— As conventional energy sources – namely oil, gas, and coal – are increasingly limited in their respective quantities, many countries are encouraging the development of renewable energies. Wind energy, already a well-developed technology, is quickly becoming the most attractive renewable energy source. Cables used in the wind energy industry are facing challenges caused by the harsh environments where the wind can be harnessed most efficiently. The current status of wind power industry is introduced herein, as well as the performance, requirements, and testing procedures of cables used in wind turbines.
There has been increasing interest in renewable energy due to conventional energy resources, such as oil, gas, and coal, being limited while demand still increases. Numerous types of renewable energy, such as hydropower, wind power, solar power, geothermal power, tidal and ocean wave power, and biogas power, have been emerging for decades. Although these renewable energies only cover a small portion of the entire energy demand, the promising future of these new energy sources is recognized globally.
In addition to the dwindling amounts of conventional energy sources, another factor accentuating the attractiveness of renewable energy is the pollution caused by fossil fuel power plants. Wind energy, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, and produces no greenhouse gas emissions during operation. At the end of 2009, worldwide nameplate capacity of wind-powered generators was 159.2 GW. Energy production was 340 TWh, which is approximately 2% of worldwide electricity usage  and has doubled in the past three years.
Regarding the existing wind turbine maintenance and new wind farm construction, engineers and designers meet the inevitable situation of having to choose the correct cable for such applications. For example, inside the wind turbine nacelle, continuous flexing control and data cables should be used; torsional flexing cable should be used within the wind turbine tower. In addition to the flexibility requirement, thermal resistance, abrasion resistance, and resistance to oil and other chemicals should also be considered. The complexity of cable selection could easily lead to a wrong decision and result in unnecessary downtime and costly maintenance.
II. Cable requirements for wind power applications
Generally, wind farms are located in harsh environments with extreme weather conditions, including wind, ultraviolet light and salt spray. Because of this, the performance of the cables used in wind power applications is arguably more critical than that of other applications. The moving parts of the wind turbine also add to the already heightened level of importance of proper cable selection.
Premium-grade power cables, data and control cables, and communication cables, need to be considered for existing wind farm maintenance and new, large-scale wind farm development to determine the interconnection quality for the power grid and communication system. The amount of cable needed for just one wind generator is not quite as minimal as one might imagine. For example, a 90 m high 1.25 MW wind power generator requires approximately 1 km power cable. As such, 40 km cable would be considered necessary for a wind farm with 50 MW capacity.
Wind generators work in tough environments that typically feature a wide temperature range (around -40ºC to 50ºC) and extremely large amounts of exposure to UV radiation. Therefore, the specified cable needs to be able to withstand -40ºC and ultraviolet light for desired longevity. For the moving parts of the wind turbine, the cable should have good torsional and bending flexibility with a small bend radius. The cable also needs to be resistant to fuel, coolant, oil, corrosive chemicals, and abrasion. Should the wind farm reside close to the ocean or off-shore, the cable must also be salt water resistant. In addition to the aforementioned requirements, flame retardancy is also required due to the safety considerations. In certain cases, extra features such as low smoke, zero halogen (LSZH) materials and EMI protection may also be required.
Taking all this into account, cable used in a wind power application should generally meet the following criteria:
To maximize flexibility, it is recommended that design engineers specify annealed soft copper conductors with high strand counts, with a short concentric-lay construction for bending flexing applications and a long concentric-lay construction for torsion flexing applications. For conductors larger than 6 mm2 (10 AWG), a rope-lay construction is required.
To increase flexibility in low temperatures, thermoplastic elastomer (TPE), ethylene propylene rubber (EPR, a type of EPM or EPDM), or silicone rubber (SiR) are common choices for insulation material to resist ozone corrosion and aging due to heat. PVC/nylon insulation is also widely used due to its high dielectric strength.
Cable jackets may be thermosetting compounds such as chlorinated polyethylene (CPE), polychloroprene (neoprene), chlorosulfonated polyethylene (CSPE) synthetic rubber (SR), or thermoplastic compounds such as TPE, TPE-PVC alloys, and polyurethane (TPU). These materials are oil-, fuel-, solvent-resistant with superior flexibility under low temperatures. Such properties make them ideal jacket materials for wind power cables.
It should be noted that cable structure is also a determinant factor of the cable’s flexibility. Symmetric conductor design with a balanced structure will typically lead to a high degree of flexibility.
Even if a cable is built following these general guidelines, full testing is strongly recommended to simulate the “real world” application.