Successfully Operating 200 kV Superconducting HVDC Transmission Highlights Reliability, Efficiency

Sponsored by

Jean-Maxime Saugrain, Nexans and Jack McCall, American Superconductor

Already overtaxed metropolitan power grids around the world are being challenged to meet the rising electricity demands associated with today’s growing, digitally based economy. As a result, the threat of power outages is ever-present, particularly in dense, urban areas where massive amounts of power must be carried through congested, underground ducts.

Effort must be made to increase the bulk electricity system’s reliability, and new commercial technologies must be used. High-temperature superconductors (HTS) in high-voltage direct current (HVDC) applications are among the most promising, cost-effective solutions.

The 138 kV AC superconductor power transmission cable has been operating since April 2008 in Long Island Power Authority’s grid. Courtesy of American Superconductor.

Nexans, at its high-voltage laboratory in Hanover, Germany, completed in July the first successful demonstration of an HVDC HTS cable operating at 200 kV DC.

This prototype cable, together with new terminations designed for this application, passed tests based on International Council on Large Electric Systems (CIGRE) recommendations. Researchers proved the design, operating at nominal ratings and over voltages as high as 360 kV, represent 1.8 times the nominal rating. The superimposed overvoltage tests, designed to simulate lightning or switching events, were successful.

Passing the tests is significant for the 200kV DC terminations because they are different than those used in previous AC systems. The next step for Nexans is to adapt this HTS cable system to the high currents required to transfer several gigawatts of power. DC terminals enable real-time power flow control. In addition to improving the overall stability of the AC grid, the ability to change power flow direction quickly provides opportunities to enhance the operation, reliability and economics of the grid. Nexans will also develop suitable joints to enable the installation of long lengths of HTS cable as well as to carry out repairs in the field.

Excellent Near-term Market Potential

Operating a single, synchronous alternating current (AC) power grid on a continental scale is not practical. For example, in the case of the U.S., the electrical grid operates as three self-contained, large-scale power grids, or interconnection areas. HVDC is the primary way to interconnect these asynchronous grids. Using HTS cabling with HVDC terminals is an ideal technology blend to transmit large amounts of power over long distances. New merchant transmission company Tres Amigas LLC is planning a Clovis, N.M., SuperStation that will use superconducting HVDC to interconnect America’s three electrical islands.

In transmission and distribution applications, the HTS industry has advanced beyond full-scale power equipment prototypes to demonstration projects—in New York City; Long Island, N.Y.; Columbus, Ohio; Tokyo; Seoul, South Korea; and Moscow, for example—that are undergoing in-grid evaluation. HTS is a permanent, working part of the grid.

For example, since 2008 an HTS transmission cable manufactured and installed by Nexans has been operating at 138 kV in the Long Island Power Authority’s (LIPA) AC grid. This cable system is constructed of HTS wire produced by American Superconductor (AMSC). It can conduct 150 times the electrical current of similarly sized copper wire. The cable is part of LIPA’s primary transmission corridor. At full capacity it transmits up to 574 MW, or enough power for 300,000 homes.

In 2006, National Grid and American Electric Power (AEP) energized distribution voltage-level superconductor cable systems in their commercial power grids in Albany, N.Y., and Columbus, Ohio, respectively.

AMSC’s high-temperature superconductor (HTS) wire is used in the construction of superconductor power cables. AMSC’s HTS wire can transmit 150 times more power than similarly sized copper wire. Courtesy of American Superconductor.

The success of the Long Island HTS installation led LIPA to undertake an extension of the 138 kV system that will be manufactured by Nexans using AMSC’s second-generation (2G) HTS wire. The Department of Energy is providing up to $9 million in cost sharing for the $25 million Phase II project.

Ultralow-loss transmission of electricity is not the only operational benefit of HTS cabling in transmission and distribution applications. In AC applications, HTS power cables have low impedance, which means they can draw power flow away from overtaxed conventional cables or overhead lines, thereby relieving network congestion. They also can be designed to have fault current limiting characteristics and, when deployed in strategic locations, these types of superconductor cables rapidly can absorb additional power flows when conventional power grid components are damaged during electrical storms or other events. The fault current limiting HTS cables automatically suppress damaging power surges to create resilient, self-healing grids that can survive attacks and natural disasters. It makes them an ideal modernization tool for metropolitan power grids. They are expected to become core components of intelligent, more secure power networks by functioning, as needed, as fault current limiters (FCL).

HTS cabling success at LIPA and sites around the world and the announcement of the Tres Amigas SuperStation signify a turning point in grid modernization. The SuperStation will interconnect U.S. Eastern, Western and Texas, officially the Electric Reliability Council of Texas (ERCOT) grids, creating a national network for power marketing. This landmark project will be accomplished using superconductor HVDC cables and the most advanced HVDC terminals employing voltage source converter (VSC) technology.

While an electrical connection among the three grids generally requires DC power links to properly synchronize power flow among the regions, AC-to-DC converters must be used to convert AC power from one region to DC power and then to invert the power back to AC in the adjacent region. The Tres Amigas project will consist of three multigigawatt VSC-based DC terminals; one for each grid interconnection. A triangular electric circuit with about 10 km of HVDC superconductor cable will tie the three terminals together, enabling the transfer and balancing of many gigawatts of renewable and conventional power. The DC terminals enable real-time power flow control. In addition to improving the overall stability of the AC grid, the ability to change power flow direction quickly provides opportunities to enhance the grid’s operation, reliability and economics.

The watershed nature of this application of HTS in the SuperStation is seen, too, because this interconnection will allow renewables producers, which are abundant in the southwestern U.S., to bring power to market across the much larger national grid.

Superconductor cables offer a number of advantages in the creation of stronger, more reliable, and more efficient power grids.

Saugrain is managing director of the superconductivity activity at Nexans. McCall is director of business development, HTS T&D systems, at American Superconductor.

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