air, technicians need to be able to get their hands on the necessary tools fast.

And it’s not just about getting tools to the jobsite, it’s about getting the right combination of tools without leaving any crucial pieces behind. Figure 1

Using mobile tool cribs that come with tool-control technology, as well as devising specialized tool kits for specific jobs, can help technicians work more effectively and productively to get the job done quickly.

Making Tools Mobile
Sometimes getting the tools to the jobsite itself is half the battle, especially if it’s located halfway across the world. A mobile tool crib can be an efficient and economical solution by allowing tools to travel from one site to another, instead of incurring the high cost of supplying both sites with the same set of tools. When managed properly the mobile tool crib is an integral component of a jobsite, especially at larger wind farms or sites in remote locations. 

The idea and practice of a tool crib is nothing new, of course. What is new, however, are well organized tool cribs that offer a totally controlled environment, increasing jobsite productivity while reducing costs. Next-generation mobile tool cribs from Snap-on Industrial offer the latest technical advances including asset management software that reduces tool replacement costs while increasing jobsite efficiency. 

Mobile tool cribs are also becoming more mobile. Instead of having technicians go back and forth to a tool crib, which is often in an existing facility a distance from the actual jobsite, tool cribs are now coming to the workers. Mobile tool cribs can be customized and set up directly on a jobsite. Modular in design, mobile tool cribs include storage and control systems, cabinets, and shelving, plus electrical components that include lighting, air conditioning, and electronic keyless entry systems. Some suppliers even provide trained tool crib attendants to manage the mobile tool crib on site. 

Mobile tool cribs house all tools and equipment, typically in conex-type containers of 20 to 40 feet in length. To aid in tool organization, mobile tool cribs come with custom-built cabinetry and shelving, detailed with foam inserts to keep even small tools in place. All tools housed in the mobile tool crib have been specified by the customer. This allows the user to avoid paying for unneeded additional tools, and ensures that the requested tools will be integrated into the organization system before the crib arrives.

Keeping Inventory
Once the mobile tool crib is operational on the jobsite, keeping track of it allows workers to quickly access tools when needed. Traditional tool cribs are notoriously difficult to navigate, with a take-it-when-needed tool checkout policy that can lead to wasted time and lost tools. To alleviate this problem, mobile tool cribs use asset management software to help increase tool accountability.

Asset management software keeps track of the tools and also enables the tool room attendant to track usage trends, output valuable management reports, and provide a clear indication of what truly is available in inventory. When new tools are added, the tool crib attendant assigns them a bar code and updates the system to reflect an accurate count. Figure 2

Security also can be an issue at job sites. Newer tool cribs use ID cards instead of keys, so that managers know who accesses the crib and when. The same key-card is used in connection with the tool control system to identify who is checking out which tools. This way lost tools can be traced back to the worker that last accessed them, whether or not they were returned to the crib, reducing misplaced tools and costs. When the job is finished, mobile tool cribs can be loaded onto a tractor trailer for delivery to the next jobsite. The company also receives a printout of what tools need to be replaced, recalibrated, or recertified before the mobile tool crib moves on.

Custom Kitting
Nothing is more frustrating to a technician than when that one crucial tool to get a job done has been forgotten, even if it’s simply back at the tool crib. Consider the technician working 300 feet in the air on a wind turbine. It’s common for technicians to carry a bag with more than 100 tools from turbine to turbine, but what happens if he forgets a tool on one turbine after scaling another? That last thing he wants to do is scale that other turbine just to retrieve a tool. Whether the technician is up in the air or on the ground nothing wastes time, productivity, and money more than needlessly walking back and forth from the job to the turbine or tool crib to retrieve a forgotten tool. However, one way to make sure that the tools needed for specific jobs make it there is by creating a custom tool kit.

Here’s a common application for how a custom kit works. For example, a piece of machinery that undergoes routine preventive maintenance every three months would have a kit created and ready to check out from the tool crib with all the necessary tools needed to complete maintenance on that job. By kitting those tools in the crib, a technician needing to do that repair doesn’t have to check out the 10 tools individually. Rather, he simply checks out the kit, knowing he has all the tools to complete the job. Figure 3

An ideal use for a custom kit is preventive maintenance. That’s because many facilities have equipment, machinery, and systems that require regular ongoing preventive maintenance as part of their operation. A custom kit can be developed to house all the necessary tools required to perform preventive maintenance on specific components. All the technician has to do is check out the kit from the tool crib to know that he has all the necessary tools to complete the job.

Getting Started
The first step for a customer in creating a custom kit is to work with a Snap-on Industrial representative to identify the tools needed for the particular job, and in the process look for efficiencies in the way maintenance is performed. A larger customer may have up to 15 different applications requiring kits of various sizes. The key is identifying those needs and developing kitting solutions that improve efficiencies and ultimately save time and money. Figure 4

Just as organization and accountability are important aspects of the mobile tool crib, these attributes are also sought after in custom tool kits. All kits come with cutout foam placements for tools, which provide a higher level of organization and accountability. This added level of organization offers inventory control for safety and theft prevention advantages and ensures the technician can account for every tool in the kit at a glance.

The days of lost tools are quickly becoming a thing of the past. Every day mobile tool cribs and custom kits are saving technicians time and money. As companies look to continually improve efficiencies, today’s tooling solutions offer the combination of good asset management processes and high tool security. It’s a winning combination. 

“He who stops being better stops being good.” What is the relevance of this quote from Oliver Cromwell, founder of the English Republic, to the wind industry? Fully developed upgrades are a performance booster for any wind turbine generator (WTG) since the optimization of turbine components leads to significant improvements, such as the reduction of recurring faults and the increase in availability and performance. Upgrades can also prevent serious damage and help to avoid high repair costs.

The development of WTG upgrades should be an integral part of every independent service provider’s (ISP) portfolio. By changing only a few parameters of a WTG control system, a turbine can often be optimized quickly and easily. The capabilities of ISPs can be measured by their ability to master the challenges faced in the development and introduction of WTG upgrades. Turbine owners and operators should check whether their ISP has proven processes in place for developing and implementing optimizations.

Optimization Approaches
It is necessary to recognize and describe a problem at the beginning of the upgrade development process. The catalyst for such an analysis can come from many directions including the ISP, as a result of fault analyses, from on-site installers reporting about recurring irregularities within certain turbine components, or from operators suggesting possible optimizations.

The first step is to determine the cause of the problem. Neither evident nor obvious, problem analysis is typically one of the greatest challenges in the upgrade development process. ISPs that provide continuous fault analyses on contracted turbines are the most knowledgeable and have access to key data necessary to identify and analyze problems. A detailed cost/benefit analysis can determine whether the development of a turbine upgrade makes economic sense for the owner. To develop a concrete solution, an ISP should have in-house mechanical and electrical engineering knowledge. They also need access to very specific competencies from partners during the development process as well. Figure 1

A continuous documentation process is essential to seamlessly retrace every development step of an upgrade. This is important even when carrying out simple design changes, particularly if a solution unexpectedly fails to produce the desired on-site improvements. A risk assessment is also recommended, since an upgrade may lead to design changes that could affect a turbine’s operational safety.

Finally, the prototype of an upgrade must demonstrate that it is actually beneficial. This sometimes requires longer on-site testing and evaluation to ensure practical suitability, as well as sustainability.

Defining DMAIC
The path to an upgrade can be reduced to a simple formula known as “DMAIC”—or Define, Measure, Analyze, Improve, Control—which is a core process of Six Sigma. Since the 1970s Six Sigma has a proven track record in the industry as a statistical quality management system and is a recognized and successful method for defining DMAIC processes. The systematic use of such methods as part of the development of WTG upgrades is a reliable way to realize measurable improvements in turbines. Decision makers should not only check whether ISPs use such systems for development processes, but also whether or not they support them with further quality assurance measures.

Critical Situations
Turbines containing three-point bearings provide an excellent upgrade example. With the traditional bearing configuration, grave consequences may result if the main bearing fails and the damage is not detected in time. The damage produced by the failure allows the main shaft, including the rotor, to move toward the back of the nacelle. The wind pressure acting on the rotor pushes the entire gearbox up to nine millimeters backwards until the displacement of the drivetrain is finally halted by a steel block. As the gearbox rests against the steel block, the pressure of the drivetrain is transferred to the planetary carrier bearings. The result is the complete destruction of the planetary carrier bearings and, therefore, the gearbox.

Continuously monitoring the gearbox’s position prevents any serious or costly damage from occurring. Availon developed a new upgrade, mainly consisting of two inductive proximity switches, to solve this problem. Using control system error messaging, along with the remote monitoring system, they constantly monitor the position of the gearbox. If the drivetrain deviates from its original position, a fault message is transmitted to the remote monitoring system. To prevent any further damage to the gearbox and main bearing, the turbine is then immediately shut down by the control system. The turbine can then only be restarted on-site once the position of the gearbox has been inspected.

An on-site inspection can evaluate the degree of damage to a main bearing. Replacing the main bearing is usually inevitable, and already represents a considerable investment, but it is less than the cost incurred from a complete gearbox replacement. To prevent such situations from becoming critical, this upgrade is readily available.

Replacement Problems
During maintenance, replacing carbon brushes in Vestas’ V80 and V90-2 megawatt turbine generators is time consuming because of issues that arise from the original attachment of the brushes. When removing the brush attachment, other components may fall into the slip ring compartment. Due to the arrangement of the generators, the fallen components are difficult to reach without dismantling the slip ring even further. The added work contributes to lost time and higher service costs. Figure 2

A new attachment solution significantly decreases the time needed to replace carbon brushes. It also prevents parts from falling into the slip ring compartment during maintenance and repairs. In addition, the new upgrade increases the service life of the carbon brushes. The original mounting system consists of only one spring, causing the brush sides to wear unevenly during operation. As a result the brushes have a limited service life of approximately 12 months. The new mounting upgrade, coupled with a configuration change, prevents this irregular wear of the carbon brushes. Availon field trials have shown that the upgrade increases service life from 12 to 18 months, on average.

Improved Ventilation
Availon developed a ventilation management improvement for the transformer used in the Vestas 2 MW turbines. The upgrade applies to turbines installed in extreme weather locations, such as along coastal areas where transformers fail and need replacement every two years. Such frequent replacement represents a substantial investment, but is necessary because of unfavorable air circulation and contamination due to the salt in the air. The salt accumulates on the transformers and leads to corrosion of other components.

The upgrade uses special deflectors to direct the air through the nacelle so less enters the transformer room. Using a specially controlled fan, only 20 percent of the cooling air is channeled into the transformer, which is sufficient for cooling. The new air conditioning system for the Vestas 2 MW turbines aims to fully prevent the development of salt deposits in the transformers in the future, thereby increasing service life considerably.

Portable Control Box
Maintenance or other work performed in the nacelle of Vestas turbines has repeatedly been a problem for operators. In the past, one person had to operate the WTG control system from the tower base so that work could be carried out in the nacelle. With the new control box Availon has developed, this procedure is no longer required. The control box is a portable operating console connected by the Opto interface in the nacelle and can control the turbine directly from the nacelle.

Extending Blade Bearings
The life of rotor blades mainly depends on their seals. Due to constantly changing weather conditions, seals must meet specific requirements. If they don’t, it won’t be long before the blade bearing suffers damage.

Generally, a distinction is made between contact and non-contact seals. Depending on the type of relative movement of the components being sealed against each other, a distinction can be made between static (no movement), translational (linear movement), and rotary (rotating movement) seals. A blade bearing seal is a contact-sealing element with a rotational relative displacement caused by the displacement of the blades. Located on the stationary outer surface of the pitch bearing, the seal should prevent water intake and grease leakage onto the moveable inner race of the pitch bearing. Figure 3

But what happens if a seal does not work correctly and doesn’t prevent unwanted substance transfer? A damaged seal often allows a discharge of bearing grease and contaminates the equipment or ground with grease residue or dripping grease. As a result, grease must be replenished often.

A greater hazard is presented if rain enters the rotor blade bearing and mixes with the lubricant. This will shorten the life of the bearing, partially due to corrosion. Water that penetrates the seal can also cause an increase in error messages from the pitch controller and failure of pitch drives or pitch bearings. Replacing sealing profiles is nearly impossible due to their installation positions, so it is incredibly costly to fix the issue. To avoid such problems, a rotor blade bearing seal must be protected against environmental influences. The solution is a cover, or sealing hood, that meets the following special requirements:

• Protects the bearing against water intake;
• Is resistance to oil and grease;
• Has excellent aging resistance;
• Offers easy installation;
• Has an extra-secure seal, despite significant centrifugal forces;
• Compensates for large tolerances, and;
• Is easy to service.

The hood upgrade consists of several half shells that are glued and bolted together at the joints and positioned around the rotor blade. A tension strap positions and secures the hood profile of the cover on the rotor blade. The sealing hood is made from shapeable chloroprene with special auxiliary tools and tempering furnaces required to achieve reliable shaping. Figure 4

Dichtomatik GmbH and Availon partnered to create the hood upgrade. Headquartered in Hamburg, Germany, Dichtomatik has many years of experience in the field of sealing systems for the wind industry. Through the development of upgrades to various turbine systems, the service life of the turbines can be extended and unplanned maintenance costs reduced. The greater reliability of wind turbines will allow for continued implementation of this renewable resource in the coming years.  

The North American wind turbine market has experienced rapid growth in the last six years. Support for wind energy at the federal level in both the United States and Canada has fluctuated, but many states and provinces have responded with their own incentive programs and renewable energy standards. These initiatives, coupled with comparatively stable federal support, have been recent major drivers of the boom in wind turbine installations across North America.

Figure 1

But what does this mean? Simply put: Adaptation. Most wind turbines installed in North America and Canada are actually designed by European companies or based on European designs. These designs were typically performed to IEC standards and European electrical requirements, which are often not compatible with the requirements of U.S. and Canadian electrical code and product standards. Unlike utility-owned power plants, most wind turbine installations in the U.S. and Canada are owned by Independent Power Producers (IPPs) and fall under the requirements of the National Electric Code (NEC) or Canadian Electric Code (CEC). Historically, though, many jurisdictions have not enforced the requirement of the NEC or CEC, which means wind turbine designs have not been adapted to North American requirements.

Designing to Comply
Wind park installations are more prevalent than ever. As a result, the enforcement of existing electrical codes and development of wind turbine specific standards is becoming more stringent across all markets. Wind turbine manufacturers face the daunting task of redesigning their products to meet an evolving set of requirements. The challenge for manufacturers is to adapt their designs without compromising key design features and differentiators. Additionally, manufacturers need to distinguish between aspects of the design that must be modified to comply with North American regulations and where they can avoid making changes so as to minimize the time and cost of the redesign effort.

There are many aspects of the wind turbine electrical system that require evaluation to determine the consistent elements and differences between IEC requirements and the corresponding U.S. and/or Canadian requirements. One key area is the requirements for the major critical components of the system. These critical electrical components include switchgear, control systems, power conversion equipment, pitch and yaw drives, power distribution, cooling systems, and most importantly, the generator.

The generator is the most critical element of the wind turbine design. Why? The generator is not only more expensive, but it is the single piece of a turbine that is the most difficult to replace, or redesign, to achieve North American compliance. While there are significant differences between the standards for generators with the same design across markets, an understanding of the differences—as well as the similarities—can enable compliance for the European Union, the U.S., and Canada.

Maintaining Standards Across Markets
The main IEC standard for generators is IEC 60034-1 [1], along with other parts of the IEC 60034 series that cover specific aspects of construction and performance. In addition, there are standards for component parts of the generator, such as the insulating materials and overall electrical insulation system. The full list of applicable standards will depend on the type and construction of the generator, but the following are typical:

• IEC 60034-5 [2] covering Ingress Protection (IP) classification;
• IEC 60085 [3] for thermal evaluation of electrical insulation systems;
• IEC 60034-18-1 which provides general guidelines for the functional evaluation of insulation systems, and;
• IEC 60204-1 [4] for general electrical safety.

In the U.S. the corresponding standards are UL 1004-1 for rotating equipment and UL 1004-4 with specific generator requirements. Other related standards include:

• UL 1446 [5] for electrical insulation systems (random wound generators);
• IEEE 1776 [6] for electrical insulation systems (form wound generators, more common for large wind turbines);
• UL 50E [7] for enclosure requirements, and;
• UL 746C [8] for properties of polymeric materials supporting live parts.

In Canada, CAN/CSA C22.2 No 100 is the standard for motors and generators. Rather than a separate standard for the generator electrical insulation system, C22.2 No 100 includes testing to qualify electrical insulation systems. Other applicable Canadian standards include:

• CAN/CSA C22.2 No 0.17 [9] for polymeric materials properties;
• CAN/CSA C22.2 No 0.4 [10] for grounding and bonding, and;
• CAN/CSA C22.2 No. 0 providing general guidelines related to the Canadian Electrical Code.

Apart from the differences in specific constructional or testing requirements, there is a conceptual difference between compliance in the E.U. and in North America that can affect the steps that need to be taken to demonstrate compliance with North American standards. In both the U.S. and Canada, equipment such as generators is certified by third-party companies that must verify and document complete compliance of the equipment to all the applicable requirements from the relevant standards.

Figure 2

These third parties are called Certification Organizations (CO) and Testing Organizations (TO) in Canada, and Nationally Recognized Testing Laboratories (NRTL) in the U.S. These companies, such as Intertek, are accredited to perform testing and issue certification. Any equipment bearing a certification mark from one of these third parties has been fully evaluated and there is documented evidence of compliance.

In the E.U. manufacturers are required to comply with E.U. directives, but the use of standards is optional and manufacturers are able to self-certify without a third party’s involvement. Self-certifying manufacturers must document compliance with the essential requirements of the directives and, assuming standards are used as the means to determine compliance, the manufacturer will perform tests as defined in the normative standards and document the results to demonstrate compliance. Since standards are optional and manufacturers are free to make their own judgments about methodology, applicability and results of testing, this information often cannot be used for demonstrating North American compliance even where the testing or constructional requirements overlap. When third-party testing data from an accredited source is available, equivalencies may be drawn between the IEC test results and the U.S. or Canadian standards that would allow use of the IEC results to satisfy the North American requirements.

Generator Construction
The principle of third-party qualification applies not only to the overall generator assembly, but also to the components that comprise the generator. Certain components are deemed “critical” when the generator is evaluated for compliance. A critical component is one that must satisfy requirements mandated by the product standards in order for the generator to be considered compliant. This can include aspects of the component such as dimensions, materials used, material properties, electrical ratings, and even the shape or the color. The generator manufacturer cannot rely solely on ratings and data from the component manufacturer to demonstrate compliance. Instead, components used in the generator should either be certified, indicated by the CO or NRTL mark on the component. If not indicated by either mark, then the component will have to be qualified for use by the CO or NRTL evaluating the generator.

Another key difference between most European and North American standards with respect to wind turbines is that European standards are more performance-based. Simply put, this means there are certain performance criteria that must be met in order for the product to be considered acceptable, while North America standards are more prescriptive, providing specific constructional requirements. For example, both the U.S. and Canadian generator standards have specific minimum thicknesses for cast metal, sheet metal, and polymeric enclosures. They also specify minimum design criteria for the grounding, ventilation openings, corrosion protection, wiring connections, and wiring types. The European standards applied to generators do not provide this level of specifics, but the do rely on the performance of the product in the intended use to drive the specification of the construction and the components, materials, or equipment that goes into the generator. The term suitable is used throughout the IEC standards for electrical equipment with suitability left up to the manufacturer.

A key component of any generator is the electrical insulation system (EIS) or systems employed in the stator—and depending on the type of generator—in the rotor. The EIS of the rotor and stator can be different. The EIS consists of various electrical insulation materials (EIMs) in conjunction with conductors, which form the windings that are inserted into the stator and/or rotor core. The EIS provides ground-wall, phase-to-phase, and turn-to-turn insulation. Failure of the EIS can result in a phase-to-phase or phase-to-ground short circuit resulting in shock or fire hazards. The EIS between manufacturers vary greatly and can be a simple assembly of a few materials or a complex arrangement of many layers of differing types of materials. These EIMs are chosen for both mechanical and chemical properties. The combination of EIMs that comprise the entire EIS can behave very differently than individual materials would in isolation.

Figure 3

The EIS is subjected to various stresses in application including electrical (continually varying voltages), mechanical (vibration, abrasion, expansion/contraction), environmental (humidity, contaminates) and thermal (heating and cooling cycles during operation). In the U.S., the standards for EISs (UL 1446 and IEEE 1776) emphasize thermal stress as the primary stress factor. The test methodologies of the U.S. standards are designed to provide a thermal class rating, which is a measure of the maximum operating temperature that the EIS can be utilized at on a continuous basis and still maintain its electrical properties over the expected lifetime of the generator. The testing specified in the Canadian standard (C22.2 No. 100) also emphasizes thermal stress but does not necessarily include all of the materials comprising the EIS.

In the E.U. the evaluation of the acceptability of an EIS may be based on either thermal, electrical, or a combination of both as the primary stress factors and is referred to as a functional evaluation. There are several IEC standards that provide test procedures that may be used. Also, there are options for waiving certain environmental and mechanical stress tests during the functional evaluation based on the end product application. The basic methodology for conducting the thermal aging tests for both U.S. and IEC standards is not dissimilar and share a common basis. The methods for determining the thermal classification of an EIS are based on analyzing the failure profiles using the method of Arrhenius for both schemes. Therefore, it is not unreasonable that a similar result would be obtained regardless of which thermal aging scheme was utilized. The challenge in bringing an IEC design to North America lies in documenting the methodology, decisions and test results used to qualify the EIS per IEC requirements so that equivalency with the North American requirements can be determined.

Making the Grade
The list of required tests for generators specified in North America and European standards is very similar. In both cases they require testing of the entire product for temperature (thermal) performance: testing to demonstrate the prevention of electrical breakdown of equipment, and thus the prevention of electrical shock or fire caused by such breakdown; tests for the ability of personnel to access both moving parts and electrically live parts; and tests for abnormal conditions such as “over speed” or short circuit. Due to the difference in electric grid frequency between North American and Europe (60Hz versus 50Hz) some testing done for European compliance may have to be repeated at 60Hz. For generators such as permanent magnet generators (PMG) where the full output of the generator goes through a power converter the generator output frequency does not change for a 60Hz application so test results can likely be re-used for North American compliance.

While both the E.U. and North American standards contain over-speed testing, there is a distinct difference in the test methods. The U.S. standards require that the testing be conducted with the generator operating at 100 percent rated output at 120 percent of the rated speed. The E.U. standard allows waiving of the over speed test when agreed to between the manufacturer and the user. When performed, the E.U. over-speed test does not require the generator to be producing 100 percent of rated output. Both the E.U. and North American standards require testing to verify the rating of enclosures but the methodologies differ significantly. The E.U. testing is based on the concept of International Protection (IP) rating (see IEC 60529). This is typically a two-digit code that indicates the capability of the enclosure to prevent the ingress of objects such as tools, wires, and fingers, and contaminates such as dust or liquids.

Figure 4

A specific IP rating is not specified for generators, but is left to the discretion of the manufacturer and the intended use of the generator. In North America enclosures are rated according to an enclosure type designation using a number between 1 and 13. These ratings are mainly for applicability of the enclosure to various environmental conditions. Some enclosure type ratings are considered suitable for outdoor use and some are only for indoor use. Each rating has specific tests designed to determine the level of resistance of the enclosure to dust, ice, water or vapors. In order to verify performance under North American requirements, materials such as the gasket, seal, or other material from property performance are required for the enclosure to meet its rating. IP ratings do not require this type of testing so where these materials are employed additional testing may be required for North American compliance. While there is not a one-to-one correlation between IP rating and enclosure type rating, it is possible for IP rated enclosures to be qualified for use in North America. Similar to other components, the key is the availability of third party test data from an accredited source.

North American standards specify some tests that are not required by EU standards. These include tests to demonstrate that the generator output power is within 5 percent of the ratings of the nameplate for generator, specific testing for the ability of generator enclosures to withstand physical abuse, mechanical stresses and impacts during use, and tests to verify the capacity of generator lifting lugs.

Conclusion
While North American and European generator standards differ both in their concepts of compliance and in specific requirements, there still remains a significant similarity that enables the adaptation of European designed generators to the North American market. The adaptation can often be made without significant physical changes to the generator. The keys elements to accomplish this include thorough documentation of the components used in the generator construction particularly third-party test data, emphasis on the key constructional and testing differences between the standards such as qualification of the Electrical Insulation System(s), component certifications/qualifications, enclosure ratings, and testing such as over-speed and output power rating. A detailed analysis of the generator construction and documentation will determine which North American construction and test requirements can be satisfied with existing documentation and where additional information, analysis, or testing is required.  

References:
1) IEC 60034-1 is a standard and is applicable to all rotating electrical machines except those covered by other IEC standards, for example, IEC 60349.
2) IEC 60034-5 is Part 5 of IEC 60034. It is a standard for rotating electrical machines. Part 5: Degrees of protection provided by the integral design of rotating electrical machines (IP code)-Classification.
3) IEC 60085 distinguishes between thermal classes for electrical insulation systems and electrical insulating materials.
4) IEC 60204-1:2005+A1:2008 is applicable to the electrical equipment or parts of the electrical equipment that commences at the point of connection of the supply to the electrical equipment of the machine and operate with nominal supply voltages not exceeding 1 000 V for alternating current (a.c.) and not exceeding 1 500 V for direct current (d.c.), and with nominal supply frequencies not exceeding 200 Hz.
5) UL 1446, Standard of Safety for Systems of Insulating Materials-General, outlines the test procedures required for the thermal evaluation of electrical insulation systems (EIS).
6) IEEE 1776 is a test procedure for comparing two or more insulation systems in accordance with their expected life at rated temperature.
7) This standard applies to enclosures for electrical equipment intended to be installed and used in non-hazardous locations in accordance with the Canadian Electrical Code.
8) These requirements cover parts made of polymeric materials that are used in electrical equipment and describe the various test procedures and their use in the testing of such parts and equipment.
9) This is the standard for evaluating the properties of polymeric materials.
10) This is the standard used for the bonding of electrical equipment

The grounding transformer is an often-neglected component of the wind farm, shunted off to a sidebar issue. However, those who neglect to adequately plan for grounding transformers do so at their peril. In reality, millions of dollars in liability and loss can be attributed to ground-fault arcing, so grounding-related issues should top the checklists of any electrical contractor developing a wind farm.

Proper construction to meet the specific needs of wind farms is absolutely essential. In addition, be sure your grounding transformer considers such essential parameters as primary voltage, the size needed to carry the rated continuous primary phase current without exceeding temperature limits, and fault current and duration. Finally, select from among the variety of options available based on the application’s site-specific needs.

What is a Grounding Transformer?
Simply put, a grounding transformer is used to provide a ground path to either an ungrounded Wye or a delta-connected system. Grounding transformers are typically used to provide a relatively low impedance path to the ground, thereby maintaining the system neutral at or near ground potential; to limit the magnitude of transient over-voltages when re-striking ground faults occur; to provide a source of ground fault current during line-to-ground faults; and to permit the connection of phase to neutral loads when desired.

If a single line-to-ground fault occurs on an ungrounded or isolated system, no return path exists for the fault current, so no current flows. The system will continue to operate, but the other two un-faulted lines will rise in voltage by the square root of three, resulting in overstressing of the transformer insulation and other associated components on the system by 173 percent. Metal Oxide Varistors (MOVs)—solid state devices used to suppress voltage surges/spikes (lightning arresters)—are particularly susceptible to damage from heating by leakage across the blocks, even if the voltage increase is not sufficient to flash over. A grounding transformer provides a ground path to prevent this.

Grounding transformers are essential for large multi-turbine wind farms, where the substation transformer frequently provides the sole ground source for the distribution system. A grounding transformer placed on the turbine string provides a ground path in the event the string becomes isolated from the system ground.

When a ground fault on a collector cable causes the substation circuit breaker for that cable to open, the wind turbine string becomes isolated from the ground source. The turbines do not always detect this fault or the fact that the string is isolated and ungrounded. The generators continue to energize the collector cable, and the voltages between the un-faulted cables and the ground rise far above the normal voltage magnitude. The resulting costs can be staggering. The loss of revenue alone for a string of 10 turbines can exceed $10,000 per day. Considering removal and replacement, costs of equipment could approach an additional $40,000 per transformer.

For example, a typical wind farm configuration is somewhat analogous to a carriage wheel with a ring, hub, and spokes. The wheel’s outer ring is like the fence around the wind farm and the hub in the center is where the collector is located, which connects to the grid. The spokes are radial lines where each wind turbines sit. Typically each radial string of turbines will connect to a grounding transformer. Figure 1 provides a generalized diagram of this layout.

Construction Configurations
Grounding transformers are normally constructed with one of two configurations: a Zig-Zag (Zn)-connected winding with or without an auxiliary winding, or a Wye (Ynd)-connected winding with a delta-connected secondary that may or may not be used to supply auxiliary power. Figure 2 shows these two possible configurations. The current trend in wind farm designs is toward the Wye-connected primary with a delta secondary (Figure 2b). There are several reasons why the two-winding Wye-connected grounding transformers are seemingly more popular than Zig-Zag designs:

• Though not actually the case, two-winding transformers are perceived to be more readily available for replacement or upgrade;
• Lack of familiarity with the design basics required for the Zig-Zag configuration means designers tend to fall back on the more familiar configuration;
• The Wye-connected two-winding design allows for secondary loading and metering while Zig-Zag designs do not;
• Not all manufacturers provide Zig-Zag grounding options to potential customers, even those for whom that configuration might be most appropriate.

The Zig-Zag connection’s geometry (Figure 2a) is useful to limit circulation of third harmonics and can be used without a delta-connected winding or the four- or five-leg core design normally used for this purpose in distribution and power transformers. Eliminating the need for a secondary winding can make this option both less expensive and smaller than a comparable two-winding grounding transformer. Furthermore, use of a Zig-Zag transformer provides grounding with a smaller unit than a two-winding Wye-Delta transformer providing the same zero sequence impedance.

Wye-connected grounding transformers, on the other hand, require either a Delta-connected secondary or the application of four- or five-leg core construction to provide a return flux path for unbalanced loading associated with this primary connection. Since it is often desirable to provide auxiliary power from the grounding transformer secondary winding, this benefit may make it preferable to use a two-winding grounding transformer instead of a Zig-Zag connection. Both Zig-Zag and two-winding grounding transformers can be constructed with auxiliary power capabilities, and this can be either a Wye- or Delta-connected load.

A solidly grounded system using a grounding transformer offers many safety improvements over an ungrounded system. However, the ground transformer alone lacks the current limiting ability of a resistive grounding system. For this reason neutral ground resistors are often used in conjunction with the grounding transformer to limit neutral ground fault current magnitude. Their ohm values should be specified to allow high enough ground fault current flow to permit reliable operation of the protective relaying equipment, but low enough to limit thermal damage.

Essential Parameters
When selecting and specifying a grounding transformer for your wind farm, be sure to consider the following key parameters.

Primary voltage—This is the system voltage to which the grounded winding is to be connected. Don’t forget to specify the transformer’s basic impulse level (BIL), which measures its ability to withstand lightning surges. In some cases the BIL will be dictated by equipment considerations, such as 150 kilovolt (kV) BIL ratings on 34500 volt wind farms, because of the limitation on dead front connectors.

Rated kilo-volt amperes (kVA)—Because the grounding transformer is normally a short time device, its size and cost are less compared to a continuous duty transformer of equal kVA rating. For this reason grounding transformers are often not sized by kVA, but by their continuous and short time current ratings. Regardless of how you rate it, the grounding transformer must be sized to carry the rated continuous primary phase current without exceeding its temperature limit. This load includes the magnetizing current of the core, the capacitive charging current for the cables, and any auxiliary load, if applicable.  The higher this value, the larger and more costly the transformer will be. Typical continuous current values can be as low as 5 amps to as high as a few hundred. Be sure to include any auxiliary loading requirements.

Continuous neutral current—The continuous neutral current is defined as three times the phase to current. Or, in other words, the zero sequence current. This is usually considered to be zero if the system is balanced. However, for the purposes of designing a grounding transformer, it is a value that is expected to flow in the neutral circuit without tripping protective circuits (which would force the current to be zero) or the leakage current to ground that is not a symmetrical function. Again this value is needed to design for thermal capacity of the grounding transformer.

Fault current and duration—This value is needed to calculate the short time heating that results from a fault on the system and should be determined from an engineered system study. Typical values range from a few hundred amps to a few thousand amps, with duration times expressed in seconds and not cycles. For instance, a value of 400 amps for 10 seconds is typical. The fault duration is a critical parameter for the transformer designer. Where protection schemes use the grounding transformer for tripping functions, a relatively short time duration is specified (5-10 seconds). On the other hand, a continuous or extended neutral fault current duration would be required when the grounding transformer is used in a ground fault alarm scheme.

Impedance—The impedance can be expressed as a percentage or as an ohm value per phase. In either case it should be chosen so that the un-faulted phase voltages during a ground fault are within the temporary over-voltage capability of the transformer and associated equipment, such as arresters and terminal connectors. Values, which can vary from as low as 2.5 percent to almost 10 percent, must be provided by the system designer.

Primary winding connection—Be sure to specify the type of primary connection, either Zig-Zag or grounded Wye. Consider the factors discussed above concerning situations for which a particular configuration might be most appropriate before making the decision. Secondary connection – specify the secondary voltage and connection when applicable. Also be sure to consider the size of auxiliary loading to be connected for either Zn- or Wye-connected primary windings.  If the option is to have a two-winding transformer with no secondary load, determine if the delta winding can be “buried” (that is, not brought out) or if only one bushing is to be brought out for grounding to the tank or testing.

Features and Options
In addition to the design characteristics discussed, there are a number of other considerations or desirable features you should consider when building your wind farm grounding transformers.

First, advise the supplier whether you need a compartmental pad-mount transformer with integral tamper-proof compartment or substation design. Also consider whether the grounding transformer will be located outdoors or indoors, since even outdoor units need special attention when placed near other structures. Select the proper fluid type for the particular application—options include mineral oil, silicone, and Envirotemp® FR3™ fluid, a natural ester-based fluid with exceptional fire-resistant properties and favorable environmental attributes. Consider connectivity choices, and select the best one for the site. Options vary from dead front, live front, and spade terminals. Terminal location can be under a cover or on a sidewall, exposed or enclosed. In addition, temperature rise is assumed to be 65°C, so adjust design if necessary.

Consider site elevation or any special environmental concerns, and use special paint as required. Last, consider neutral ground resistors. The rated voltage of the NGR should be equal to the grounding transformer’s line to ground voltage. The current rating and duration should match the grounding transformer ratings. Remember to set the current rating high enough to be above the cable charging current and grounding transformer magnetizing current. 

Wind turbine generator OEMs, as well as suppliers of other plant equipment, generally offer a warranty for their products. In the wind industry it has become common to offer a warranty of one to two years after installation. Owners need to address how to manage those warranties, both for the wind turbine generators as well as for Balance of Plant (BOP) equipment. The reason why may be obvious regarding a wind turbine generator that has an installed cost of several million dollars, but BOP equipment is also warranted by its manufacturer. Because wind stations contain multiple repetitions of the same equipment, any single failure may be the precursor to many others.
 

During the warranty period repairs are usually performed by the OEM or a competent service provider, and almost always performed at zero cost to the owner. For example, output breakers, wind turbine generator pad-mounted transformers, and energy collection cables can all be visually inspected, and tower foundations can be inspected using an optical transit to detect settling, especially in regions with non-homogeneous geological strata.

Figure 1

Before the expiration of the wind turbine generator warranty period, owners need to prepare for accepting the OEM-supplied equipment. Ideally the equipment is turned over in the best condition possible. The owner needs to consider which of the following approaches best suits their needs.

First, the owner can simply accept the OEM report of the current wind turbine generator status, and receive the equipment “as is” at the end of the warranty period. This option is generally inadvisable, for a number of obvious reasons. Second, the owner can conduct an end-of-warranty (EoW) inspection just prior to the expiration of the warranty. This will establish the “as is” condition of the OEM-supplied equipment before the OEM leaves the site. For a fairly small investment of $3,000-$5,000 per wind turbine generator the status of each wind turbine generator is established by conducting a full inspection, an NDE (nondestructive evaluation), an NDT (nondestructive testing), or all three.

Third, in addition to the EoW inspection the owner can conduct periodic “benchmark” inspections, or spot checks, on select turbines throughout the warranty period to establish wind turbine generator and component status independently of any OEM reports. A benchmark inspection includes the same aspects of an EoW inspection.

Inspection Important
The EoW inspection is best performed three to six months before the warranty period expires. This way the owner can present results to the OEM and reach agreement on who is responsible for resolving material condition issues before the OEM leaves the site.

A two-man crew requires about five hours per wind turbine generator to conduct the necessary EoW inspections, evaluations, and testing. The cost and duration of these activities depends on which components are inspected and at what level of detail. Those decisions can be made based on a “cost versus benefit” analysis, the maintenance history of each wind turbine generator at the wind station, or both. The ideal inspection team consists of one technician with an electrical background and one with a mechanical background to ensure that all areas are covered by knowledgeable personnel. Prior planning and scheduling minimizes the impact on the wind station and its availability. The EoW inspection typically includes the physical inspection of all the items listed in the sidebar accompanying this article.

Figure 2

The resulting EoW inspection report should include: the factual conditions found; when and exactly where the sample, reading, or photographs were taken; and acceptance specifications. For findings that are not acceptable, describe any requirement(s) that may have been violated. Along with presenting the facts it is best if the inspector includes an opinion of the cause of deficiencies, as well as providing information regarding any additional inspection or maintenance that should be performed prior to acceptance by the owner.

Proactive Preparation
The most proactive approach is to periodically perform benchmark inspections or spot checks on select wind turbine generators throughout the entire warranty period. Performing annual or bi-annual benchmark inspections to supplement routine planned maintenance activities and establish component material conditions over time provides the owner with the status of each wind turbine generator. This way, when the warranty expires the owner can more easily predict when each major wind turbine generator component may degrade to the point of requiring major repair or replacement.

Conducting benchmark inspections also provides the opportunity to gather all maintenance and inspection activity in a Computerized Maintenance Management System (CMMS) to create a database of information that shows maintenance trends over time. This information can be used to show trends in wind turbine generator performance, repairs, and ROI (return on investment). If an EoW inspection is also performed, that adds one more data point for such trending data.

End of Warrenty Checklist

Performing Benchmark Inspections: It is best to supplement the recommended OEM preventive maintenance activities with other benchmark inspections on select wind turbine generators such as non-destructive generator windings impedance tests, foundation settlement measurements, and energy collection system (underground) impedance readings.

Such inspections should be performed at least twice prior to the EoW inspection to provide, at minimum, three inspection data points to create a suitable trending graph that is fairly equally spread across the years of the warranty period. These trends are used to clearly demonstrate both the historical condition and performance of a component or system during the warranty period, as well as providing the rate of any degradation. This way the owner is not saddled with an expensive repair of a failed component immediately upon expiration of the warranty.

Figure 3

Trending with a CMMS: Trending the multiple criteria and systems of multiple wind turbine generators is feasible only when performed by computer analysis. Typically, a work order is used to record material condition information each time a planned or corrective maintenance activity is performed. Work orders are best originated, worked, and stored in a CMMS system, as maintaining paper copies of work orders such as in a three-ring binder located in the O&M building or inside the door of each tower are almost useless, especially for trending.

“As Found” Information
Periodically during operations, and as part of the EoW material condition report, “as found” conditions should be trended in addition to the results of special NDE or other inspections. For example, hydraulic and lubrication oil sample results, filter loading differential pressures, alignment readings, torque readings, running temperatures, and results of visual inspections can and should be trended.

Root Cause Analysis
Parts replacement can and should be trended to identify which wind turbine generators show excessive wear and tear that has not been resolved. For example, if the yaw motors or gears have been repeatedly replaced in a wind turbine generator, the condition of that wind turbine generator at EoW turnover may appear acceptable. But until the root cause of the continued replacements has been identified and corrected, the need for replacements will continue.

Alarms and Resets
Alarms and resets can and should be trended in a similar manner, with the goal of early identification of any pending failure or substandard wind turbine generator performance. This is an area requiring more work than is typically performed. The goal is to gather and analyze the data on multiple alarms and resets to determine their cause, and so that the mean time between failures can be understood. This statistical information can be used to drive down the alarm/reset incident rate for each wind turbine generator. Lower levels of “mean time between failures” translates into more reliable wind turbine generator performance over time.

Retaining OEM Data
Some OEMs separate SCADA-monitored data points into two categories: owner information, and information that is proprietary to the OEM. The proprietary data may be deleted when the OEM departs, or it may be made available at an additional cost. During the EoW inspection, the owner can “mine” or analyze all of the data held in the SCADA system. In addition to performing this analysis, it is an opportune time for the owner to review exactly what historical information is contained in the SCADA system and to learn whether that information will remain available after the warranty expires. The proprietary SCADA data may be removed coincidently with the end of the service and maintenance contract, and when the OEM departs the wind station.

Being armed with the facts—that is, all of the material conditions, maintenance history, and trends of every aspect of each wind turbine generator and its components—places the owner in a strong position for negotiating with the OEM when the warranty period ends. This ensures that the wind turbine generators are turned over to the owner in the best condition possible and minimizes the risk of major repair expenses immediately after expiration of the warranty.