Think tank assesses electricity transmission design
Underground superconducting DC cable system would boost efficiency, safety
BY ROSE RAGSDALE FOR GREENING OF OIL
Moving huge quantities of electricity over vast distances could become easier over the next decade if utilities adopt a new type of power transmission system using DC, or direct current, and superconducting cables, according to the Electric Power Research Institute.
The institute describes the design of a superconducting direct current cable system capable of moving thousands of megawatts of electricity between regions in a 252-page report published March 3.
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The proposal is timely in light of the U.S. government’s focus on upgrading the nation’s outdated electric transmission grids and distribution systems by 2030 with a network of powerful superconducting cables. The move comes in response to increasingly frequent power failures and other power supply-related problems that have plagued the country’s power systems in recent years.
EPRI is a research and development association with members representing more than 90 percent of the electricity generated and delivered in the United States, and international participation from 40 countries. It said the report is aimed at engineers and researchers involved in designing new power transmission systems.
The two companion superconducting reports, 144 and 88 pages, respectively, highlight the practical issues of integrating a long-distance, high-power superconducting DC link into existing, lower-power AC transmission and distribution systems, and states that the operation and control of this link will be critical to the viability and acceptance of the concept.
Enhance safety, reliability, efficiency of existing AC grid
Long-distance, high-capacity power transmission circuits operating at power levels possibly around 10,000 megawatts per circuit might be needed to bring power from remote sources such as large wind farms to urban load centers, according to the report.
“In the future we may see the development of generation facilities, such as large wind farms or nuclear “farms” capable of producing five to 10 gigawatts, but located far from urban centers of demand. It will then be necessary to move large amounts of power over long distances,” said Arshad Mansoor, vice president of Power Delivery and Utilization for EPRI.
As designed, the superconducting cable system outlined in the report would provide 10 GW power capacity with a nominal current and voltage of 100 kiloamps and 100 kilovolts.
The report also points to the cable system’s potential to enhance the safety, reliability and efficiency of the existing AC power grid.
More power for end users
Ultrahigh-voltage AC or DC transmission lines would be the conventional solution. However, using superconducting cables and underground power with superconducting DC cables could be viable alternatives to high-voltage overhead lines in the near future. If the cables are interfaced with voltage source converters, it should be possible to connect many converters to the cable, all along the cable route, which removes a frequent barrier to application of high-voltage DC for transmission applications.
Such a system is practical and ready for commercial development, using today’s technology, the institute said. Its analysis points to significant efficiency gains using superconducting DC transmission lines, with the capability to reduce transmission losses at full load by more than 50 percent compared to AC, or alternating current, or high-voltage DC systems.
The improvement in efficiency means utilities would see more of the power that they transmit actually reach end users, said Steve Eckroad, program manager of Underground Transmission Research for EPRI.
Further, the report indicates that builders of superconducting DC transmission lines could rely on commercially available technology and construction methods similar to those used in natural gas pipeline construction. These include factory manufactured, transportable sections of an outer carbon steel pipe containing inner stainless steel piping for the flowing coolant and superconducting cable, and trucking to the site for assembly, welding and burial.
New system for Seoul
Eckroad said EPRI’s research took into account ground-breaking demonstrations and projects under way around the world.
“Our research was and is well aware of other demonstrations,” he said. “Lessons learned from those projects that are relevant to our design were incorporated (in the analysis).”
South Korea, for example, is gearing up to install what may be the world’s longest distribution voltage superconductor cable system near Seoul by mid-2010. In April, that nation’s largest power cable manufacturer, LS Cable, ordered about 50 miles of 344 superconductors – American Superconductor Corp.’s second-generation high-temperature superconductor wire. It plans to strand that wire into a 22.9-kV cable system as part of Korea Electric Power Corp.’s commercial power delivery network. The proposed cable system will be nearly half a mile in length and capable of carrying 50 MW.
“As is the case in many countries around the world, Korea has been experiencing sharp increases in electricity demand and is relying more and more on renewable energy sources to meet that demand,” KEPCO CEO Kim Ssang-Su said in announcing the project in 2009. “With their ability to carry a vast amount of power in a small pathway, we see great promise for superconductor cable technology and look forward to energizing this first system.
A power cable made with high-temperature superconductor wire inside can conduct up to 10 times the amount of power of the same-diameter cable made with copper wire. By replacing copper cables with high-capacity superconductor cables in cities using existing underground tunnels and ductwork, utilities can avoid digging up city streets while also relieving grid congestion and increasing the reliability and security of power networks.
“However, those projects are AC (alternating current) whereas our line is direct current (DC), which introduces significant changes in some aspects of the design,” Eckroad said. ”Nevertheless, the development of a DC cable is a natural progression of the ongoing developments in AC superconducting cable.”
Demand would boost capacity
Moreover, production capacity of superconducting wire today is limited but, given substantial demand capacity, could possibly be expanded sufficiently for longer lines. Refrigeration and vacuum requirements of the line might be met by equipment and methods utilized in the industrial gas industry, EPRI said.
Lighter, thinner, higher-capacity superconducting cable might be fabricated, shipped and installed with methods and equipment now used for conventional underground transmission cable.
Assuming the current trend in cost-performance improvements in superconducting wire continues, EPRI said a superconducting DC transmission line could become an option within a decade, along with the extra high voltage AC lines that are currently being used to move large amounts of power over long distances.
Links of interest
Electric Power Research Institute
Program on Technology Innovation: A Superconducting DC Cable
Program on Technology Innovation: Study on the Integration of High Temperature Superconducting DC Cables Within the Eastern and Western North American Power Grids
Program on Technology Innovation: Transient Response of a Superconducting DC Long Length Cable System Using Voltage Source Converters
DOE’s Office of Electricity Delivery & Energy Reliability
American Superconductor
LS Cable
Sumitomo Electric
Contact Rose Ragsdale at editor@miningnewsnorth.com
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