For anyone who doesn’t realize the important role of wire and cable in wind farm reliability and performance, consider the fact that a 125 MW to 150 MW wind farm with approximately 50 turbines encompasses about 3 million feet of cable for the nacelle, tower, collection system and transmission needs. That means that a larger-scale 625 MW to 750 MW wind farm – with approximately 250 turbines – relies on 15 million feet of cable spanning roughly 2,800 miles. When it comes to satisfying the demand for renewable energy, every foot of cable counts.
As an intermittent source of power that requires high reliability with low operation and maintenance costs, a wind farm must be extremely efficient to ensure that as much power as possible can be generated and delivered to customers whenever the wind is blowing. Any downtime or delays can result in enormous losses in revenue. Consequently, all of the cables within the system – from the rotor to the electrical grid – must be engineered to ensure maximum efficiency, cost-effective performance and long-term reliability. There are also several conditions within a wind farm that require industry-standards-based cable designs, which have been qualified and tested to withstand the operating and environmental demands throughout the life of the system.
Conditions and considerations
There are many sizes, types and voltages of wire and cable used throughout a wind farm. It is not unusual to find up to 50 different cable constructions within a system, all of which play a critical role in overall wind power generation, transmission and distribution.
In the nacelle and tower, a wide range of power, control and signal cables is needed for a variety of interdependent systems, such as the generator, gear box, pitch system, yaw gear, transformers, controllers and other devices required to effectively monitor, control or convert the wind into electricity. In the collection system, medium-voltage underground cables are needed to provide efficient low-loss transmission of power from multiple turbines to the substation. Throughout the entire system, high-bandwidth fiber-optic cables make up the supervisory control and data acquisition system needed for continuous monitoring of the performance and efficiency of every turbine. And once the power reaches the substation, overhead transmission conductors deliver the renewable power to the grid.
Cable systems deployed within the nacelle and tower of a wind turbine are faced with a rigorous environment that includes vibration, flexing, high-torsion stress, electromagnetic interference, oil, ozone and extreme temperatures.
In the nacelle, cables need to be properly sized and selected to fit seamlessly with accessories and maintain constant connections as the nacelle turns into the wind. Power cables that are undersized could overheat during peak loads and potentially cause the device they are powering to do so as well. For low-voltage signal and communications cables, knowledgeable cable manufacturers can select proper shielding to prevent interference from electric motors and help to reduce or eliminate attenuation of the signal. This can be critical to maintaining signal transmission integrity for monitoring and control of auxiliary systems within the nacelle. The nacelle cables must also be resistant to oils and ozone, as well as flame-retardant, to maximize safety.
The tight space within the nacelle requires cables that are highly flexible with an appropriate bend radius of four to six times the cable’s outer diameter for easy installation. Once the electrical energy moves from the generator in the nacelle to the vertical tower, which can be anywhere from 40 to 90 meters high, there are additional cable flexibility and torsion requirements unique to wind turbine generators. Cables that connect within the nacelle and run down the tower will experience extreme torsional stresses during their lifetime due to the nature of the rotating nacelle. The cable transition inside the tower that experiences torsional movement is known as a “drip loop.” It provides the needed slack to minimize mechanical stresses exerted on the cables when twisting occurs.
The flexibility of a cable is primarily driven by the conductor strand type, as well as by insulation and jacket stiffness and cable geometry. With smaller-diameter cables, the need for flexible conductors is less of an issue. Larger-diameter cables require stranded fine-wire rope conductors for better flexing and bending. Because torsional stress is dependent on radial thickness, jacket and insulation stiffness, and torque (i.e., force of twist), the overall roundness of the cable is also a consideration. If a cable is not properly manufactured to be completely round, or if it features irregular radial thickness from its center throughout its length, stresses from the twisting force will also be irregular. This can place more stress on the internal components of the cable and lead to premature cable failure. Because wind farms can be located in a variety of terrestrial and offshore locations, wind cables require a broader operating temperature of -40°C to 90°C, while maintaining stable electric properties.
Although cable type is ultimately dependent on the turbine design and transformer location, cables within the nacelle and tower must meet stringent industry standards and specifications. In North America, the trend today for turbine manufacturers is to specify cables that meet both Canadian Standards Association (CSA) and Underwriters Laboratories (UL) standards.
For example, to meet the -40˚C cold bend requirement, cables must pass UL Standard 2556 or CSA Standard C22.2 No. 0.3 cold bend testing. Some of the cable design attributes required for nacelle and tower cables are outlined in Figure 1.
In wind farm projects, the collection system is one of the most critical aspects. To capture as much renewable energy as possible and eventually reach a return on the high capital investment involved in building the wind farm, the collection system (including the 35 kV medium-voltage cable portion and the associated substations and transmission lines) must be designed and built for efficiency, endurance and economics.
Unlike traditional power generation plants that are located adjacent to substations, wind farms can be located several miles from a substation. Some of the renewable energy produced will be lost from the point of power generation to the substation due to resistance of the wires and equipment the energy passes through. Any amount of energy lost in the collection system is lost revenue. While some losses are inevitable, reducing the percentage of loss can be achieved by having properly sized and engineered cables with advanced materials that are fully qualified and adhere to industry standards.
For example, traditional medium-voltage utility cables have been re-engineered to use cross-linked polyethylene jackets that offer reduced, optimized neutrals to provide a higher temperature rating and better efficiency over the life of the cable through cooler operation, lower line loss and greater resistance to deformation.
Because the collection system is a static and underground system, repairs are extremely costly. To minimize the risk of expensive repairs, medium-voltage cables used in the collection system should be constructed of qualified materials and tested and proven to demonstrate long-term reliability and performance for extended cable life in underground installations. They must be able to withstand the rigors and challenges associated with direct 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. For maximum moisture protection, these cables should comply with the longitudinal water penetration resistance requirements of ANSI/ICEA S-94-649 Part 6 and ANSI/ICEA T-34-664.
Unplanned delays during deployment can adversely affect the construction of a wind farm, potentially resulting in penalties and a loss of qualifying tax credits that are contingent upon delivering power by a specific date. Therefore, it is imperative that the collection system cables install seamlessly with connectors, fittings and other accessories in order to avoid costly delays. And once installed, terminations and splices must remain stable to prevent failures and revenue-losing repairs and maintenance. Compliance with standards is vital in this regard.
For example, off-centered core dimensions can compromise cable functionality and accessory compatibility.
Like nacelle and tower cables, collection system power cables should be qualified to meet or exceed industry standards. Qualified power cables must meet industry standards UL Standard 1072 and CSA Standard C68.5, along with ANSI/ICEA S-94-649 and AEIC CS8. Working closely with cable manufacturers to select the right design can minimize the risk of failures and avoid commissioning delays.
With the deployment of wind energy sources in North America and the high reliability and low operation and maintenance costs, combined with North American-based wind turbine manufacturing and the need to source local cables, the demand for cables that meet both CSA and UL standards has become paramount. Turbine manufacturers and utilities are calling for standard-compliant cables that have been tested and proven to meet design criteria for torsion, flexibility and temperature ratings while providing lower line losses and extended cable life. Certified test reports that can be traced to applicable national industry standards are highly recommended when specifying collection system cables for wind farms.
As the renewable energy market evolves, wind cable industry standards are also evolving. Renewable energy developers will soon have improved guidance and standardization when defining cable types for wind applications. Industry standards organizations, such as the National Fire Protection Association, CSA and UL, are recognizing wind farm cables in published codes and standards.
For example, UL Standard 6141 will require all wiring that resides within a wind turbine that is accessible to service personnel, or that runs vertically up the tower, to be certified to certain UL ratings for flame performance.
Industry standards organizations are also recognizing traditional cables with proven long-term performance and reliability. Most traditional standards-based North American cable types will soon be specifically permitted within wind turbine standards, which will help level the selection and performance ratings. Cable manufacturers with quality manufacturing capabilities and long-term experience developing cables for harsh environments are well positioned to offer these cables for wind applications.
As the wind energy market continues to grow, standards-based, tested and proven cables will go a long way to ensuring reliable wind power generation, transmission and distribution – and ultimately revenue. w
Marketplace: Wire & Cable
Why Cable Matters In Wind Reliability, Revenue
By Tim Clancy & Joe DeBolt
Although cables may sometimes be an afterthought, they play a critical role in generation, transmission and distribution.
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