As wind turbine development continues to increase around the globe, many designers have already developed preconceived notions about what their cabling should and should not include to have the turbine produce the most power at the least expensive cost. As wind turbine technology advances, we are finding out that some of the simplest and most basic principles of wind turbine cabling are not necessarily true.
Here are a few myths that are floating around the wind turbine community and an explanation to challenge these previously held industry beliefs:
Myth No. 1: Copper is the only conductor material for use in wind turbines.
Reality: Traditionally, cable and wire manufacturers have used copper as the conductor material of choice for cables used throughout wind turbines. However, with copper prices being extremely volatile due to varying degrees of demand, cable research and development engineers use alternative conductor materials or copper alloys that perform comparably to pure copper when they can to combat the effects of copper’s changing price in order to keep turbine costs under budget.
One such material is aluminum, which traditionally has been incredibly stiff and not easy to use. Some manufacturers, however, have developed a cable with flexible-aluminum conductors that are lighter (up to 60%), safer and easier to install/replace.
These flexible-aluminum cables, which are diesel-locomotive-like, are best used in the tower as well as in the down tower area (DTA) of the turbine and have performance characteristics comparable to standard copper cables, but at a fraction of the cost. The cost savings of using aluminum goes beyond just the conductor material and influences both installation and maintenance costs, because the complete power cabling system – generator to inverter – is interrupted only in the loop. This maximizes cable safety and reliability compared to the conventional installation method, which interrupts the cable at every tower section. Furthermore, installation time is reduced from days to merely a few hours.
Myth No. 2: Class 5 standard copper conductors are optimal for all wind turbine applications independent of the kind of stranding or layering.
Reality: In theory, the functions and applications of a cable are very simple. However, this is not the case. A clear guidance on the application and operating conditions should be well defined in order to make a recommendation on other cable and wire criteria by cable specialists. Knowing the conditions for the application is the basis when deciding which insulation materials to use for individual conductors and the outer jacket.
In applications with moderate flexing movements, flexible-aluminum cable is used. Once the motion changes in type and frequency, copper is used as the conductor material and is available in a wide range of dimensions and designs. For example, take the cross section of a 500 kcmil (240 mm²) to show the various possibilities. A single or multi-conductor cable with Class 2 stranding has a structure of 37 strands with a diameter of 2.87 mm² and is used for non-flexing applications such as direct burial or cable platforms in industrial plants and buildings. A flexible, fine-wire conductor like Class 5, which has 1,225 strands and a diameter of 0.5 mm², is used for flexing applications with high levels of frequent bending and directional change cycles. Finally, there is a highly flexible Class 6 structure with 1,905 strands and a diameter of 0.4 mm². This design is used in applications when extremely small bending radii and extreme bending cycles are required.
Within the different structures of the copper conductor, there a few additional critical aspects that must be considered: the length of the lays, the lay direction and the compactness of the conductor construction. Cables with a stranding of Class 5 and Class 6 with very short lay lengths and special lay directions are typically used in drag chain applications. Long lay lengths are preferred for torsion applications (loop cables), and each manufacturer designs cables with stranding that follows these basic principles. While each manufacturer has a unique method of construction, each cable should lead to the same result – withstanding a minimum of 10,000 cycles in the loop section of a wind turbine.
Myth No. 3: The best insulation material for wind turbine cables is rubber.
Reality: The selection of insulation materials plays an essential role in applications that are outside the norm or have undefined operating parameters. The cable loop of a wind turbine is a perfect example of this. The cable loop is located at the transition from the nacelle to the tower, where the cables experience axial stress (torsion). The resulting friction from up to 24 power cables causes an extreme abrasion on the surface of individual cables. This usually causes power cables to lose their required insulation properties. Sometimes after an operating period of two to three years, these abrasive effects are already apparent.
In the realm of insulating materials, the mind-set that the “good old rubber cable” is the best available outer jacket material does not always hold true. Rubber materials possess adhesive properties on the cable’s surface, which restricts cables from easily sliding as they rub against each other. This increases friction and the chance that rubber cables will create wear on the cable jacket. Experienced manufacturers prefer thermoplastic elastomers (TPEs) such as polyurethane or special PVC compounds, which have better sliding properties.
A special production process, combined with the chemical properties of the TPEs, gives the outer jacket a surface appearance similar to that of an orange – which reduces friction and allows for easy cable movement during contact, thereby reducing cable abrasion. Although friction is going to occur as outer jacket surfaces rub against each other, factors such as speed, temperature and pressure play a large part in determining the damage encountered by friction.
Another factor is thermal overloading. In the wind turbine, very high currents – depending on the power performance of the wind turbine (up to 8,000 A) – cause the conductor temperature to rise up to the maximum allowed temperature of the cable. Cable insulation must withstand not only the conductor temperature, but also the climatic influences of the different regions, which can range from -40°F to +140°F (-40°C to +60°C). Finally, engineers must also consider the fire properties of the insulation materials by using halogen-free and low smoke (LSZHLS) compounds, for example. Other resistances include oils, greases and UV rays (sunlight). If all of these factors are taken into account in the cable design, an operational lifetime of 20 years is realistic.
Myth No. 4: Bus bar systems in wind turbines are more economical than cable solutions.
Reality: A new electrical bus bar system seems to be a relatively attractive alternative to cable systems, solely from the aspect that a bus bar system offers an uninterrupted connection to the DTA. For wind turbines with a tower height up to 500 ft. (150 m), the handling and fastening of stiff aluminum cables is a difficult task. New generations of cables, such as highly flexible aluminum conductors, enable a seamless connection directly to the inverter in the DTA. The installation time, as well as the installation process, is simple and efficient, and it also allows for quicker and more reactive responses to turbine design changes such as the tower height. A corresponding bus bar system is more rigid and not as easily changeable when design plans are rapidly being altered.
There is no existing industry in which technical innovation is changing as fast as in the wind sector.
From one wind turbine generation to the next, components and subsystems are never permanent and constantly evolving. Generators and transmissions improve in their performance and become more efficient; drive concepts also are adapted to be state of the art. The requirements of the utilities are a little more difficult to anticipate but must be taken into account. With so much technology to take into account in different tower systems, turbine manufacturers need a flexible solution to transmit the power, which is currently only possible with an intelligent cable solution. w
Marketplace: Wire & Cable
Dispelling Cable Myths: All Lines Are Not Created Equal
By Uwe Schenk
Despite an opinion shared by more than a few wind developers, all cables are not the same.
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