Second Wind's Triton wind profiler uses sodar to collect wind data.

Ultrasonic sensors function without moving parts. On a typical sonic anemometer, a transducer sends a pulse of ultrasonic sound from a ‘North’ facing side of the sensor. A microprocessor measures the time it takes to travel to a ‘South’ transducer. The wind speed is calculated from the time it takes the ultrasound to travel to the opposite transducer. Measurement times are affected by the wind speed and direction blowing along the line between the transducers. Without moving parts, measurement is said to be immediate and precise.

In the cases above, the instruments are small enough to mount on a nacelle. Larger, ground-mounted sonic instruments, however, can take the place of a met tower and measure wind speed and direction at several elevations.

This latter device, also called a sonic wind profiler or a sodar (sound detection and ranging) unit, detects wind speeds and directions at several levels up to about 300 m. The unit is said to work unattended to capture accurate wind data at turbine heights in any weather and location. One model runs on as little as 7 W from a battery recharged by a solar panel, and it can be relocated by one man with a truck. Readings from these devices look like anemometry results and so need no expert analysis. Users can often access data in real time from a computer over a satellite wind-data service.

Sodar uses short-wavelength sound waves to measure the Doppler shift of emitted sound and calculate wind speeds. Sodar units are reported have performed well in tests.

Laser-based wind sensors use laser Doppler velocimetry – an optical remote-sensing technique similar to Doppler radar – to measure minute frequency changes of light reflected by microscopic air particles moving with the wind, which precisely determines wind speed and direction. One laser wind sensor mounts atop the turbine nacelle (pointing through the rotor) to measure real-time horizontal and vertical wind speed and directions in front of the turbine. This sensor looks 300m ahead of the turbine to measure wind speed and direction as it approaches the turbine rotor. It transmits that data to the controls in time (20 sec of lead time for a 35-mph wind) to reorient the turbine. The system is comprised of a base laser and a remote lens. The base unit, housed in a separate assembly, can be mounted inside the turbine’s nacelle. The remote lens mounts atop the nacelle. According to one report, reacting to oncoming wind before it reaches a turbine improves power production by about 10%.

Windpower Engineering & Development

rotor shaft bearing

Turbines use a wide variety of bearings. Large ones include huge 2-m diameter main-shaft bearings with two opposing rows of cylindrical rollers to handle enormous thrust and radial loads. These support the shaft that holds the hub and rotor. Slew bearings mount between nacelles and towers to let nacelles rotate as needed. These can have 3 to 6-m diameters with gear teeth machined into outer ring surfaces.
Another bearing, a hybrid design, aimed at wind-turbine generators provides insulation against electric currents. Their use can minimize the risk of premature bearing failures due to erosion from electric currents. The manufacturer says the units can maximize lubricant effectiveness for long-term performance, even under poor lubrication conditions.

These ball bearings are said to benefit from a deep groove that combines steel rings and bearing-grade silicon nitride (ceramic) rolling elements. The balls are lighter, harder, less dense, and more durable than all-steel bearing counterparts, and they conform to standard bearing dimensions.

Benefits to its use are said to include higher reliability than conventional bearings along with reductions in life-cycle costs, total operating costs, and maintenance requirements. The bearings can supposedly upgrade existing wind-turbine generators without redesigning them.

Custom bearing may be a solution where conventional designs have come up short. Products of several bearing manufacturers support pitch, yaw, and gearbox applications in systems from 200 kW to 5.0 MW. The firms can design bearings to meet performance and life requirements with the most economical bearing design. Experience with offshore applications help specify appropriate corrosion resistant coatings such as zinc, paint, or other surface treatments. Material requirements can be met for operating temperatures down to –40C and below.

Other bearings appear conventional but have been modified. For example, in an electric environment, electrolytic corrosion from stray currents threatens bearing performance. One design uses coated angular-contact ball and cylindrical rollers to insulate the bearings from electric current. A ceramic coating applied to the outer surface and side faces of the outer ring prevents current from passing though the bearing. The design provides an insulation resistance of at least 2,000 MΩ. Under normal operating temperatures, this alleviates electrical arcing and early failure. The bearings come with or without seals or shields, and are interchangeable with standard, non-insulated bearings. The units are usually available with bores of 50 to 160-mm dia.

Another way to handle stray currents provides a discharge path to ground. One solution would provide a low resistance path from shaft to frame. A device called a bearing-protection ring meets the criteria. It uses principles of ionization to boost its electron-transfer rate and promote the efficient discharge of the high-frequency-shaft currents induced by many wind turbine generators. It channels harmful currents away from the bearings to ground.

The ring surrounds the generator shaft with many of conductive microfibers. The stiff yet flexible fibers provide high-density contact points — parallel paths of least resistance from the motor shaft to ground. The fibers reduce voltage buildup on the generator shaft by conducting instantaneous currents of many tens of amperes and discharging from tens to thousands of volts with MHz frequencies. The ring is especially suitable for use at high frequencies because its fibers tend to compensate for variations in roughness of the shaft surface, or microscopic misalignment of the ring and shaft, or both. When a microfiber looses mechanical contact with the rotating shaft, electric contact is quickly re-established somewhere else along the ring, due to local field emissions.

Windpower Engineering & Development

The day shift from the Gamesa nacelle assembly line.

Turbine OEM Gamesa Technology Corp. has announced the completion and shipment of its 1,000th nacelle manufactured at its Fairless Hills plant in southeastern Pennsylvania. The finished nacelle is among the 152 G90X-2.0 MW wind turbines comprising the 304-MW Blue Creek Wind Farm in Van Wert and Paulding counties in western Ohio. The project, which is owned by Iberdrola Renewables, is under construction and expected to be online by end of 2011.

“Completion and delivery of the 1,000th nacelle is a significant milestone that reinforces our leadership in the North American wind market,” said Luis Miguel Fernandez, Chief Corporate Officer of Gamesa North America. “And with this great milestone, we reaffirm our long-term commitment to the North American market, which continues to hold vast potential.”

Gamesa was the first overseas wind manufacturer to set up full production facilities in the United States, officially selecting Pennsylvania in September 2004 as the site for its U.S. headquarters, East Coast development office and two North American manufacturing facilities.

Gamesa initially invested $34 million to convert 20-plus acres of a former U.S. Steel industrial site from a brownfield into a state-of-the-art manufacturing center for nacelles. The company continues to invest in that facility in Bucks County. Renovations at the plant began Spring 2006 with production starting later that same year. In 2010, the company completed upgrades at the Fairless Hills plant that included the introduction of a moving production line as part of their lean-manufacturing strategy.

Gamesa started manufacturing G90X-2.0 MW turbines in the United States in 2009. More recently, the nacelles facility started producing the newest addition to the G9X-2.0 MW platform, the G97-2.0 MW Class IIIA nacelle for use in low-wind sites.

Gamesa has about 800 MW scheduled to go commercial by Dec. 31. To date, the company has installed more than 2,500 MW in North America, or about 1,285 turbines at 27 wind farms in 13 states.

Gamesa
gamesacorp.com/en

Windpower Engineering & Development

GE says its experience with superconducting equipment from its healthcare MRIs is applicable to superconducting generators for wind turbines. High torque at low rotational speeds may let wind turbines product up to 15 MW without a gearbox. See where the direct-drive superconducting generator is located in the wind turbine nacelle.

The technology development arm of a large electrical firm says it has begun work on the first phase of a two-year, $3 million wind project from the U.S. Department of Energy. GE Global Research (ge-energy.com/wind) says it will begin work on a next-generation generator for wind turbines that could support applications in the 10 to 15-MW range.

Conventional wind turbines often use a gearbox to increase slow rotor speeds to a higher rpm required by conventional generators. While such drivetrains are effective, they become expensive as they scale to larger wind platforms due to their additional weight and maintenance needs. It is possible to get additional power from larger drivetrains, only with an increase in the cost of electricity.

“New technologies will be needed to support larger-scale wind platforms,” says Keith Longtin, Wind Technology Leader, GE Global Research. He says the company will apply its experience with superconducting magnets used in healthcare MRI equipment. “Field windings are where we want to use the superconducting materials and cryogenics. So to leverage MRI experience, we will go with the topology of a rotating armature, sort of the opposite of a conventional generator.”

Longtin adds that superconducting technology may allow significant improvements to the generator and eliminate the gearbox. For example, magnetic fields would be larger from superconducting coils, even larger than those from rare-earth magnets. Hence, greater outputs from a similar size. The key is in reducing generator size and weight while dealing with lower shaft speeds and high torque. For size comparisons, Longtin says, “Our offshore turbine is rated for 4.1 MW, has a diameter of about 6 m, and weighs about 85 metric tons. We think with superconducting technology we can get 10 to 15 MW from about a 5-m diameter and the same weight. So that’s about three times the output.”

GE says the superconducting machine will use commercially available cryogenic coolers (for temperatures below 77°K) to improve the reliability of the complete machine. “We will investigate use of superconducting materials such as niobium-titanium, niobium-tin, MgB2, YBCO, and other second generation materials along with liquid nitrogen, helium, and neon to get the generator to superconducting temperatures, and techniques for staying there,” says team member Kiruba Haran, Manager of the Electric Machines Lab at GE Global Research.

The proposed superconducting machine aims to have more then twice the torque density of competing technologies and will further reduce dependence on rare-earth materials prevalent in permanent-magnet generators that are finding favor in recent turbines. The greater potential power from superconducting generators, coupled with better energy-conversion efficiency leads to more favorable economies of scale. For example, fewer towers would be needed for a given wind-farm output, which will help reduce the cost of energy produced by wind turbines.

The generator wind project will have two phases. Phase I will focus on developing a conceptual design and evaluating the economic factors associated with it. Phase II will explore potential commercialization of the technology. The Oak Ridge National Lab will be a partner on the generator wind project, helping investigate and mitigate high-risk technology challenges. Oak Ridge has facilities for more fundamental research, so they will run reliability testing on cryogenics. “It’s relatively easy to make something once and get the power needed, but can it be done in a reliable and cost effective manner over and over?” asks Haran. “We must do component tests to find the answer along with vibration and environmental tests for life data.” WPE

Windpower Engineering & Development

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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Windpower Engineering & Development

The Airgenesis design uses two rotors, one at each end of a nacelle. Gearboxes and generators mount at ground level for easier access.

There is more to the new design than two rotors. “Most of the heavy equipment is mounted at the base of the turbine so maintenance work can be simplified. Furthermore, using multiple generators provides the capability of replacing generators without shutting down the turbine. Clipper Windpower uses a design of four generators for similar advantages but mounts them in the nacelle. Troxell says many details covered by several patents are being held confidential. The design is said to target a constant electrical output at low wind speeds, conditions in which traditional wind turbines cannot operate.

The company did reveal details of the design to power-plant maintenance expert Jerry D. Casteel who then commented, “Placing components that are costly to maintain to the ground will make the Airgenesis design competitive with fossil fuel and hydro units of the same size.”

The plot from Airgenesis is said to illustrate the potential advantage of its design over conventional wind turbines.

The company says it is confident the design represents 80% more efficiency than most conventional commercial systems, three times the megawatt production of current ground technology, increased energy to grid, reduced tonnage and stress loads on towers, and complete construction costs of less than $400,000 per MW when in full production along with overall reduced operational costs.
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Windpower Engineering & Development

A variable-ratio transmission that uses gears and chains, but no belts, has been introduced for licensing and production by inventor Gary Lee, CTO of VMT Technologies (www.theuniversaltransmission.com). The transmission works by expanding or reducing the diameter of a “moon gear” that drives the chain, thereby changing ratios. It does not rely on friction, which means the transmission can handle more torque than a conventional CVT without it over heating and melting components. “Our high-torque engaged CVT will also improve transmission efficiency,” says Lee.

Universal Transmission inventor Gary Lee

Conventional continuously variable transmissions (CVTs) use a pair of cone-shaped pulleys on the same shaft that are driven together or pulled apart to change the radius on opposing surfaces. But the belts or chains used in them depend on friction to transmit torque.

The company recently unveiled a working prototype for the wind power industry. “Wind power gearbox manufacturers told us, ‘Improve wind turbine efficiency by just 2% and the industry will beat down your doors,’” says VMT’s Sales Director Mike Agrelius. “Our engineers project improvements of 5 to 10%.” VMT received a patent for the transmission on February 22, 2011.

Lee says the Universal Transmission overcomes problems of conventional CVTs by using a metal chain with teeth that are always engaged with the moon gears on which it rides, so it transmits power more efficiently than a friction-based design. He says the moon gear solves what has been called the partial tooth integer problem.

CVT inventor Gary Lee envisions a two-stage gear box to handle the high-torque, variable speed duties in wind turbines.

Agrelius adds that the transmission has great promise for the wind industry because it can work as a speed variator. For example, it would keep the generator turning at an optimum speed while the blades are kept at a pitch to provide maximum aerodynamic efficiency, thereby capturing more energy from the wind than a conventional drivetrain. The transmission would also allow eliminating the power electronics in the nacelle that are needed to turn the variable current and frequency from the generator into more manageable power, according to Lee. Another plus: the transmission will absorb the power in wind gusts rather than transmit them through the drivetrain, hence, lowering load on drivetrain components and improving time between maintenance and replacements. One of the larger problems facing the wind-power industry is the high cost of capital and maintenance regardless of transmission.

But what advantage might the transmission have with respect to direct drives, turbines without transmissions? Agrelius says direct drives might try to regulate shaft speed by adjusting pitch, and thereby sacrifice efficiency and power capture. Another plus for VMT’s transmission is that it would make better use of induction generators as opposed to permanent-magnet versions that rely on politically sensitive rare-earth materials, the availability of which have been trending downward.

A nacelle cut away shows the space saving possibilities of the Universal Transmission.

Lee sees his Universal Transmission used in other power and transportation-based industries as well, trucking in particular. The company has no plans to manufacture the transmission, but rather license it to others. Over the past year, the company has met with transmission manufacturers such as Allison, Dana, and Ford.

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Windpower Engineering & Development

Baumer
www.baumer.com

Windpower Engineering & Development

Parker’s Precision-Cooled Rack Solution features patented two-phase evaporative cooling for things that must stay cool in turbine nacelles.

The flexibility of Parker’s technology and its use in racks, cabinets, and containers has been implemented in many other industrial markets where it has been easily configured to cool a variety of applications such as power electronics, motors, transformers, and batteries. Dual-phase liquid cooling continuously cycles a refrigerant within a sealed, closed loop to cool a wide range of devices. The cooling system uses a small pump to deliver just enough coolant to the evaporator – usually a series of one or more cold plates optimized to acquire the heat from the warm device. In so doing, the coolant begins to vaporize maintaining a cool uniform temperature on the surface of the device. The resulting two-phase coolant is then pumped to a heat exchanger where it rejects the heat to the ambient and condenses back into a liquid, completing the cycle.

The cooling rack works well in wind turbines where the size and weight in the nacelle continues to grow with total capacity. In addition, the Parker precision-cooled rack is the only product available today offering the potential to double the power density of key major subsystems such as the generator and power conversion system, and whose modular thermal management elements greatly simplify onsite maintenance when required, while reducing overall maintenance costs.

Parker Hannifin
www.powersystemscooling.com

Windpower Engineering & Development

Wind turbines transmit power from their generators to ground-based equipment through large cables capable of handling many Amps. But there are no slip rings to get this power out of a turning nacelle. To accommodate the necessity of yaw, the cable is run up the side of tower, over a 180° strain relief, into about 3-m long loop and then into the nacelle. This “slack” accommodates nacelle motion. But as you can imagine, in cold weather, cables stiffen, so the stranded copper conductors and insulation are a composition that should stay flexible. “Tests our company conducts in Europe involves 5 million twists at about five rpm,” says Lapp USA Senior Product Manager Rick Orsini.

Improved tolerance to cold weather is the cable trend. “At -40°C, cable insulation can become so brittle it will break off and expose conductors. One lab test calls for dropping a small weight on a cable frozen to -40°C. It shatters the insulation as if it were glass. That temperature is extreme but possible in turbines in cold weather regions,” says Orsini.

Another cabling trend is toward insulation that tolerates gear oil. Depending on the insulation material, with sufficient exposure to oil, insulation can swell and flake off, or become brittle and break off, again exposing conductors. Although gearboxes have improved in recent years, they still use oil that must be changed on occasion and that invites drips and spills that can find their way down that big cable loop below the nacelle. One company says oil spills in the nacelle are unavoidable.

No surprise that OEMs are looking for price concessions, perhaps the most universal trend. Turbine manufacturers setting up production in the U.S. often ask for halogen-free cable insulations. Halogen-free materials are often considered because if exposd to flame, they would not outgas harmful compounds to nearby technicians. “However, considering that it is unlikely that anyone would be in a nacelle should a fire break out, the necessity of more expensive halogen free materials becomes less important,” says Orsini. PVC based insulation are less expensive and provide the oil and temperature performance needed. So far, adds Orsini, his company has been able to produce a halogen-free insulation good to -25°C.

Of course as turbines are getting larger, their voltage outputs are also going up. UL WTTC (wind turbine tray cable) standard 1227 got frequent sited for 600V cables it describes. The more recent WTTC 2227 now describes 1,000V cables.

What happens when a cable is exposed to flame is on the mind of other manufacturers as well. Lubrizol says it developed a thermoplastic polyurethane for cable jacketing and molding that delivers a Limited Oxygen Index (LOI) of 40. The material has a Shore hardness of 53D, and is UL-94 rated for V-0 on the vertical burn of a 75-mil sample thickness.

Getting cables through bulkheads while maintaining water and oil tightness is also getting attention. One multi-cable bushing system is said to provide better way to seal and clamp cables than competitive designs, and with simplified service. Users install a frame to an enclosure wall just once. The frame has individual passage for each of 8 to 10 cables, each accommodates standard 16 or 24 pole connectors. Manufacturer Lapp USA says each cable has high pullout resistance with compression modules. The device’s advantage is faster repairs because the frame remains on the housing at all times, and it requires no special tools, sealants, or lubricants.

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Windpower Engineering & Development