After many years of promise and disappointment, high temperature superconductor (HTS) power transmission has arrived. PEi was on Long Island in the northeatern United States for the commissoning of the longest HTS link in the world.

Tim Probert, Deputy Editor

The joyous back-slapping at the opening ceremony told its own story – a major milestone in power engineering had been reached. More than 22 years in the making, Greg Yurek’s dream of utilizing high temperature superconductor (HTS) technology in a power transmission cable in a commericial power grid had finally come true.

Interest in the field of superconducting power cable dates back to the 1960s, but because conventional metallic superconductors required cooling with liquid helium, these cable system designs were unduly complex and cost prohibitive. Interest in the field, however, was renewed following the discovery in 1986 at the Massachussets Institute of Technology (MIT) of ceramic materials with high temperature superconducting capabilities, which enabled the use of liquid nitrogen as a cooling medium at about -200 °C.

The completed HTS link at the Holbrook substation end with three phases emerging from the ground to connect to the HV terminations (the vertical parts)
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Yurek founded American Superconductor Corporation (AMSC) in April 1987, with three fellow MIT professors, a year after the discovery of these ceramic materials. The challenge for Yurek and AMSC to commericalize these problematic materials into flexible power cables has proven a tough one indeed but, on 25 June 2008, he celebrated the commissioning of the world’s first transmission voltage HTS system at Long Island, New York, USA.

The HTS cable system installed in the Long Island Power Authority’s (LIPA) power grid contains hair-thin, ribbon-shaped HTS wires that conduct 150 times the electricity of similar sized copper wires. This power density advantage enables transmission voltage HTS cables to utilize far less wire and yet conduct up to five times more power – in a smaller right-of-way – than traditional copper-based cables. The 138 kV, 600 m power link, comprising three HTS cable phases running in parallel was energized on 22 April 2008 and is operating successfully in LIPA’s Holbrook transmission right-of-way. When operated at full capacity the HTS cable system is capable of transmitting up to 574 MW of electricity – enough to power 300 000 homes.

The main project objectives were twofold. The first one was to identify the key issues and concerns that needed to be addressed to enable HTS cables to be operated effectively in a power grid at transmission voltages. The second was to develop, design, produce and operate this HTS cable system. The Department of Energy (DOE) previously funded $27.5 million of the $58.5 million total project cost, which advances the DOE’s ongoing efforts, through the Office of Electricity Delivery and Energy Reliability, to modernize the US electricity delivery infrastructure. HTS power cables are envisioned by the DOE as a component of a modern electricity superhighway – one that is free of bottlenecks and can readily transmit power to customers from remote generation sites, such as wind farms.

Project success down to expertise

LIPA’s installation, which is the longest and most powerful superconductor cable system in the world, includes three phases connected through six outdoor terminations. It was designed, manufactured and installed by French cable manufacturer Nexans. The cable cores utilize HTS wires produced by AMSC.

The liquid nitrogen refrigeration system was manufactured by Air Liquide. Three 600 m long vacuum-insulated flexible cryostats provide high quality thermal insulation maintaining the cable cores at cryogenic temperature.

When Nexans was contracted for the Long Island project, the Group decided to make full use of its technological assets and industrial tools. Nexans is a world leader in flexible cryogenic envelopes. This expertise was originally dedicated to the transfer of liquefied gas (Cryoflex® product). The company subsequently adapted it to the world of superconducting cables.

As host utility, LIPA provided the site engineering and preparation, as well as guidance on the design and testing of the cable system. AMSC, as the prime contactor of the project, provided project management, technical input, as well as the HTS wire for the cable.

After an initial operational period, subsequent performance and economic assessment of the cable system, LIPA plans to retain the new superconductor cable as a permanent part of its grid and eventually install HTS cables elsewhere to address the growing power needs of Long Island.

Reducing future grid upgrade costs

HTS cables can carry three-to-five times more power than conventional copper cables of a similar size. Thermally-independent, compact HTS cables can be installed into existing rights-of-way, thus helping to reduce the cost and environmental impact of future grid upgrades. With much lower impedance and resistance than conventional technology, superconductor cables can be strategically placed in the electric grid to draw flow away from overtaxed conventional cables, or overhead lines, thereby relieving network congestions.

The trench shows the reduced right of way needed for HTS compared with the conventional overhead cables on the left
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Nexan’s cable design employs concentric layers of HTS wire and a dielectric material, providing electrical insulation, compatible with cryogenic temperatures. Liquid nitrogen coolant flows over and between both layers of wire, providing cooling and contributing to the dielectric insulation between the center conductor layer and the outer shield layer. As the dielectric material remains at about -200 °C, this cable architecture is commonly referred to as a coaxial, ‘cold-dielectric’ design. Such cables offer several important advantages, including higher current carrying capacity, reduced AC losses, low inductance and the complete suppression of stray electromagnetic fields outside of the cable assembly. The reduction of AC losses enables wider spacing of cooling stations and the auxiliary power equipment required to assure their reliable operation.

Recently published research on several cable development programmes and reliability issues highlights the dramatically lower impedance of coaxial, cold-dielectric cables. Impedance in an electrical transmission circuit determines the power flow division among many cables connected in parallel. Power flow in a circuit is inversely proportional to its impedance. Thus, other factors (applied voltage and phase angle) being equal, a coaxial HTS cable will carry more load than a conventional cable connected in parallel to the same two points on the grid. The inductance of cold-dielectric cable is up to six times lower than that of conventional cable, and 20 times lower than an overhead line of the same voltage.

next generation technology

The commissioning of Holbrook is just the first phase of what will eventually be a three-phase project. Already the LIPA Phase II extension is well underway. The second phase of the LIPA HTS project involves the same partners. Yarek says the introduction of second generation (2G) HTS tapes, which are designed to be significantly cheaper than the first generation HTS conductors used in the initial project, will lead to a more cost effective cable system – an important step towards commercialization of HTS power cable technology.

LIPA’s existing HTS cable is 600 m in length. As future multi-km power links are expected to be composed of cable sections of similar length, the second phase also encompasses the development and demonstration of a suitable cable joint. The new project calls for the replacement of one of the existing HTS cable system’s phases with a 600 m long cable made with AMSC’s proprietary 344 superconductors, the company’s brand name for 2G HTS wire. The cable system will also incorporate Secure Super Grids technology.

Introduced by AMSC in May 2007, Secure Super Grids is a system-level solution that uses customized 2G HTS wires, HTS power cables and ancillary controls to deliver more power through the grid and suppress power surges that can disrupt service. AMSC is leading a separate, parallel project to demonstrate and deploy the first distribution voltage (13 kV) Secure Super Grid solution in the power network of Consolidated Edison in midtown Manhattan.

A HTS cable phase is unspooled from the drum on which it was delivered and pulled into position in the cable duct
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“The response to our Secure Super Grids solution since its launch in May 2007 has been tremendous,” said Yurek. “Utilities worldwide are seeking ways to relieve choke points and instantly suppress power surges in their grids, and Secure Super Grids accomplishes both goals simultaneously.”

Secure Super Grids utilize multiple paths for electricity flow in metropolitan power grids to ensure system reliability when individual circuits are disrupted because of severe weather, traffic accidents or willful destruction. In addition, they use the special properties of AMSC’s 344 superconductors to not only relieve grid congestion, but also instantly suppress power surges that often damage utility equipment and disrupt customer service.

In addition to being the manager for this turnkey project, AMSC will supply approximately 60 000 m of its 344 superconductors, needed to manufacture the power cable. As was the case in the original LIPA cable project, AMSC has chosen Nexans as the cable manufacturer and Air Liquide as the provider of the cryogenics system. Apart from being the world’s first transmission-voltage cable system powered by 2G HTS wire, AMSC and its project partners will be developing new repairable cryostat and cable joining technology and a low cost, reliable and efficient refrigeration system.

The DOE, through its National Energy Technology Laboratory, is expected to provide AMSC with $4 million in federal funding through completion of its first project budget period, expected to end in September 2008. Upon successful completion of key project milestones and sustained execution of a viable business strategy, as much as $5 million in additional DOE funding may be made available for continued implementation of this two and a-half-year project through March 2010, subject to availability of funds appropriated by the US Congress. By 2015 it is expected that Phase III will be completed, a 6.5 mile (10.5 km) HTS link between two LIPA substations – Port Jefferson and Holbrook.

At present it far from certain that HTS power transmission cables will become a common part of a transmission system designer’s repertoire. The utilization of 2G cables will undoubtedly bring down costs, but the high cost of cryostats – at around $7.8 million – which need to placed at regular intervals mean that it could be some time before HTS systems will become economically competitive.

It may be that they remain as a niche product, harnessing the unique ability of superconductor cables to deliver large amounts of power through small corridors that offer a key solution for congested metropolitan power grids such as New York City or even Shanghai in China. The success of Phase I of this project, however, shows that Nexans are ready to deploy HTS cable technology more broadly in utility power grids.