SUPERCONDUCTING POWER TRANSMISSION CABLES -- OVERVIEW

Although the energy crisis of the 1970s is now past and demand has increased considerably, the motivation for developing HTS power transmission cables comes primarily from the need to increase the power-handling capabilities of existing underground circuits, which are filled to capacity. HTS cables not only offer a doubling of the power per circuit, they also provide an environmentally attractive solution, because a leak in an underground HTS system would cause the benign release of nitrogen, whereas a leak in existing oil-filled high voltage cables could result in devastating soil contamination. Where oil-filled cables are used underwater, such leaks could produce even greater environmental damage.

Upgrading a power system by retrofitting existing ducts with HTS cables is most likely to occur in dense urban areas where the costs of trenching to install higher-capacity conventional systems would be prohibitive. In Tokyo, for instance, where demand for electric power is increasing at a rate of 2-3% per year, use of HTS cables is attractive since space is extremely limited and most underground ducts are filled to capacity. The opportunity in Tokyo alone provides a tremendous development incentive. There are ten large cable tunnels in Tokyo, each 20 km long and each containing three cables. If these cables were replaced with HTS cables at the rate of only one of the three cables in several tunnels each year, the project would require 600 km of cable and last ten years. The HTS conductor alone needed for such a venture would exceed 100 million meters and represent a business opportunity of several billion dollars. And if the relative economic value of the joints and terminations required for the cable follows today's pattern, then the business opportunity for these cryogenic components is at least ten times greater than that of the conductor business itself.

Development of LTS cables and cable concepts in the 1960s was pursued by industrial giants like Siemens, GE-France, BICC, and Westinghouse, and by several academic and government laboratories, including important contributions from the Technical University of Graz, Austria, and Brookhaven National Laboratory (BNL) in the United States (Giese 1993). In Japan, members of the MITI's Electrotechnical Laboratory carried out an economic study and concluded that superconducting cables were especially attractive for high power dc applications. Early testbeds used LTS materials in a variety of configurations.

In Germany, Linde studied the ac loss characteristics of rigid Nb tubes and built a 7-meter-long cryostat to measure these losses. Later the Linde team proposed a composite conductor of Nb, copper, and invar. In a collaborative program between the Technical University of Graz, AEG, Kabelmetal, and Linde (Munich), the superconductor was formed by coating the inner and outer walls of concentric corrugated tubes with a layer of Nb so that the layer on the outside of the smaller tube faced the Nb layer on the inside of the larger tube.

The BNL project employed Nb₃Sn superconducting tapes, and for the demonstration cable Intermagnetics General Corporation (IGC) manufactured a composite tape that had layers of copper, Nb₃Sn, Nb, and stainless steel. This work resulted in design of a 1,000 MVA, 138 kV, 4,200 amp system and in a preliminary solution to the problem of terminations. Progress in phase two of this work went well; however, the project was terminated for economic reasons.