Cold dielectric at LIPA

In the autumn of last year, the US Department of Energy (DOE) announced the next round of projects chosen for funding under its superconductivity partnership initiative with industry, the aim of which is to accelerate the commercial introduction of HTS (high temperature superconductor) technology.

One of the projects involves a proposal to demonstrate what is seen as the next step in developing HTS power cables: a pre-commercial, 77 MVA, half-mile underground superconducting link.

The flexible HTS cable will be installed in the LIPA power grid near East Shoreham on Long Island, close to Brookhaven National Laboratory. It will run in parallel with an existing line.

The $18 million LIPA installation will be the longest HTS cable to date. It will also be the first coaxial (cold dielectric) design to be manufactured and tested in the United States.

In addition to host-utility LIPA, and the DOE, co-sponsors of this project include Pirelli Cables & Systems, American Superconductor Corporation, EPRI and Air Liquide.

Details of the demonstration project and the contracts between the parties are currently being worked out, with a view to having the cable in operation in 2004.

In addition to single-core (warm dielectric) HTS cables, such as those installed at the delayed Detroit Edison Frisbie substation project (see panel opposite), Pirelli has already manufactured 150 ft long (45 m) coaxial (cold dielectric) cables that are expected to begin operating at utility test facilities in France in 2002. The Long Island pre-commercial cable will build on this experience.

In the cold dielectric design, two concentric HTS conductors are used per phase, the phase and the return conductor, separated by insulation. The entire conductor assembly, including insulation and central liquid nitrogen duct, operates in the cryogenic environment (ie is inside the cryostat, which is essentially an annular vacuum chamber or elongated thermos bottle, providing thermal insulation).

In the warm dielectric configuration a single HTS conductor (with central nitrogen duct) runs inside the cryostat. The electrical insulation is then applied to the cryostat itself.

Although the single-core, warm dielectric, type may prove best for retrofitting certain existing duct or pipe systems, the coaxial, cold dielectric, design promises the greatest level of transmission capacity and efficiency. This efficiency gain is due to the presence of a return current in the coaxial return conductor that is equal and opposite to the current in the phase conductor, completely shielding the magnetic field generated by the current in the phase conductor, and reducing losses associated with the field. The amount of energy that the cryogenic system must remove is therefore reduced.

The cold dielectric system potentially opens up the possibility that underground cables could have the power carrying capabilities that today are only available with overhead transmission lines.

Load growth

LIPA's interest in the HTS cable demonstration stemmed from its goal to be prepared to deploy HTS cable systems broadly throughout its network to address problems due to continuing load growth. Some areas of its service territory already boast load densities of up to 3.73 MW per square mile. The utility projects that eastern Suffolk County will experience 62 per cent load growth over the next 20 years, with overall projected load growth of 27 per cent.

HTS cables may offer the possibility of accommodating such significant growth while minimising disruption and aesthetic impact.

To transmit electricity from new generating plants planned in the area, LIPA had been looking at the development of a new 345 kV transmission system. As a result of permitting restrictions on Long Island for overhead transmission, the utility expected that the entire 345 kV system would be underground cable and require the construction of 345 kV/138 kV step-down transformer substations to connect with the existing transmission grid.

However, the availability of HTS cables capable of transmitting large amounts of power at 138 kV could greatly simplify the system design by avoiding new 345 kV substations and the associated equipment.

Paul Grant, a research physicist and EPRI science fellow for strategic science and technology, said that the Long Island project proposal to DOE was formed on the basis of discussion of applications with a number of large utilities about criteria for demonstrating a pre-commercial HTS cable system. "The Long Island cable proposal pushes the envelope on the two most critical fronts in achieving market penetration of HTS technology," says Grant. "First, the selected application is a commercial-length, cold dielectric coaxial design that will provide broad operational and economic benefits by enabling bulk power transmission through congested rights of way and suburban corridors. And it will be the first flexible coaxial cable specifically designed for pipe-type retrofit in the United States.

"Second, the proposal calls for demonstrating an innovative liquid nitrogen refrigeration technology developed by Air Liquide that promises to reduce capital costs, enable implicit standardisation, and minimise maintenance requirements," Grant adds. He described the Long Island cable project as "the next step on the path to utility deployment of superconducting cable technology - this effort will literally take HTS to the street."

Other HTS power applications

As well as cables, the projects chosen by DOE in the round of funding it announced in autumn 2001 include generators, transformers, and magnetic separators for mineral and chemical processing. The co-sponsoring organisations will receive about $57 million in DOE support, over three to four years, which the teams will match with an estimated $60 million.

In addition to the LIPA cable, other power projects announced included the following:

• GE will lead a team that will design and develop a 100 MVA superconducting generator. New York State Energy Research and Development Authority, PG&E National Energy Group, American Electric Power (AEP), and Praxair are also part of the team for this $26 million effort.

• A team led by cable manufacturer Southwire will provide a three-phase HTS power cable at a Columbus, Ohio, substation of AEP. The project team also includes Nordic Superconductor Technologies, 3M Company, Integration Concepts Enterprises, and Oak Ridge National Laboratory. The planned 1000 ft (304 m) HTS cable will replace an existing, oil-filled underground cable at AEP's Bixby Road substation that has limited current-carrying capacity. This estimated cost is $18 million, to be evenly shared between DOE and the project team.

• A team led by Intermagnetics General-Superpower, an HTS wire manufacturer, Waukesha Electric Systems, Southern California Edison (SCE), and Air Products will develop and demonstrate a prototype, utility scale HTS transformer to convert electricity from 66 kV to 12 kV at the end of a typical transmission system. The project is valued at about $31 million.

• A proposed superconducting flywheel power risk management system is to be developed by a team led by Boeing Phantom Works in a $14.7 million effort. Praxair Specialty Ceramics, Ashman Technologies, Mesoscopic Devices' Boulder Cryogenics subsidiary, SCE, and Argonne National Laboratory will collaborate on the 35 kWh flywheel system, which will feature low-loss, superconducting magnetic bearings to produce an efficient storage system that manages both power cost and reliability risks. The system could provide emissions-free, silent, and efficient storage and generation for distributed energy and power quality.

All-superconducting substation

Meanwhile, a new EPRI project is supporting the construction of a demonstration facility to investigate the costs and benefits of deploying HTS components into an all-superconducting substation.

Superconducting magnetic energy storage (SMES) has already been identified as a potentially beneficial technology for the enhancement of power delivery systems, both for transmission stability and distribution power quality. Beyond SMES, however, the introduction of other superconducting components (such as cables, transformers, and current limiters) into a substation is expected to yield benefits in extended life, lower maintenance costs, increased power throughput, and reduced space requirements. EPRI has identified significant cost savings for the cryogenic subsystems when multiple superconducting components are co-located (eg, at a substation).

A preliminary EPRI study in 2000 estimated that a superconducting substation would require only 75 per cent of the area and 50 per cent of the maintenance of a conventional substation, and would offer up to three times the power-handling capacity of a conventional substation of the same area. Other advantages projected in the study include fewer mechanical systems, insensitivity to environmental variations such as elevated temperatures, and fewer exposed bushings (and thus fewer arcs and other failures associated with environmental conditions).

The new EPRI project will support construction of a demonstration SMES device at the Center for Advanced Power Systems (CAPS), now under construction at Florida State University in Tallahassee under a combination of funding grants from the US Department of Defense and the National Science Foundation. The SMES will be integrated into the power system feeding both the Center and the National High Magnetic Field Laboratory (NHMFL) and will eventually become part of an all-superconducting substation proposed at CAPS.
Tables

High temperature superconductor projects around the world