Wind turbines use many types of signal and low-voltage cables for a variety of applications. Most applications are in the nacelle and tower.
In the nacelle, cables connect sensors to controls and to power devices. Flexibility there is important to route the cables in confined areas, such as for sensors in the blades. Flexible cables also perform better with the high vibrations caused by rotating machinery in the nacelle. In addition, gearboxes can leak oil so using cables with oil-resistant insolation helps prevent cable failures. Wind techs will appreciate the feature when a gearbox needs replacing but its related sensor cables will not. Power applications in the nacelle include pumps, fans, pitch systems, and drives. Ethernet cables also run from nacelle to tower base.
In the tower, low-voltage power is needed for lighting, while higher voltage cables are needed to bring the generated power to the base of the tower for a connection to switchgear and the grid. A three-meter drip loop in the power cable, just below the nacelle, lets a turbine yaw or make several rotations in either direction as controls work to keep the turbine pointed into the wind. So a flexible power cable must tolerate torsion or twist.
All of these cables must be UL listed with a good flame rating, such as FT4. One UL type, WTTC (Wind Turbine Tray Cable) is rated for 1,000V. Oil resistance is useful here too because gearbox oil can drip down the power cable should the gearbox leak.
Another cable feature often required in towers is one that offers flexibility at low temperatures, especially for turbines in cold climates. There, cables that twist in the drip loop during normal operations must remain somewhat flexible. Flexibility lets cables sustain vibrations in colder environments. A few insulation materials perform well down to -40°C.
The section above provided by Rick Orcini, Technical sales with SAB Cables.
Low-voltage signal and control cables also require proper shielding to prevent interference from electric motors and to reduce or eliminate signal attenuation. This is critical to maintaining the integrity signal transmissions for monitoring and control of auxiliary systems. Regardless, all cables within the nacelle must offer resistance to oils and ozone, as well as flame retardant to maximize safety.
Smaller low-voltage cables used within the nacelle and tower are designed to withstand torsion and vibration, while providing stable electrical properties over a broad temperature range.
Where are high-voltage cables used in a wind turbine?
High-voltage cables connect the generator to power conversion panels. Typical voltages from the generator range from about 650 to 800V. A single cable is typically not large enough to handle the current generated for each phase, making it necessary to combine several cables in parallel to accommodate currents as high as 2,400A per phase. Space constraints and latent heat within the nacelle require that the cable combine a high-wire count of flexible conductors with a large cross-section and a thermosetting insulation.
When voltage coming down tower is less than the 35 kV for the collection system, a step-up transformer at the base of the tower increases the voltage to meet the 35-kV requirement. At the substation, voltage is again stepped up to reach the 138 to 345-kV operating range typical for bare overhead lines.
What are the mechanical properties for underground and overhead cables?
The 35-kV underground-collection system (35-kV cables) must be rugged and suitable for the mechanical requirements associated with direct-buried applications. These cables have to withstand changes in ground conditions, such as dry soil that causes cables to run hotter, or wet soils where moisture can affect the long-term life of the cable. Overhead transmission lines must withstand windy environments expected with wind-farm locations and environmental concerns, such as ice loading.
While overhead transmission lines are used for a final connection to the grid, the collection system within the wind farm is static and underground. As with any underground system, repairs are extremely costly. Medium-voltage cables used in wind-farm collection systems should be designed with qualified materials, tested, and proven to demonstrate long-term reliability and performance for extended underground life. They must withstand the rigors and challenges associated with buried-installation techniques and potential hazards, such as water ingress. Water-blocked conductors, concentric neutrals, jacket, and completed cable are critical features that can prevent longitudinal penetration and migration of water along the conductor and beneath the outer cable jacket.
What new developments in cables should wind technicians be aware of?
To establish a more efficient, environmentally friendly collection system for wind-power applications, wind technicians and site electrical engineers should note the re-engineered utility medium-voltage cable, EmPowr Link CL Advantage. It is a more ruggedized, 35-kV cable for harsh soil conditions. It features a smaller, lighter design with flat concentric neutrals to protect the core of the cable from installation and environmental damage. This provides an added measure of cooling, an efficient shielding system, lower line loss, and better resistance to deformation.
For bare overhead transmission lines, E3X Technology lets utilities optimize the power grid by adding more capacity and control losses with significant first-cost and long-term operational savings. TransPowr with E3X Technology features a thin, durable coating that applies to the surface of General Cable bare overhead conductors. This heat-dissipating coating increases emissivity and reduces absorptivity, improving energy effectiveness and efficiency by allowing for a higher ampacity rating, reduced operating temperature, and lower losses for a given conductor size, or a reduced conductor size for a given ampacity rating. Depending on operating conditions, a bare overhead conductor with E3X Technology can offer up to a 20% reduction in project first costs, up to 25% increased ampacity, up to 25% lower line loss, and up to 30% reduced operating temperature.
The latter sections are supplied by Karen Wilkinson at General Cable.
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