It’s no secret that wind-turbine capacity, particularly for offshore turbines, continues to grow each year with 6 to 10 MW on the horizon. Even with efficiency improvements, key power generation subsystems —including generators, power-conversion electronics and transformers—are challenged to manage ever increasing heat within limited nacelle space. In addition, even if incurred power losses are as little as 3 to 5%, thermal management systems would have to dissipate 200 to 300 kW and more of heat.

While traditional air and water-cooled systems have provided low-entry costs, water cooling is becoming more challenging to implement. Installation and maintenance costs required to safely distribute enough water to adequately cool ever larger power systems are a major concern. Rising capacity and corresponding power losses are driving thermal-solution designers to consider more advanced thermal management to minimize the overall growth of the nacelle and wind-turbine infrastructure.

Limitations to air and water
Air-cooling has served small-scale wind turbines well over the years, but it’s not practical for removing the heat produced in a megawatt-scale unit. Its thermal capacity is so low that it’s difficult to blow enough air across a motor or through the converter to maintain reliable operating temperatures. That’s why water cooling is selected more often over air for larger wind turbines.

However, water systems are relatively large, and their thermal-efficiency limitations force the size and weight of power generation sub-systems to essentially track their power throughput. That is, the power density is almost constant due to the thermal performance limitations of water, making power-generation components of a 10-MW wind turbine nearly twice the size and weight of a 5-MW model. This is largely because water cooling cannot adequately remove additional heat loads without spreading them out.

Water cooling provides some reduction in size but 2-phase cooling allows a greater additional reduction.

In addition, water’s inherent electrical-conductivity potential poses the risk of a short circuit in the event of a leak, which can be catastrophic around high-power equipment. Also, because wind turbines are often in areas where temperatures routinely drop below freezing, additives such as glycol are mixed in to lower the freezing point. However, this tends to decrease the thermal performance of the coolant. Lastly, system designers must carefully select similar metals that will contact the water. Even with a deionizer or careful monitoring of inhibitor concentrations, water is corrosive. To avoid galvanic corrosion, expensive stainless steel is often selected for all plumbing and manifolds throughout the water loop to reduce the need for long-term maintenance, especially in offshore installations where remoteness and access issues require “maintenance free” operation.

Evaporative cooling
To address the challenges of cooling high-power systems in wind turbines, a few companies have developed alternatives. One in particular, uses a noncorrosive, nonconductive coolant (refrigerant) that evaporates on contact with hot electronics, in a small, light-weight, and highly efficient closed loop.

The loop has the same basic components as a water-cooling system: pump, reservoir, cold plate or cooling coils, and condenser. The big difference however is that water doesn’t change phase as it passes over the device being cooled–it simply heats up– whereas the refrigerant liquid turns to a vapor.

By taking advantage of the more efficient evaporation, two to four times the amount of heat can be removed for the same temperature difference (°C/W) than by single-phase water cooling. This directly increases power throughput, a limitation dictated by the amount of heat that can be removed from the system at the maximum reliable operating temperature.

The two-phase evaporative approach also eliminates safety and maintenance issues associated with water cooling, while allowing greater power densities. The process’ isothermal nature also reduces thermal cycling, which increases the lifespan of the turbine’s electrical components.

Sub-systems such as generators, transformers, and power-conversion electronics can be reliably driven to support up to 40% more power for the same size or weight, simply because additional thermal loads are removed without raising the subsystem temperature.For example, a 1MW power inverter is reduced in size by a factor of 3:2 when converted from air to water cooling, and then 2:1 when converted from water to evaporative-refrigerant cooling. Given the same power throughput, fewer power modules, and supporting mechanical and electrical infrastructure are required, resulting in reductions of size and weight (up to 50%), as well as overall system cost.

Size, efficiency, and benefits
The table, Cold plate performance compares a standard module cooled by air, water, and evaporative-refrigerant methods. With ambient conditions being equal and power modules limited to the same maximum surface temperature (120°C), the total measured thermal losses reached were limited to 600W for air cooling, 1,070W for the best water cooled cold plate, and 1,461W for evaporative cooling. In addition, temperature uniformity, known to impact the reliability of electronic assemblies, was much better with the evaporative-cooling system (6°C variation) than the water cooling system (19°C variation).

The evaporative system’s footprint, smaller and lighter than that of alternative thermal management equipment, coupled with its ability to reduce the size and weight of power systems, frees up valuable space in the nacelle. And, with only one-fifth the fluid flow-rate of traditional water systems, evaporative cooling presents significant performance benefits. This is because the refrigerant’s two-phase thermal-cooling capacity is significantly greater than that of single-phase water, so less fluid and space is needed. Also, two-phase precision cooling uses smaller and lighter pumps that draw less power, as well as simpler and smaller diameter hoses and manifolds that hold less coolant.

Although the comparison of cooling methods focused on power-conversion electronics that would be used in a typical wind-turbine converter, the same thermal benefits are available when comparing evaporative cooling for liquid-cooled generators and transformers. Most generator stator and transformer windings use copper-coiled water jackets to remove their heat.

Due to water systems’ lower thermal efficiency, engineers have to continually increase the size of higher-capacity generators and transformers to effectively spread out the heat. Using a pumped evaporative refrigerant unit with the same copper coils already embedded in the generator or transformer, the power throughput capacity can increase by as much as 30 to 40%, usually without a system redesign.

For small, compact equipment, Parker Hannafin’s Precision Cooling Systems developed the system in a rack-ready modular design to cool high-density power converters and inverters at capacities starting at 1.5 MW.

Almost maintenance free
Wind-turbine operators will appreciate that evaporative precision-cooling equipment requires no regular service. This is of particular importance with offshore wind farms, where accessibility for routine servicing is a major challenge and often results in costly downtime. In harsh winter conditions, entire wind farms may be inaccessible for days.

Two-phase precision cooling equipment is almost maintenance-free because:
• Pumps are more than twice as reliable as comparable water pumps.
• It is leak-proof. Should someone inadvert-ently damage the system causing a leak, the nonconductive coolant will flash to gas and not damage electronic components.
• The coolant neither freezes by nature nor requires additives or deionizers.
• The noncorrosive coolant does not react with metals.
• The only filter included in the system is a “dryer” to remove residual water or humidity from the system upon initial charge, which eliminates corrosion potential.
• It can be equipped with dry-break con-nectors for ease of module replacement, minimizing downtime during component failure replacement.

Building it in
The ease of integrating a new cooling system cannot be overemphasized. The rack-ready thermal system can be designed directly into custom cabinets or racks in nacelles, or provided in a drop-in configuration to retrofit legacy water or air cooling systems. The drop-in replacement consists of a stand-alone cooling unit, coupled with configurable plug-and-play cold-plate kits to build into various subsystems—an ideal design where a central cooling loop can support the generator, power conversion electronics, and reactor.

Rack-ready modules show how it’s possible to design high-density power converters and inverters at capacities starting at 1.5 MW. Modular inverter sections can be paralleled for high power installations. The system features electrical connectors to power bus and no-leak refrigerant connectors for an easy plug-in replacement. In addition, it scales up to 100 kW of heat rejection.

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A view inside the hub, before a blade goes on, shows three pitch motors on gearboxes. Ultracapacitors are inside the three smaller enclosures. The large enclosure (center) holds controls.

On modern wind turbines, each blade is adjusted by an independent electro-mechanical pitch-control unit. The electronic controller checks the turbine’s power output several times each second. Should the power output become too high, the controller signals the blade-pitch mechanisms to turn the rotor blades slightly out of the wind. Conversely, when wind velocity decreases, the blades turn or pitch to capture more wind. Such adjustments optimize energy-generation efficiency and avoid stress or damage to the drivetrain, thereby reducing maintenance and extending turbine life.

Future designs for multi-megawatt turbines may include a cyclical pitch control, one in which the pitch adjusts during rotation to compensate for stronger winds above the hub height. For added safety, pitch controls incorporate aerodynamic braking. The rotor attains the full braking effect with a 90° “off” position for all three blades. Even if one blade-pitch unit fails, the other two rotor blades safely complete the braking process. Each autonomous pitch system is equipped with an independent backup power supply to ensure fail-safe operation of the pitchs control and braking functions in the event of a total power failure, or for maintenance.

Energy storage
Electric pitch controls use batteries or ultracapacitors for backup power. Batteries must be sized to satisfy peak-power demands and adjust the rotor blades or activate braking, even when demands occur only for a few seconds. Other battery characteristics pose design challenges for pitch-system engineers. Poor performance at extreme temperatures and a relatively short operational life can lead to periodic replacement throughout turbine life. In contrast, ultracapacitors perform reliably over a wide temperature range (-40 to +65°C), and their long operating life reduces or eliminates need for replacement over the typical life of a wind turbine.

Integrated ultracapacitor power packs
A growing number of wind-turbine manufacturers and leading integrators of pitch-control systems have designed ultracapacitors into pitch drives to take advantage of their all-temperature reliability and long operational life. Pitch controls and drives are located in the rotating rotor hub of the wind turbine or in the blade itself. The power supply and control signals for pitch drives are transferred by a slip ring from the non-rotating part of the nacelle. The slip ring is connected to a unit that includes clamps for distributing power and control signals for the three individual blade-drive units. Each unit consists of a switched-mode power supply, field bus, motor converter, emergency system, and ultracapacitor bank. Switching on the power supply charges the ultracapacitor module to its nominal voltage. Typical charging time is about one minute. The capacitor module has sufficient energy to run the system for more than 30 seconds at nominal power. The ultracapacitor module directly connects to the dc link of the motor inverter, which then drives a 3-phase, 4-pole asynchronous motor mounted directly to the gearbox of the blade drive. The motor delivers its maximum torque at low rpm. Each blade has sensors that control the blade position.

The product line of ultracapacitors from Maxwell Technologies consists of several purpose built modules.

Manufacturers continue designing larger wind turbines. Megawatt-class versions dominate much of the world market, pushing the average installed capacity per turbine near the 2-MW mark. The largest turbines produce up to 7 MW with rotor diameters up to 126 m. Ten-MW units are on the drawing boards. Larger emergency power packs are required to ensure proper blade-pitch functions for such large installations. Ultracapacitor modules rated for 75V are well suited to fulfill the requirements of megawatt-class turbines. To obtain the standard nominal voltage of 300 Vdc used for such wind turbines, four of the sub-modules are connected in series.

This is one way a bank of ultracapcitors could be wired into a DC Link.

Working offshore
Ultracapacitors’ high reliability in extreme temperatures, long operating lifetime and minimal maintenance requirements make them particularly attractive for offshore and remote wind-power applications, with difficult and costly maintenance visits. Unlike batteries, which require ongoing evaluation of their state of health (SOH) and state of charge (SOC), ultracapacitors do not need costly test runs and expensive management systems. Hence, inspection cycles can extend to several months. What’s more, specialized heating systems for cold-climate versions are not required.

Although wind energy contributes about 2% of the total world electricity supply, it is estimated that by 2020 wind’s contribution will grow to over 4%. So within a decade, another 230 GW of new capacity could be installed, which represents a market potentially worth $250 billion. Ultracapacitors have already been installed in nearly 20,000 wind turbines and will play a major role in this expansion. Current trends suggest that ultracapacitors will be instrumental in supporting industry growth by providing a simple, cost-effective, long-life solution for reliable, low-maintenance functioning of pitch control and braking systems.

The 16-V small-cell module powers wind turbine pitch controls.

Looking ahead
A bank of ultracapacitors can also provide static VAR compensation, dc-link backup, and startup power, or any combination thereof for wind turbines and wind farms. For example:

Static VAR Compensators (STATCOM) in wind-energy systems synchronize the current and voltage (i.e. unit power factor) from a generator by supplying reactive power to the system. With excellent cycling and power capability, ultracapacitors are ideal for energy storage that improves the power-quality output of individual turbines or full wind farms. As wind conditions vary, the power output and resulting voltage levels can vary by up to 10% of average output power. Ultracapacitors absorb energy to keep voltage spikes down and release energy to prevent dropouts and so smooths power delivery to the grid. As most events range from milliseconds up to a minute, ultracapacitors provide a cost-effective, maintenance-free solution for power quality issues. STATCOMs have traditionally used other types of capacitors, but the longer life and high capacitance of ultracapacitors provide a better solution.

DC-link backups provide voltage stabilization for wind turbine outputs (power quality) and ensure the wind turbine stays electrically connected to the grid. Grid disconnection can occur from disturbances on the grid side, but more commonly interruptions are on the turbine side due to low winds or from power used by the turbine dragging down the voltage (e.g. pitch motors activating). An ultracapacitor system can provide “low-voltage ridethrough” by maintaining the constant voltage required by the inverter to bring uninterrupted power to the grid. As an added benefit, the same ultracapacitor system can also provide general backup power for the dc-dc link.

Startup Power, a new concept, makes a wind turbine rated for a higher wind conditions usable in lower winds. For example, a Class 3 wind turbine could be usable in Class 4 winds by using a small motor and bank of ultracapacitors to start a lower-wind rated turbine and begin producing in lower winds. This would overcome the main issue of the power required to get the wind turbine started.

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How many of you are raising your children to be truck drivers?” a speaker asked at a recent AWEA conference. No one raised their hand. “This is partially why there will be a shortage of skilled drivers,” he teased. The other reasons he suggested were that this lifestyle is not attractive to young people today who are looking for more of a white collar way of life which rewards them financially and affords them a homelife that driving a truck does not. Others followed Doug Miller, VP of Operations for Lone Star Transportation LLC, detailing the frequent challenges of delivering large heavy loads in the wind industry. To make the work more difficult, such loads are growing larger and heavier with multi-megawatt turbines.

Railroads seem the ideal way to move huge and heavy goods, but even these are hampered by turns too tight for long loads, such as blades, and a few tunnels too small for the 12.5-ft wide nacelles. Here’s what else that keeps logistic directors up at night as they struggle to get turbines and towers to sites on time, and how OEMs are responding to their boat-in-the-basement problem.

Trouble with trucking
Before a driver even starts a diesel to deliver a turbine, a detailed logistics plan must be laid out. Inexperienced planners have a habit of overlooking this detail until the end of the project. Trucking managers warn that it can take months to formulate. In a nutshell, the plan involves assigning lots of specialized and expensive equipment, filing for and receiving permits from several states, and then finding drivers and support personnel.

The Schnabel trailer, built by Trail King Industries (trailking.com) shows the complexity of equipment needed to haul large wind turbine towers. The $300,000 trailer is intended only for moving large base and lower-mid tower sections. Trailer sections attach to opposite ends of the tower.

And consider this: moving just one complete turbine takes 9 to 10 trucks, most of which are specialized trailers. Different trailers are needed for the nacelle, blades, and towers. Lone Star Transportation’s (LoneStar-LLC.com) Doug Miller says hauling nacelles and tower sections require equipment that cost up to $425,000 or more. For instance, three vehicles are needed to move each blade set, one specialized trailer moves the nacelle, and up to four specialized trailers transport tower sections. In some cases, the latter group requires even more specialized rigs, called Schnables. These trailers are used as the diameter of the tower increases and to lower the transport’s overall height to obtain more direct permitted routes. Generally, the smaller sections, ship on more conventional specialized trailers. Finally, one or two more trailers are often needed for other smaller components.

Let’s keep counting. Depending on the distance the components are transported over, delivering six turbines per week could requires up to six or more trailers and rigs, plus 18 ore more blade trailers. So a trucking company may have $12 to 13 million tied up in assets. “Furthermore, that shipment calls for 54 drivers to lead the glamorless life style I described, not to mention support equipment such as escorts, pilot cars, and others needed for a safe delivery,” says Miller.

As a specialized 48-state carrier over irregular routes, the complaint Miller hears most often from drivers is short home time and not enough money. “The two don’t go together because spending time at home means you can’t make money. You cannot fix the monetary compensation without raising costs for everyone else. We have to balance that problem if we want an industry to attract people.”

Logistic Director at Suzlon (suzlon.com) Gary Kowaleski also recognizes the trucking problem and highlights a few more details that OEMs and wind farm developers should appreciate. For instance, the drivers that move super loads are the more senior drivers and few younger drivers are moving up to replace retirees. An average truck driver salary is about $40,000/yr, and tops out at about $60,000.

To make matters worse, rules and regulations are changing with respect to work hours. Lone Star’s Miller says a driver can work 70 hrs in eight days under current regulations. That person can spend 14 total hours working on the job in the course of a day, 11 of which may be spent behind the wheel.

Proposed regulations also say drivers can work 70 hrs in eight days, but a daily total cannot exceed 13 hours, with 10 behind the wheel, and two mandated 30-min. breaks. “On the surface, the new rules are not too bad. But this industry is over-dimensional so it is already restricted by curfews in metro areas,” says Miller. “So an average day actually runs from about 9 am to 3 pm because you only drive in daylight hours. Factor in the new deal with two 30-minute breaks and drive times drop to five hours or less each day. So this regulation will be detrimental. All it does is require us to buy more equipment and hire more people because of the short cycles.”

On the plus side, a recent standard from the Federal Motor Carrier Safety Administration (FMCSA) shows some promise. “It’s probably the best measurement tool we have in the industry,” says Miller. “It will weed out some bottom feeders that don’t do the job right. But it needs some refining and tweaking.” For example, he says, a measurement system puts companies into peer groups. Lone Star is a specialized carrier, but the FMCSA groups it with van carriers that have the same population of trucks or drivers. “We operate 1,000 special trucks while JB Hunt, for instance, operates 1,000 vans. But we are graded the same. That makes no sense.”

Likewise, the points assessed for violations needs adjusting. “For instance, a driver involved in a fatal accident is assessed a violation of two points. But if an inspector find a frayed strap or securement device, the company is assessed a penalty of ten pts. “It seems the fatality should carry a lot more weight or point value than a frayed strap. So the point system also needs an adjustment,” he adds.

The nacelle arrives at a wind farm on a 13-axel rig operated by Lone Star Transportation. Almost each part of a wind turbine requires specialized equipment for transport.

In port Keeping up with advances in the wind industry is one challenge ports face. They must foresee what cargo will be offloaded and provide storage for it, as well as infrastructure that gets cargo to a final destination. “That means anticipating changes that might take place, such as how wind industry loads influence port facilities,” says Alastair Smith, Senior Director of Marketing Operations for the Port of Vancouver, Wash. For example, in 1999 turbines were mostly 660-kW units and weighed about 19 metric tons (1 metric ton = 1,000 kg or 2,200 lb). “Now we handle nacelles that weigh about 90 metric tons and some manufacturers hint they could be up to 132 metric tons in the near future,” says Smith. “So we’ve had to make sure we have the capability to handle those.”

One rising issue is having enough space to handle large components. For ports, it’s not a matter of handling a single construction project, but balancing multiple projects at one time. For example, Smith finds turbine shipments are frequent in a year’s first quarter. But construction firms don’t get access to wind farms until second quarters. Hence, the port has to stockpile a lot of material prior to construction firms taking stuff out. “Construction companies want the turbines all in one place when they get access into wind-farm sites. Then they want components delivered at a steady pace.”

Smith says his team allocates acreage for all projects and ensures that if they do not have enough acreage available, they will develop more. This year, the port developed 25 acres and will add 24 more.

With regard to costs, most shipping charges do not come from ports. Smith says the Port of Vancouver has an advantage over others in that it has the best access to about five major areas in the U.S. where wind is growing: Pacific NW, California, Colorado, Texas, and Upper Midwest. “The transit time from Asia, where a lot of towers are made, to the West Coast is about 15 days. “Taking a shipment to a Gulf port would take 30 days, so the port has a 15 day advantage,” he explains. Cost estimates for the extra 15 days include the vessel charter rate, fuel consumed, and fees for going thru the Panama Canal. Thus, the charter rate for a vessel can be $25,000/day for an extra $375,000. Fuel cost is close to another $225,000. Then add $100,000 for the canal.

Wind on the rails
A few OEMs have addressed their logistics problems in part by locating factories on the Great Plains. From there, shipping by rail across the relatively flat expanse is less problematic than over mountains. However, although railroads are generally considered more efficient than trucks, a few tunnels in the East and West are too small.

Wide nacelles are problematic because tunnels and tight mountain turns are a constraint on rail shipping. For instance, if turbines must be transported through some parts of West Virginia, Pennsylvania, and Maryland, the 13-ft max width drops due to smaller tunnels and tight turns. Long blades, 55m and more, are also difficult to transport through some tunnels.

Railroad limitations also fall into structural and operational categories. Structural limitations refer to loads that cannot exceed the 13-ft width. Flatbed rail cars are 10 ft 8-in.wide, so nacelles can exceed that by a small margin, but 12.5 ft is about max. Width is so critical that trains with wide loads cannot pass each other on adjacent tracks.

Unit trains are generally least expensive to operate. These have much the same freight, such as all turbine components. To haul such equipment, private companies such as Kasgro Rail (kasgro.com) and TTX (ttx.com) have built and own most of the about 400 8-axel cars capable of 400,000 lbs or 200 tons. The car owners would like to keep that stock busy.

At times, it has taken days to unload trains because of limited storage, or the construction firm is not ready for the equipment. Offloading a train at a spur requires a crane hired by a third-party logistics operator.

Unit trains have a few challenges. For instance, a train of nacelles might exceed the design rating of some bridges, an evaluation that calls for limiting a load per distance. Railroads get under the limit by putting an empty car, a spacer, between each 8-axel flat bed. Hence a 25-nacelle train would require 25 spacer cars if it must use an older bridge.

As with other transport companies, railroads would like advanced notice to plan big shipments and involvement in the development of new turbines. GE, for example, had the foresight to consult with CSX while designing its 2.5-MW turbine which the company manufacturers in Florida. The company involved the railroad in 2009 when work began and shipments now proceed without difficulty.

A few solutions
Almost every transportation representative sees solutions to logistics problems. For example, all agree it’s a good idea to partner with your shipper. Lone Star’s Miller says his company would like to act as a transportation consultant. “We ask, ‘can your components become modular?’ If so, what does that do you for the reassembly tasks? And, can sourcing and suppliers be more localized? Also, we suggest opposing regulation changes and support lifting other restrictions we face with respect to over dimension loads.”

Miller says his company would rather spread its assets around and do more projects with less equipment per project by having other transportation modes such as rail bring components close to the site and let his trucks work the last few miles. “That would let us put one truck on site instead of six. Then I can assign the remaining five to other projects. We work with other modes of transport because we know one carrier can not do it all. Strategic partnerships are a huge part of this business.”

This year, the port of Vancouver Washington allocated 70 acres just for wind energy. It takes about one acre to park about six towers (24 sections) with room between to effectively move trucks and trailers. Two years ago the port purchased Terminal 5, which adds 100 acres for project cargo, enough for 600 tower sections.

Energy Transportation Inc. Business Developer and Logistics Manager Shelli Short (energytran.com) says her company found a successful and similar strategy in building a large flat gravel lot near a rail spur chosen because it’s within 200 mi of several wind farms. The lot is intended to hold large parts that will be trucked the final leg to sites. She reports that the strategy has been working well. Short also suggests that country-wide permitting for transports would be a good place to start fixing the problem of turbine shipping and its expenses. Each state differs in planning, permits, and costs, and some charge up to $50,000 for the permit alone.
Maintaining nacelle sizes and weights will allow fitting new turbine designs on existing 13-axel trailers, equipment acceptable in most states. But if nacelle weight exceeds 165,000 lb, 19-axel trailer might be an option, while actual transport and timing becomes a research project. Suzlon’s Kowaleski says designers at OEMs are listening. “For example, Suzlon recognized an opportunity to expand its existing tower line from an 80-m design to include 90 and 100 m. There were no difficulties with the 80-m tower relative to transport, but two years later, a 90-m tower design was introduced which could be manufactured at our supplier in Mexico. However, it would not have been possible to transport it by rail to North America due to a wide base diameter that could not have passed over a particular bridge. During the design of the 100-m tower, transport details were requested and supplied to engineering relative to North America logistics. When the design for the 100-m tower arrived, we saw it had the same base diameter as the 80-m unit, which made our 100-m unit viable for rail transport from Mexico. The OEM engineering team is listening.”

If OEMs don’t listen to the logistics people, things can get messy. For example, a construction manager recently explained that if a load at a rail stop is too heavy for one truck, as with one recent nacelle, they’ll take pieces and parts off until it’s under a weight limit. Just keep the reassembly manuals handy.

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Dave Clark/Condition monitoring specialist

There are three methods for monitoring wind-turbine operations: wind-conditions monitoring, performance monitoring, and condition monitoring. The terms are thrown around so much that many in the industry are confused. Although the terms describe different tasks, they have the same goal: performance and reliability of the wind assets.

An automotive analogy may be useful. For instance, if your car is not getting optimal fuel mileage, you get a tune-up. If that doesn’t work, you take the car to a mechanic for a more detailed analysis. So diagnosing the mileage problem is analogous to monitoring wind conditions. A tune-up is analogous to examining SCADA data, or performance monitoring. And the mechanic is akin to the analyst who finds meaning in the vibration data. Here’s more detail on each.

Monitoring wind conditions

Suppose a turbine’s power production or output is less than its power curve says it should be. Does the problem concern wind conditions or the turbine? Wind conditions monitoring may provide a clue. A wind-resource assessment of the site is performed prior to construction and at least for a year. If the resource assessment is correct, the turbine should produce predictable power. If it doesn’t make rated power, (rated for the available wind) then you will need to benchmark the conditions versus the rated power, essentially plotting the wind versus the turbine output.

The Triton from Second Wind is one of several sodar-based (wind sensor using sound) remote wind sensors. The unit can measure wind speed and direction at hub heights to give operators data to gauge a wind plant’s efficiency.

This is done with wind-conditions monitoring this way: Examine the site’s meteorological data to gauge or plot against the turbine’s output. Look for data from a met tower or remote sensor, such as a sodar unit. “Our systems measure wind conditions that can be used for wind-resource assessments,” says Naomi Pierce of Second Wind, a company that makes portable wind-sensing systems. “The equipment can be used for wind-resource assessments and monitoring operating farms. In the latter case, the equipment monitors wind and wind conditions to gauge the performance of an operating wind turbine.”

Performance monitoring based on wind conditions measures the wind potential that can be plotted against the rated power of a wind asset or site, to determine the production efficiency. Wind performance should show how much power the site can produce. If the turbine does not produce expected power, there might be a problem with the turbine. This also involves power-purchase agreements and warranty implications.

Performance monitoring

Performance monitoring is a great tool for wringing maximum performance out of a farm or fleet. This monitoring involves taking the sensor data from the wind turbine and mining (examining) it for information as to why it’s underperforming. You look for reasons why the rated power was not produced.

A tremendous amount of data comes off of a single turbine. Its sensors monitor characteristics such as:

• Yaw and pitch position
• Temperature
• Wind speed and direction
• Generator speed

There are more aspects, but mining the SCADA data can determine where performance issues lie. Steve Brost, a CMS Engineer and turbine prognostics and health management analyst, says this about performance monitoring:

“We use a software tool called T2 for identifying deviations between two or more data populations.  We use it to identify deviations in trends by comparing the (multivariate) means of data populations and point out the one with the most consistent deviation.  We configure a threshold in the tool to identify the ‘highest hitters’, those that deviate most. Once the tool has identified a repeat offender, a value for that data population is collected into what we call a Cusum (cumulative sum), so we can focus on locations in the turbine with the highest probability of potential failure. For example, the hottest temperature might indicate damage or confirm damage through other measurements such as vibration. The Cusum characteristic is something of an alarm threshold or a filter we can set.

After taking all Cusum values across our fleet, we examine the highest values and cross reference the findings with vibration data to focus on locations that are alarming the most frequently in vibration and temperature or other SCADA trending data such as highest fault count.”

Suppose a turbine’s temperature points to a potential problem. This element is neither sufficiently specific nor nearly as predictive as vibration-condition monitoring. But a “hot” indicator is similar to a “check engine” light in a car. It signals a problem but not specifically what is causing it.

Statistically, every tenth turbine faced relevant damage each year. Costs for a planned repair are on average less than 30% compared to the replacement of a component. (Source: DEWI). Consequential damage can be prevented.

Condition monitoring

Nothing reduces turbine efficiency more than a failed component that halts its power production. Condition monitoring using vibration in particular, aims at identifying specific component degradations. Typically, this includes sensors placed on components that are costly to repair, fail with regularity, or both. Usually and regardless of model, such components include:

• Main bearings
• Low-speed shaft, gearbox
• Planetary section, gearbox
• High-speed shaft section, gearbox
• Generator, drive end
• Generator, non-drive end

Several operational parameters (which performance monitoring can detect) ultimately increase wear on drive-train components. Performance monitoring detects these abnormalities while condition monitoring detects component wear. Hence, it is crucial to perform both. The monitoring methods work together on performance-related failures.

For example, a rated power output may be affected by a misalignment between the gearbox and generator, or a failing high-speed shaft. Yaw deviation, the difference between wind direction and nacelle direction, can also affect power output by causing undue drive train loading. Condition monitoring detects the wear.

As you would expect, not all failures or shortcomings are performance-monitoring detectable or correctable. For instance, looseness, misalignment, imbalance, and most all early detection on gear and bearing failures are impossible with factory sensors. So check the fuel efficiency on your turbine, give it a tune-up, and avoid a trip to the mechanic. Although methods for monitoring wind turbine performance have different names, all work toward the same thing: peak wind-turbine performance.

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Hugh Smith Regional product line marketing manager Miller Fall Protection / Honeywell Franklin, PA www.millerfallprotection.com

A quick-acting braking system that arrests falls within inches, not feet, has safety managers retiring conventional shock-absorbing lanyards. Instead, they favor compact, lightweight, personal fall limiters (PFLs), also known as self-retracting lifelines. As manufacturers introduce PFLs with 100% tie-off fall protection (dual or twin legs), workers can move safely anywhere on a job site without ever being disconnected and at risk of a fall. Whether on a construction site, building scaffolding, or maintaining a wind farm, it’s good to know someone has your back.

The 6-ft shock-absorbing lanyard has been a dominant tool in personal fall protection for years. These lanyards allow for up to 6 ft of free-fall distance before activating, and another 3.5 ft of deceleration distance before arresting a fall. However, a personal fall limiter requires less than 2 ft to arrest free falls. If you have 15 ft of fall clearance or less, you’d better have a retractable on or you’re going to hit the ground.

Safety managers say the primary driver of their decision to swap out lanyards for PFLs is fall-arrest clearance, which is critical to worker safety. Personal fall limiters have evolved into more compact, lightweight, and affordable devices developed in response to workers’ needs for quick stopping action at low fall clearances, and for greater mobility around barriers. Today’s high-strength, high-impact materials let product engineers build smaller units that can withstand required fall forces. Some models accommodate workers up to 400 lbs including tool weight. Advanced designs incorporate a built-in swivel mechanism, and D-ring connectors that easily adapt two lightweight PFLs for continuous 100% tie-off fall protection. This eliminates a need for double-legged shock-absorbing lanyards. With the reduction in size and weight comes a reduced price, making PFLs more affordable than ever.

The latest advancement is the ANSI Z359.1 compliant line of Miller brand retractable lifelines by Honeywell Safety Products–the TurboLite and TwinTurbo personal fall limiters, and the Turbo T-BAK and Twin Turbo T-BAK tie-back personal fall limiters.

Recently one of the nation’s largest manufacturers made a concerted effort to reduce fall hazards, 150 personal fall limiters were added to their arsenal of fall protection equipment in a move that will eventually outlaw lanyards on their job site. All employees, contractors and sub-contractors will be required to comply with the new policy, which incorporates rigorous, hands-on training and inspection, and encourages those working at height to take personal responsibility for their own safety.

Wondering if it’s time to transition from lanyards to PFLs? Think of it this way: for years cars were not equipped with seat belts, then came lap belts, and after that lap-belts plus shoulder harnesses. Then both were incorporated together with quick-acting pretensioners. Seat belts eventually moved around passengers when the car door closed. We complained about the changes, but got used to them. Now we buckle up without even thinking whenever we get in our cars.

Advances in personal fall protection have taken us from the safety belt, to the 6-ft shock absorbing lanyard, to personal fall limiters. You may think you’ll never be in a car accident, and you may think you’ll never actually fall. But if a person does, it could lead to an incapacitating injury or worse. However, with the right fall arrest equipment, a person can go home safe after work. Isn’t that worth it?

WPE

The wind industry recognizes the importance of proper bolt tension. The acknowledgement will challenge some of the industry’s torque-tightening practices as it matures and adopts more sophisticated technology to improve reliability and reduce maintenance costs.

To start, let’s review current practices. For instance, bolts on turbines are often tightened in one of two ways: either by torque or by hydraulic tension tightening.

Torque is a rotational force applied to a lever and multiplied by its distance from the centre of the tightening end (fulcrum). Torque, measured in Nm or lbs-ft, rotates a bolt head or nut to tighten and stretch the bolt so it clamps two surfaces together. Hydraulic torque wrenches allow the operator to control the torque applied and are extensively used to tighten wind-turbine bolting.

One design for a tell-tale bolt comes from UK-based James Walker’s RotaBolt. A simple mechanism inside the bolt provides an indicator of the real load or tension. RotaBolt fasteners are guaranteed to consistently indicate within ±5% of the specified load regardless of environmental influences.

Although simple to use, the technique suffers from inaccuracy due to inconsistent friction. Even on lubricated threads and nuts, the level of friction is uncontrollable. This means there is a critical lack of tension control that can lead to bolt failure, and ultimately compromise joint integrity.

Hydraulic tension tightening is also used to tighten large diameter bolts on wind-turbine structures. A hydraulic ‘jack’ pulls the bolt axially. When the bolt is stretched by a specified amount, the nut inside the tensioner rises off the flange creating a gap between flange faces. The gap is closed by tightening the nut. Releasing the hydraulic pressure transfers tension to the bolt and flange.

If you can turn the indicator cap, the bolt is not tensioned.

The drawback here is that tension loss occurs when releasing the hydraulic pressure. It’s an effect called “load transfer relaxation.” To compensate for the loss, the hydraulic pressure originally applied is increased a bit, a “hydraulic overload value”. However, the amount of load-transfer relaxation is not accurately predicted and so varies from one bolt to another. Consequently, hydraulic tensioners are unable to control or measure the tension achieved for each bolt. The device is also unable to provide an in-service reliability check.

One way around the shortcomings of both tightening methods is with a bolt that indicates when its specified load or tension has been reached during tightening. The RotaBolt can be used in either torque or tension tightening. When this bolt is tightened, the cap locks at exactly the right amount of preload in the bolt. This allows for more control during tightening while ensuring that an OEM’s design criteria is met independent of the tightening system. It also allows checking the bolt’s status in service (if a cap on the bolt does not turn, it’s still tight) something traditional bolt retightening checks are unable to do.

Accuracy is not affected by friction, as in torque control, nor by load transfer errors, as in hydraulic tensioners. This bolt-tension solution can ensure joint integrity on wind turbines. Joint integrity, in turn, allows for greater reliability and the more efficient production of renewable energy.

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Bill Hindman Industrial Marketing Systems Fountain Hills, Arizona

While the wind-swept ridge tops of the West Texas plains are ideal for generating wind power, harsh site conditions can degrade turbine blades due to erosion from dirt, hail, lightning strikes, insects, and other airborne particles. Plus, fatigue from the sun’s UV rays and other natural elements leave hairline cracks and gouges in the leading edge. If not mended, water can seep through these minute cracks and cause damage that could require major blade repairs in the future. After years of service and millions of blade rotations, one company decided it was time for some major inspection and maintenance on the turbine blades of their large wind farm near Sweetwater, Texas.

The management company decided to perform a complete inspection and leading-edge refurbishment on all blades on the farm, rather than repair blades on an individual case-by-case basis. Performing maintenance in this manner would minimize shutdown time and reduce the likelihood of major problems that require shutdown for extended repair in the near future.

The first phase of inspection and refurbishment began on a group of 1.5-MW turbines in an area damaged by a violent lightning storm. Because a typical 1.5-MW turbine outputs upward of 36 MW of power per day, the cost to shut one down can run $800 or more a day. Minimizing downtime and keeping turbines running efficiently are critical factors in maximizing ROI. Also, having a clean, refurbished leading edge on the blades lets the turbine perform at a higher output and maximizes long-term blade performance due to reduced friction when the blades move through the air.

A view from the aerial work platform when almost 300-ft up. The red outriggers are visable, which stabilize the truck carrier. Workers can position the vehicle away from the tower base, yet the lift still reaches the top of the nacelle.

The refurbishment required workers to physically inspect all blades on site, document required work, and then repair and refurbish the leading edge as well as other areas on the blade. First, workers sanded the blade, then filled any cracks and gouges with putty. They applied two layers of a leading-edge protectant and finished the area with two top coats of polyurethane. Two different colors of protectant (first red, then white) were used so when blade erosion occurs, it will be easier to determine the extent of repair needed when viewed from ground level.

When the first phase of the project went out for bid, the management company contacted a number of contractors to determine the most thorough and economical way to perform the work. In addition to cost, minimum turbine downtime was also considered. There were a number of ways to inspect and repair the blades such as working from suspended scaffolding, rappelling down from the nacelle, or using a basket suspended from a crane. The companies used a combination of all techniques.

After the contract was awarded and work began on the site, the contracted company performed most of the inspection and refurbishing from suspended scaffolding. However, to keep the work running on schedule, they chose TGM Wind Services of Abilene, Texas (tgmwind.com) to help. TGM specialized in this type of work but used a different technique to access the overhead areas: aerial work platforms. Aerials are relatively new to North American wind farms. However, they have been used to maintain turbines in Europe and globally for many years, and time-tested in harsh conditions. With the extra productivity the aerials provided on the job, progress proceeded on schedule.

With the aide of the aerial device, workers from TGM Services (tgmwind.com) are able to get close to a blade for maintenance work. This worker is performing the first step of leading edge repair by sanding the nicks and gouges prior to applying putty.

When it was time to solicit bids on the refurbishment of blades in the next phase of the overall project, site management remembered the aerial work platforms they had seen in the initial phase and added TGM Wind Services to the list of companies to whom they sent requests for quotations. The project included inspecting and refurbishing the blades on 25, 1.5-MW GE turbines.

The machines management saw TGM operating were Bronto Skylift Model S-90 HLA truck-mounted aerial work platforms. TGM owns four of the large aerials and has used them on towers since they took delivery of the first two machines in 2010, with two additional units earlier this year. Through their experience and knowledge from earlier work on the site, TGM knew they could meet management’s expectations and agreed to submit a bid.

Mounted on a 6-axle Kimball chassis, TGM’s 90-m working height Bronto machines can drive directly to a turbine. Also, advanced outrigger controls and one-button automatic leveling allow positioning, setting up, and elevating the machines to the overhead area in 15 to 20 minutes, or less, from the time they arrive on site. This can save considerable time and lower transportation and set-up costs when compared with some other methods on a multi-tower site.

The machines can withstand winds speeds up to 12.5 m/s (28 mph) when elevated. They can lift up to 1000 lbs of men and materials in an 8-ft by 3-ft, enclosed platform to a 90-m maximum working height with a maximum horizontal outreach of 33m.

TGM’s Bronto S-90 HLA machines are also equipped with electrical, pneumatic, hydraulic, and water lines that run inside the telescoping boom from the ground to outlets in the platform. The lines let technicians easily operate power tools and washers from the platform. This saves time and is much safer because it eliminates lines or hoses running down from the overhead platform to ground level, thereby reducing the chance of accidental contact by workers or passing vehicles.

These capabilities result in faster, safer inspection and maintenance of turbine exteriors and blades at a lower cost than other currently used methods. Some methods, such as suspended scaffolding, require fastening cables inside the nacelle and letting down from a hole in the nacelle body. But aerials are non-invasive and don’t require any access inside the nacelle to reach the blades.

The Bronto aerial device in a “ready to raise” position. The outriggers on the Kimble chassis are extended and the vehicle is leveled and off the ground (note front tires).

Aerial work platforms can also be safer to operate, which was a major concern of site management. The aerial device’s platform is mounted on a telescoping boom affixed to a ground-level turntable and operated directly from the platform, so technicians have greater control in platform positioning. Also, unlike platforms suspended from overhead cables, the lifts are not as susceptible to wind forces. So techs can get up close to the blades and maintain that position while working. All TGM technicians have been factory certified to operate the aerials and received training in first aid, CPR, and other disciplines as required by the wind-farm management for all workers on site.

Because of Bronto’s aerial work platform capabilities and TGM Wind Services’ experience with similar projects, VP of TGM Kevin Darby provided site management with a fixed price quote per blade no matter what repairs or supplies were required. In addition, Darby said that TGM would inspect and refurbish an average of one turbine (three blades) per day depending on favorable weather conditions. Most other companies that provided quotes based their costs on time and materials or per diem rates.

The platforms let TGM offer a competitive package. With a fixed price, less turbine downtime, and a lower overall cost, site management couldn’t pass it up and awarded TGM the contract. Not only did TGM perform as promised, they exceeded their commitment.

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