April 2010

The AWEA WINDPOWER show always provides impressive demonstrations of the latest and greatest achievements in all the areas of science and technology that are related to wind-power generation. Wind turbine gearbox suppliers present the very best gear alloys, forging and cutting processes, the mirror-like tooth surfaces achieved by the isotropic superfinish technology, and the best bearings and lubrication systems, etc.

In contrast with these innovations, the gear tooth geometry of almost all gearboxes has become outdated. In most cases it presents century-old tooth proportions produced by the standard 20o pressure angle generating racks with some addendum modification. This tooth geometry is universal and good for general gear applications because of gear interchangeability and the availability of standard tooling. It does not fit to the specific operating conditions and requirements of wind turbine gearboxes, however, which include a long lifetime of some 20 years, relatively low speeds, high static loads, size and weight constraints, high efficiency, noise and vibration limitations, and low cost of fabrication and maintenance, etc. The gear tooth geometry must be customized to satisfy all of these conditions.

Unlike the traditional standard gear design, the proposed alternative Direct Gear Design method does not use the basic rack parameters. It uses desired performance parameters and operating conditions to define the optimized gear tooth shape. This design approach is developed for involute gears and based on the theory of generalized parameters created by E.B. Vulgakov [1], and it can be defined as an application-driven gear drive development process with primary emphasis on performance maximization and cost efficiency without concern for any predefined tooling parameters.

Gear Tooth and Mesh Synthesis
There is no need for a basic gear rack to define the gear tooth profile. Two involutes of one base circle—or, in case of asymmetric gears, two different base circles—the arc distance between them and the tooth tip circle describe the gear tooth (see Fig. 1 A-B, Fig. 1 C-D) and the equally spaced teeth form the gear. The fillet between teeth is not in contact with the mating gear teeth. This portion of the tooth profile is critical, however, because this is the area of maximum bending stress concentration.

In a wind turbine gearbox the tooth load on one drive flank is significantly higher and is applied for longer periods of time than for the opposite coast one. An asymmetric tooth shape reflects this functional difference. The design intent of asymmetric gear teeth is to improve performance of the primary drive profiles at the expense of the performance for the opposite coast profiles. The coast profiles are unloaded or lightly loaded during relatively short work period. Asymmetric tooth profiles also make it possible to simultaneously increase the contact ratio and operating pressure angle beyond conventional gear limits. The main advantage of asymmetric gears is contact stress reduction on the drive flanks, resulting in higher torque density, or load capacity per gear size. Another important advantage is the possibility to design the coast flanks and fillet independently from the drive flanks, managing tooth stiffness and load sharing while keeping a desirable pressure angle and contact ratio on the drive profiles. This allows reducing gear noise and vibration level. Asymmetric gears make it possible to simultaneously increase the transverse contact ratio and operating pressure angle far beyond conventional gear limits [2].

Tooth Fillet Profile Optimization
The tooth fillet design begins when the involute flank parameters are completely defined. The initial fillet profile is a trajectory of the mating gear tooth tip in the tight, zero-backlash mesh. The fillet optimization process [3] utilizes three methods: random search method locating fillet points; trigonometric functions for fillet profile approximation; and FEA for stress calculation.

The first and the last fillet profile points of the initial fillet profile lay on the form diameter circles (Fig. 2A, Fig. 2B) and can’t be moved during an optimization process. The random search method is used to move the fillet finite element nodes along the beams that connect the fillet center and the nodes of the initial fillet profile.

The bending stresses are calculated for every new fillet profile points’ combination. If the maximum bending stress is reduced, the program continues searching in the same direction. If not, it steps back and starts searching in a different direction. After the given number of iteration steps the optimization process is stopped, resulting in the optimized fillet profile that provides minimum bending stress concentration. Table 1 illustrates fillet profile optimization and the achievable maximum bending stress reduction for standard AGMA 201.2 gears.

The 20-25 percent bending stress reduction provided by the fillet profile optimization can be used directly to increase the tooth bending strength if it limits the gear drive load capacity. However, the gear drive load capacity and its size and weight are typically defined by the contact stress, or tooth surface durability. In this case the benefit of the bending stress reduction can be converted to reduce the contact stress and simultaneously increase gear mesh efficiency [4]. This is possible by increasing the number of teeth and applying the finer diametral pitch (lesser module) for the teeth with optimized filler, keeping the given center distance.

Table 2 presents a comparison of the traditionally designed gear pair with the 27-tooth pinion and the 49-tooth gear, the 3 mm module, and the 32 and 30 mm face widths for the pinion and gear accordingly with similar gear pairs created by Direct Gear Design with the symmetric and asymmetric tooth profiles. This table illustrates the contact and bending stress reduction and increased mesh efficiency for direct design gears. Fig. 3 presents the carrier assembly of a two-stage planetary turboprop engine [5]. Application of asymmetric gears allowed significant weight-output torque reduction in comparison with similar gearboxes with conventional symmetric gears.

Optimized gear profiles require custom tooling. For profile machining process (Fig. 4) the tool profile is the same as a space profile between the neighboring teeth. For generating machining process like gear hobbing (Fig. 4) the tool profile is defined by reverse generation when the designed gear forms the tooling rack profile. In this case the rack profile (pressure) angles are selected to provide the best machining conditions.

Summary
Unlike traditional gear design, which is based on the rack generating process and driven by manufacturing convenience, Direct Gear Design is driven entirely by application requirements, when technical and market performance of product is critical. It provides the ultimate in optimized gear geometry solutions for custom gear transmissions such as wind turbine gearboxes. The benefits of this approach include higher load capacity, reduced size and weight, extended lifetime, reduced noise and vibration, higher efficiency and reliability, and reduced cost. AKGears has implemented direct gear design in many high-performance gear drives, and we welcome your inquiries. 

References:
1) E.B. Vulgakov, Gears with Improved Characteristics, Mashinostroenie, Moscow, 1974 (in Russian).
2) A.L. Kapelevich, “Geometry and design of involute spur gears with asymmetric teeth”, Mechanism and Machine Theory, 2000, Issue 35, pp. 117-130.
3) A.L. Kapelevich, Y.V. Shekhtman, Tooth Fillet Profile Optimization for Gears with Symmetric and Asymmetric Teeth, AGMA Fall Technical Meeting, San Antonio, Texas, October 12-14, 2008, (08FTM06).
4) A.L. Kapelevich. Direct Design for High-Performance Gear Transmissions, Gear Solutions, December 2008, 22-31.
5) A.S. Novikov, A.G. Paikin, V.L. Dorofeyev, V.M. Ananiev, A.L. Kapelevich, Application of Gears with Asymmetric Teeth in Turboprop Engine Gearbox, Gear Technology, January/February 2008, 60-65.

It’s a fact of life that all electromechanical components will fail at some point. Having the right strategy in place to manage that failure can be the difference between success and… well, failure. This is especially true for wind turbine owners and operators. Without an effective strategy at the ready failure can be devastatingly expensive in terms of lost output, time, and most importantly, revenue.

Do you have a coherent strategy in place when failure strikes? When a problem arises, finding a fast and cost-effective solution is vital. Can the part be repaired? Does it need to be replaced? Is there inventory available? There is also a strong environmental argument in reusing existing parts instead of scrapping them and buying new.

Expanding Too Fast?
The rapid expansion of wind energy in the last 20 years has often resulted in a rush to get new technologies to market, and this has inevitably led to reliability issues. These issues can directly affect a wind farm’s revenue stream through downtime, plus there is a cost associated with additional operations and maintenance events. Some of these new technologies have a propensity for a higher failure rate within the control systems and power electronics compared with other components within the system. Approximately 54 percent of all wind turbine malfunctions are due to failures within the control electronics, electrical systems, and sensors. Unlike many of the mechanical components in a wind turbine, the majority of the electronics/electrical systems only have a single supply source—the OEM. Since the parts generally have a high unit cost, the cost of a new replacement part can be as much as three times that of repairing an existing part. In addition, gaps in the supply chain can result in exceedingly long and costly lead times.

Obstacles and Opportunities
As our industry continues to mature, many wind turbines are now coming out of their manufacturers warranty period. Numerous owners and operators throughout the United States and Europe have already experienced that some parts in their wind turbines have become obsolete, and there is no accompanying documentation to support their repair. Now if one of these parts malfunctions it is no longer just a case of replacing it, and engineers must find out how the part works and carry out detailed root cause analysis of why it failed. The complex electronics contained in today’s wind turbine assemblies requires a unique combination of skills and repair capabilities. The repair process must be carefully managed to ensure that the parts are repaired within appropriate time scales and in a cost-effective manner. This involves the use of detailed workflow and quality procedures so that each stage in the repair process is monitored and certified according to ISO standards. Fig. 1

Often the most difficult part of this repair process/recertification involves ensuring that parts are exercised to their full design specifications. The majority of electronic control parts are custom made for the wind turbine environment, therefore it is not practical or reliable to test these parts using standard off-the-shelf equipment. Nor is it practical to use a wind turbine to exercise the parts. This situation has necessitated the development of complex new software, tooling, and test jigs in order to carry out full testing and validation procedures. To replicate an often highly intelligent series of electronic interfaces and inputs/outputs signals need to be analyzed in great detail, and software and hardware emulation techniques are used to design tooling equipment capable of ensuring full test coverage on all areas of functionality of the parts. Finding a depot repair facility with expertise in this arena can be difficult, but not impossible. At DEX, for instance, we have facilities in the U.S. and Europe already in full operation. At every stage in the repair or refurbishment process it is crucial that the highest quality assurance standards are adhered to, and that a full audit trail can be provided showing the progress of the repair and certifying that the quality standards are in full compliance.

In all, a holistic engineering solution is necessary to report failure symptom trends, epidemic failures, third party module issues, and evidence of design quality and reliability problems. Understanding the “why” and “how” of component failures is paramount to prevent future disruptions in production.

Proactive Solutions
To maximize efficiency and effectiveness in this new environment, a coherent multi-faceted solution strategy must be in place before failure strikes. Specifically the need for a component repair or replacement strategy, asset management strategy, and an inventory management system need to be addressed proactively. Fig. 2

A coherent asset management strategy will mean that repairs are carried out quickly and efficiently, minimizing downtime. Once a part becomes defective it should be swapped out and sent out immediately for repair to ensure that a stock of working components are always available. Parts pooling schemes between operators can also help ensure that contingency stocks are kept high. The location, storage, and transportation of spare parts are crucial. Our research and experience has found that parts are frequently stored in warehouses close to the wind turbine site without adequate protection from the elements and are often in poor condition due to the adverse environment. In many cases the parts are unusable and need to be refurbished or repaired before they can be fitted. This can be avoided by storing parts in a central location under strictly controlled environmental conditions. Sensitive electronics are particularly vulnerable to temperature changes, humidity, and so forth, and can rapidly deteriorate if not stored properly.

Being able to track spare parts is equally important. Having an efficient asset management system with 24/7 visibility of where all parts are located and their condition—good inventory, WIP/defective—can save valuable time by identifying where spare part deficiencies may occur. DEX has developed a unique Web portal that clients can use to track and monitor their inventory providing them with real-time status of their operation. Fig. 3

Case Study
To help illustrate the necessity of having a comprehensive strategy in place I’d like to share what occurred with one of our clients. A major European wind farm operator whose turbines had come out of the warranty period had a series of rotor current controller parts failures, significantly reducing their production. With no advance strategy in place they simply turned to the OEM to purchase replacement parts. To their shock they learned that the lead time for their replacement parts was three to six months. In addition, the pricing structure from the OEM was going to significantly increase their costs. Compounding a worsening situation, with their reduced production the wind farmer was forced to purchase additional power from the grid to meet contractual obligations. Production loss on these turbines alone was in excess of five months, costing them an estimated $370,000. Not surprisingly, they began searching for alternative solutions to resolve this situation and alleviate any reoccurrences in the future. Fig. 4

With the wind farmer turning to our company, our DEX engineering team evaluated the defective parts and developed a full test and repair solution for the entire rotor current controller systems. A proprietary diagnostic software package was developed to ensure test reliability and repeatability during the product test phase. Upgrades were also carried out on the turbines, dramatically improving overall reliability. As a result the turbines were brought back online to a position of full production resolving their immediate need, and their material cost was reduced by a factor of three. A comprehensive solution has been set in place to prevent a repeat of long-term outages and disruptions in production.

In these challenging times wind turbine operators need to utilize any and all opportunities for maintaining the output of the operation at its highest capacity, cutting costs, and maximizing revenue and profits. Having a comprehensive strategy in place before failures occur is critical. Having the proper component repair or replacement strategy, asset management strategy, and an inventory management system in place can make a major contribution to achieving these goals. 

Nearly all utility-scale wind farm management systems have state of the art automatically controlled wind turbines, but at the electric grid intertie the substation is often managed manually. Some substations may have remote control capabilities, to be sure, but they are still completely manually controlled and require operator intervention for restoration. The real irony is that the cost to implement a full auto-restoration scheme at a modern substation with intelligent electronic protection devices is likely to cost no more than the lost revenue of one extended substation outage, and so it’s truly being “pennywise and pound foolish” to avoid implementation of such a system.

The modern wind farm grid intertie substation uses what is termed as intelligent electronic devices, or “IEDs.” An IED is any microprocessor-based unit that receives voltage, current, status, or other data from sensors and other devices and can issue commands via hardwire contacts or communication ports based upon internal programmed setpoints and logic. Substation IEDs are typically a variety of digital relays, digital logic processors, communication processors, and real-time controllers. The signals from inputs are interpreted in a systematic and predetermined way so that the system power devices are energized, de-energized, or otherwise controlled appropriately. The digitized analog values, status bits, timing functions, and logic functions that are integral elements of the substation IEDs can be leveraged to add auto-restoration capability to the substation. Although few if any IEDs must be added to perform auto-restoration, there is necessary effort to program the IEDs for auto-restoration. Standardization of the IED schemes minimizes the effort to programming, test, and commission the IEDs, and therefore the cost to implement auto-restoration. A simplified distributed wind generation substation diagram is shown in Fig .1.

A manually restored substation will have the protection relays indicated on the left side of fig. 2 under the heading “basic.” The scheme status switches are feedback indicating the system status of breakers and relays, etc. The relay IED scheme buttons are manually actuated locally or remotely to set the scheme into the desired mode. An auto-restoration may then be initiated manually using proper timing and the devices on the “basic” side of the diagram. A substation is fully automated by the addition of the microprocessor-based logic processor IED indicated on the right side of Fig. 2 under the heading “advanced.” The logic processor IED communicates with the substation relay IEDs to determine the status of all the power devices in the substation, and then upon pre-programmed logic stored in the memory of the logic processor IED the auto-restoration will be executed automatically at the appropriate time using measured data rather than timing to restore the substation.

The restoration sequence for a fully automated wind project follows the state flow diagram in Fig. 3. The normal state in which turbines are generating and providing electrical energy to the utility grid is the “normal” state at the top right of the diagram. When there is line fault or system disturbance the EPS breaker (breaker “U2” in Fig .1) will open and the system changes to the “EPS disconnected” state. The flow of energy to the grid ceases and electrical energy is no longer marketable. During the transition to the next state, two timed sequences are activated simultaneously. First a “power fail trip” status (labeled “PF trip” in Fig. 3) is declared three or more seconds after the loss of three-phase voltage. All relays for the line, feeder, and capacitor breakers are programmed with this status and all line, feeder, and capacitor breakers are opened at this time. The EPS breaker will close after the utility has cleared the fault or disturbance and the power fail trip operations have completed. The system state is now “ready for restoration.” The line breaker (breaker “U1” in Fig .1) will close if the appropriate conditions are all true after a set time delay. After the line breaker is closed, the feeder breakers will automatically close one at a time with predetermined time delays. Typically all the feeder breakers close in 10-second intervals so that all feeders are restored within one or two minutes. Once all feeder breakers are closed the collector substation returns to the “system available for generation state.” The total outage time for a fully automated collector project—turbines and substation—is typically 10 minutes or less. Any outage time due to the loss of the utility source is minimized and therefore provides optimized generation availability.

The value of automated substation restoration can be calculated by the economic advantage of reduced outage time relative to the longer time required for manual restoration. For a typical 100 MW wind farm with 33 percent generation capacity that is connected to the utility grid 70 percent of the time, the value per hour for lost delivery to the grid when generation would otherwise have been available is $3,500 per hour at 7.5 cents per kilowatt-hour. If manual restoration requires 12 hours before restoration can be accomplished, then the total energy value for this example is $42,000. It should be noted that this represents only the economic energy advantage and does not take into account the avoided cost for labor and vehicle expenses that are incurred with manual restoration. Manual restoration cost includes accounting for the time for an operator to be notified of the outage, the operator to travel to and from the collector substation, to analyze the reason for the outage, and then perform the restoration. The savings may be less than 12 hours if the manual restoration can be accomplished in good weather when an operator is readily available, but on a weekend night in inclement weather the time for manual restoration can be one or two days. After considering all these factors it is not difficult to recoup the investment for the auto-restoration investment after only one or two line outages.

The provision for auto-restoration on distributed generation, particularly for utility-scale wind farms, has been shown to be very feasible, with the microprocesser based relays and communication processors already being used to protect the collector substation that is part of every renewable energy project. With renewable energy very much dependent upon the uptime and delivery of energy to the grid, auto-restoration provides a very good way to accelerate the return on the large capital investment required for renewable generation. With only a marginal investment, the addition of auto-restoration may pay for itself in as few as one or two outages.  

Since 2001, M4 Engineering and M4 Wind Services have been offering engineering, manufacturing, monitoring, data analysis, quality control, and maintenance services to the wind industry. Since this qualifies M4 as a broad-based support company, M4 was selected by turbine manufacturers and owner/operators to conduct extensive turbine health studies at various operational sites across the United States. For over two years evaluation points have been measured using accelerometers, strain gauges, thermocouples, and lasers. Reams of data have been collected and analyzed, yielding a starting discovery: the clearest and earliest health indications came from the bending and azimuth measuring devices attached to the main rotor shaft. All other measurements indicated that a problem had already occurred. Measuring the bending moments and torsion loads of the shaft allowed for operational adjustments and general maintenance to be conducted that relieved these stresses on the turbine, preventing equipment problems and extending component life, minimizing unscheduled downtime, and increasing power generation. Hence, Rotor Redline was born.

What is Rotor Redline?
The Rotor Redline system, patent pending, is a completely vertically integrated data acquisition, transmission, reception, conversion, collection, and analysis system that evaluates main shaft loads as a precursor to equipment degradation and an indicator of inefficient wind-to-rotor alignment. State of the art electronics are paired with standard measuring techniques to provide the lowest-cost approach to condition monitoring available in the industry today. Real-time system information is analyzed and translated into executable actions for the end user to perform well in advance of equipment deterioration. Fig. 1 graphically represents the early detection of Rotor Redline, as compared to other industry methods.

Bending and azimuth measuring devices are directly attached to the main shaft to measure moments and torsion loads in a shaft-fixed coordinate system. Rotor azimuth angle is also measured, which allows the load data to be automatically transformed into a nacelle-fixed coordinate system. This information is then transferred wirelessly via the Ethernet cabling installed within the tower. These communications between all of the system’s components utilize the Zigbee Mesh Network protocol that allows for each Rotor Redline to act as its own router, eliminating the need for an on-site network receiver if tower Ethernet is available or allowing for a single network receiver to be installed for any size wind farm, regardless of acreage.

As the load data is received at M4 it is automatically analyzed, taking away the need for data stream interpretation. Abnormal loads are diagnosed for root-cause determination and the corrective action required. These actions are then transmitted to the operator to make control adjustments and schedule/dispatch maintenance personnel. If M4 is responsible for maintenance, personnel are already in route to the site if a critical alert has been generated. In addition, all data is accessible to the customer, and specific comparative data can be downloaded in real time for review and analysis. Fig. 2 provides a simplified overview of the Rotor Redline system.

The Prevailing Winds and Rotor Alignment
Before a turbine is located on a site, tremendous effort is made to determine the best placement and best orientation of the tower to maximize power production. Once erected, anemometers are placed atop the nacelle and offset calculations are made based on their locations relevant to the prevailing winds. This is fed into a software algorithm to give directional control instructions to optimize wind power production at any given time. However, winds are inconsistent and non-uniform, and the calculations are based on averages, hence directional control that is based solely on anemometer readings may have a built in inefficiency of 3-10 percent. Assets believed to be operating at 90-98 percent efficiency are actually operating with rotor-wind misalignment, and revenue is being lost. Rotor Redline fine-tunes your system and allows for recovery of this lost revenue.

For example, the data analysis indicates that the shaft is consistently bending into one direction, and the root cause is identified as yaw misalignment. The action alert sent to the operator would be to adjust the turbine X degrees in the Y/Z direction to reduce the loads and optimize power production. The inefficiency is relieved and maximum revenue generation is obtained, with the side benefit of relieving the bending moments that transmitted negative stresses throughout the turbine system. No induced stresses and strains on components means less degradation and unscheduled downtime for replacing failed parts (see Fig. 3, Fig. 4, Fig. 5).

As noted previously, wind patterns are inconsistent and non-uniform. However, the stresses and strains produced by these winds are shown in consistent patterns. Higher lifts indicate non-uniform aerodynamic imbalances from varying wind speeds. Turbulence is shown as random components of rotor loads. Lower lift tells you that excessive wind shears are present. All creating induced loads that can be offset by operational adjustments. Fig. 6 outlines the Rotor Redline remediation process in relation to directional adjustments and maintenance notifications.

Equipment and Downtime
The prevailing monitoring systems used in wind turbines today are vibration/acoustics controlled for the turbine and optical strain gauged for the blades. In acoustics the noise frequencies of pristine components are loaded into the system, and alerts are given when the sound signature strays out of a certain threshold range. This is an effective and recommended method of determining when a downstream component has already degraded, but it does not give an operator executable actions to perform that may keep that deterioration from occurring. Net result: predictive and reactionary maintenance versus the preventative maintenance of Rotor Redline. (For more information on vibration monitoring equipment see the May/June 2009 edition of this magazine, available online.)

For determining turbine blade health, optical gauges are molded into the blades during manufacture or fitted in the field. A disruption in the optical light path along this gauge indicates that there is an issue and the blade must be inspected for possible repairs. This is effective as long as the damage has occurred along the optical gauge. If damage occurs elsewhere, or if the field-fitted gauge has lost its installation integrity, no damage readings or false damage readings are given. In addition, the optical strain gauge does not give any indication of stresses being introduced to the turbine system, so gearbox—and possibly generator—degradation are occurring. Net result: inconsistent health information versus the consistent feedback of Rotor Redline.

In the previous situational example, rotor-wind misalignment was calculated and fine-tuning of the directional controls was made to improve operational performance. However, Rotor Redline is detecting other anomalies that can be used to also improve operational availability. For example, the data shows a non-uniform load each revolution at the same rotor position regardless of wind direction or strength. The fault determination is a mass imbalance in one of the turbine blades. Inspection and correction is required which will relieve stress on the system and prevent damage to the gears. An action alert is sent to the operator, and a team is dispatched to correct the problem.

An example of a downstream detection could be that there is a constant load peak being applied at the same rotation of the main gear. The fault indication is that there is an excessive buildup of dirt in the lubrication fluid, requiring fluid and filter change plus gear cleaning. This general maintenance notification again relieves the strain on the main drive system and prevents transmission of stresses throughout the turbine. Deterioration due to abnormal loads is prevented, and downtime due to unscheduled maintenance and part failures are avoided.

Determining Remaining Equipment Life
Other than the obvious benefits of being able to fine-tune rotor to wind alignment, schedule maintenance well in advance of component deterioration, or dispatch emergency work crews as soon as critical anomalies are detected, there is a another major benefit to Rotor Redline: real-time life expectancy can be determined for the major components of your turbine.

All manufactured parts have a designed-in fatigue life. Wear and tear during operation and induced stresses and strains are translated into fatigue loads. As more loads are induced, parts wear down and eventually fail (Fig. 7). Given the design characteristics of the component and its fatigue life, M4 engineers can evaluate the condition of these components at the time of Rotor Redline installation. This will establish the baseline condition of the equipment and its operation life remaining. As load data is then acquired, the Rotor Redline system calculates the fatigue loads applied to each component and updates their real-time life remaining.

Operators are now able to know when major components are going to fail and can plan accordingly. Gone are the days of an asset sitting idle as the replacement part is on backorder, or has a long manufacturing lead time. Cash is not tied up in replacement parts that are not yet needed and parts are no longer failing unexpectedly, causing severe damage or failures of other turbine components. As the operator you are now fully in control of your maintenance, replacement, and refurbishment schedules.

Return on Investment
So the Rotor Redline technology sounds good, but what’s the payback? While there are too many variables involved from site to site to make guarantees, we can estimate a maximum ROI of about eight months due to improved power generation, better wind/turbine alignment, and reduced equipment wear and replacement. How? Consider the following.

As presented at a recent AWEA conference, every 1 percent increase in efficiency or asset availability, and every $10K per tower of annual maintenance savings, has a net present value of almost $8,000,000 to the typical 100MW wind farmer. Also, every 1 percent improvement in output and every degree of corrected misalignment result in more than $30K per year per turbine in added revenue. So even the slightest improvement results in a rapid ROI and long-term increased revenue.

Summary
M4’s Rotor Redline system allows an operator to monitor all aspects of their turbine’s performance. Operational adjustments can be made that minimize downtime, extend component life, and maximize power generation. Major maintenance and repairs can be deferred or avoided, advance plans for even long-lead items can be made, and parts can be acquired as a safety net. Additional sensing components can be added, or Rotor Redline can be tied into existing up-tower equipment to provide notification of equipment deterioration. Wind-to-Rotor alignment adjustments can be made to optimize power production and relieve stresses on all of the system components, further reducing downtime and maintenance costs. Rotor Redline truly brings directional control, equipment life, equipment maintenance, and peak performance into one condition monitoring system, providing the wind farmer with operational flexibility and control of their asset’s life. 

As the telecommunication industry matures, many existing towers are being structurally optimized to improve their load carrying potential for additional antennas. Public opposition and zoning restrictions have limited the number of towers that can be built, forcing tower owners and carriers to collocate new antennas on existing structures. Towers that were originally designed for one set of antennas are frequently being structurally upgraded to handle additional microwave dishes, radios, and other equipment to increase coverage to the growing population that uses cellular technology. Collocating this equipment significantly improves the revenue potential for the tower owner, providing further incentive to structurally modify a tower to handle new loadings. In certain environments towers are also susceptible to corrosion fatigue, which lowers the structural capacity and oftentimes creates safety concerns. Tower reinforcing options also exist to remedy corrosion and reinstate the structural integrity of the tower. To further optimize the towers’ value, owners also have the option of extending the tower height to improve cell coverage and increase the vertical real estate available for lease. This approach is very attractive to tower owners as an alternative to building a new site.

Fig. 1

Just as the telecommunication industry has adopted these optimization practices, the wind tower industry is also facing similar challenges as the need to place larger turbines on existing structures surfaces and the towers degrade.

Structural Modifications
To modify a structure to carry additional loading or remedy a corrosion issue, many owners are reinforcing their towers with bolt-on structural steel members, welded reinforcements, clamp-on sleeves, or carbon fiber reinforcing. The added strength provided by the reinforcing enables the tower to support heavier loading or offset material losses from corrosion. Strengthening a structure can be challenging from design, fabrication, and installation standpoints:

• Design—Reinforcing challenges include accounting for correct tensile and buckling capacities in the proposed members, developing the loads in the end connections, and utilizing connections that can adequately transfer loads between the reinforcing elements and tower. Selection of proper reinforcing elements to expedite the installation and decrease equipment downtime is critical.
• Fabrication—Strict QA/QC procedures must be in place with the fabricator to ensure all products are fabricated to the correct design requirements. Galvanizing or other surface protection must be used to prevent long-term corrosion in the upgrade components.
• Field Constructability—Installing reinforcing elements in the field requires a competent climbing crew and strict attention to safety and quality. The reinforcing design and connection details contribute to the constructability, with bolted and adhered solutions preferred over welding to eliminate onsite fire hazards and quality control concerns. Welding may also damage galvanizing on interior pole surfaces in a way that is challenging to access and repair, accelerating future corrosion.

Reinforcing Types
Towers can be classified into three different categories: monopoles, self-supporting towers, and guyed towers. Monopoles are generally used for cell sites located in urban areas and large wind turbines. The steel tubular structures are typically fabricated using customized geometry and high strength steel plate not readily available for aftermarket modifications. Common reinforcing systems for monopoles include both bolted and welded structural steel members connected to the exterior of the pole shaft. Flat plate, bent plate, and structural channels are common choices, with structural channel providing the best value due to higher structural efficiency than flat plat. Pole-MaxTM, a monopole reinforcing product line manufactured by Aero Solutions, is a high strength channel widely used in the telecom market for this purpose. The high strength, galvanized steel channels are bolted to the monopole shaft wall with shear transferring blind bolts. The high capacity blind bolts transfer shear from the reinforcing plates into the pole wall and develop the full load carrying capacity of the upgrade in the end connections. The blind bolts are installed from the outside of the structure and do not require access to the inside to facilitate installation. The bolted connections are recommended to minimize damage to the pole and reduce the risk of fire. Once installed the steel channels can provide a significant improvement in pole capacity, enabling the tower owner to install additional loading or compensate for wall loss as a result of corrosion.