by S. “Ram” Ramachandran, Dow Wire & Cable
The Global Wind Energy Council (GWEC) recently reported 22 percent growth in installed wind energy capacity worldwide during 2010. This represented a $65 billion investment in supporting equipment and power distribution infrastructure.
To protect this investment, the balance between total system cost and long-term system reliability is crucial. If a wind farm developer, independent power provider or utility specifies quality cables made with the best materials technology and manufactured with exacting standards, the installation can provide decades-long reliability with little to no downtime for electrical losses and expensive repairs. When it comes to satisfying the demand for uninterrupted power by ensuring system reliability, power cable design and construction with validated performance-based materials is critical.
A Little History
As commercial wind farms have grown as viable alternative energy resources during the past 20 years, cross-linked polyethylene (XLPE) has been recognized as the choice material for medium-voltage power cable construction. Its popularity for direct-buried and submarine cable is predicated on quality, competitiveness and reduced long-term operating costs.
As demonstrated in traditional power distribution networks (see Figure 1), however, XLPE, although superior to materials such as high-molecular-weight polyethylene (HMWPE), has seen its share of performance issues. That led to the development of water-tree retardant XLPE or TR-XLPE insulation for underground (UG) power cables. The cables that move power from wind turbines to the grid—on-shore or off—are subject to the same mechanical and environmental stresses as those experienced in the grid. Conventional wisdom dictates that TR-XLPE becomes the choice material for wind farm 35kV UG power distribution cables.
Quality Materials Matter
An essential part of long cable life and system reliability is using quality, raw materials tested to perform according to industry specifications. Cable manufacturers look for materials that deliver easy processing while producing the performance attributes end users expect. Flexibility, stress-crack resistance and shield strippability are considerations for easy installation. Once in the ground, cables are subject to environmental stresses including water intrusion and extreme temperature variations. Corrosion kills cables and interrupts power supply. It becomes increasingly important for cable manufacturers, wind farm developers, installers, utilities and others in the value chain to understand the benefits quality materials bring to power cable design and construction.
For many years, cable has been seen as a commodity. The general feeling is that any standard utility cable will operate fine in any system. Because of this commodity mindset, cable often is purchased based on price rather than its being a critical part of an overall system investment. Shouldn’t the cable specified have a lifespan equal to the system it supports? This is possible with performance cable such as TR-XLPE vs. base cable using standard XLPE (see Figure 2). As a case in point, Dow Wire & Cable introduced its DOW Endurance MV 4202 TR-XLPE in 1983. Studies show that nearly 30 years later, buried cable made with this material exhibits little to no wear and has an expected lifespan of more than 40 years.
Many raw materials suppliers and cable makers serve the wind energy market. Research and development at the front end of the supply chain is important. Cable makers, developers, IPPs and utilities should ask about the kind of technology, clean manufacturing and packaging techniques, testing and validation that goes into raw material production. Similarly, end users should insist on specifying cable that has gone through rigorous testing and meets at least the current minimum performance standards set by utilities. Trusting investment dollars to anything less is risky.
Many testing institutes work with companies and their customers to ensure that raw materials and the cables produced with those materials meet recognized national and international standards. These organizations include: National Electric Energy Testing Research and Applications Center (NEETRAC), standards development agencies such as the Association of Edison Illuminating Companies (AEIC), Insulated Cable Engineers Association (ICEA) and Cable Technology Laboratories (CTL).
In addition, cable makers are producing cables that consistently exceed stringent, long-term testing standards such as AWTT and ACLT in North America, VDE Standards in Germany and DL/T-1070-2007 in China. These long-term testing methodologies demonstrate a proven record of ensuring long-life, reliable cable performance.
No exclusive standards exist for cable performance in the wind energy market. End users must insist on cables that meet, or preferably exceed, the current power industry minimum standards. Cables form a small percentage of the total power system cost, and polymeric materials represent an even smaller percentage. The renewable energy industry must take a broad view to focus on the needed system reliability that rests largely on excellent materials, quality cable-manufacturing processes and elevated performance standards.
Improvements at Utilities
Utilities also are implementing practices that include cable inspection, installation and operations. Wisconsin Public Service (WPS) serves more than 400,000 electric customers in an 11,000-square-mile area. As discussed in a joint white paper authored by WPS and Dow Wire & Cable, the utility has focused on cable reliability for 30 years as a preventive action to delay cable replacement. Cable reliability is determined by WPS based on strict material specifications, a comprehensive plan that monitors incoming cable quality based on those specifications, and in training cable installers. It all leads to effective system management.
Similarly, as a way to provide mutual value and insight, many utilities conduct and share field aging evaluations that provide to materials suppliers and cable manufacturers a broader view of cable and system performance vs. depending solely on accelerated aging tests in a lab. A joint evaluation conducted by Alabama Power and NEETRAC confirm the robustness of TR-XLPE cables made and installed in 1985 (see Figure 3). Results indicate that cable life in excess of 40 years easily can be projected.
It’s Time to Work Together
For anything worth pursuing, it takes a community of likeminded people to achieve success. Wind power is not new; however, to go from the power needed to turn a millstone to the power needed to light up and connect communities efficiently and cost-effectively is another story. Collaboration is essential. AWEA and GWEC are helping as they provide gathering places and information portals for all stakeholders. But collaboration must exist in the trenches, as well, among investors, developers, IPPs, utilities, equipment, cable and material suppliers, etc., to realize the energy goals that are legislated or soon will be.
Companies and their customers are working together to develop, validate and adopt superior products for cable construction and to enhance market awareness about building power systems with the right components used best to ensure optimum results for the entire value chain.
And, at a fair price that recognizes the total system cost, including the potential to specify quality cable once rather than the additional expense of repair and replacement over the system’s lifetime.
S. Ram Ramachandran is global director of end-use marketing for Dow Wire & Cable, a business unit of The Dow Chemical Co. Ram and his team interface with global end users such as utilities, communications groups, off-shore drillers and regulatory boards. He is a Senior Member of the Institute of Electrical and Electronic Engineers (IEEE) and is chairman of the Power Cable Standards Discussion Group of IEEE/Insulated Conductors Committee. He has eight patents and is the author of more than 20 papers.
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