By Paul Grad, engineering writer
Why have superconducting transmission lines not yet come into general use, despite their huge advantages compared with standard conductors made of copper or aluminium? One of the reasons is the cost of installing long-distance superconducting transmission lines.
Despite the increasing number of demonstration projects, the awareness or acceptance of superconducting lines as a mature technology among policy and decision-makers is small. Superconducting lines are more complex than standard lines. They rely on a fluid at cryogenic temperatures. The fluid cannot be allowed to transition into the gas phase and because the cooling system is powered by electricity, it requires an absolutely reliable power source. For longer-distance field installation, the cryogenic envelope and cooling system, and the joints connecting the various cable segments represent the main technical challenges.
There are many installations of short-distance superconducting lines in many countries, including the United States, Russia, China, Japan, South Korea, Germany, and the Netherlands.
Many companies manufacture superconducting cables and wires, including American Superconductor (AMSC), Massachusetts, US; Sumitomo Electric Industries Ltd, Osaka, Japan; SuperPower Inc, New York, US; Nexans SA, Paris, France; Southwire Company, LLC, Georgia, US; Superconductor Technologies, California, US; Brucker HTS, Alzenau, Germany; and Siemens AG, of Berlin and Munich, Germany.
A three-way partnership has been established between Siemens, Defence Science and Technology Group, and Queensland University of Technology, to share high-temperature superconductor technology and research. The partnership will look into all kinds of potential applications of HTS technology, including the reduction of size and weight of large-scale motors, the levitation of trains, more efficient electric motors for cars, and remote power generation.
High-temperature superconductor wire (HTS) conducts about 200 times the electrical current of copper wire of similar dimensions with practically zero power dissipation. Cables that utilise HTS wire can transmit up to 10 times more power than conventional cables or can carry equivalent power at much lower voltages. They also have an inherent fault current limiting capability.
Apart from the fact that with superconductors there are no resistive losses, the very high current densities in superconductors allow for much smaller dimensions of the conductor and cable compared with standard conductors.
Superconducting power cables are therefore especially suited to high-load areas, such as large cities or dense business districts, where purchases of easements and construction costs for traditional low-capacity cables may be cost-prohibitive.
There are various types of HTS. The most commonly used materials in early HTS were bismuth-based. These materials were known as first generation (1G) superconductors.
Second generation superconductors (2G) are based on rare earth barium copper oxide elements (YBCO), with variations in width, substrate thickness, silver and copper stabilizer thickness, plus optional insulation. These power cables typically operate at the temperature of liquid nitrogen (77K).
Another material, based on a simple compound – magnesium diboride, MgB2 – has shown great promise. The material’s critical temperature is 39K, which requires a coolant of either liquid hydrogen or helium gas, but even so, experiments have shown it would be cheaper to use the material in power cables to transport energy over long distances. Experiments have demonstrated the transmission of 3.2GW – equivalent to the output of three large power stations – at a voltage of 200-320kV and a DC current of up to 10,000A, through a superconducting cable 12.5mm across.
Recently, a reformulation of MgB2 to BMg2Ox increased its critical temperature from 39K to 85K.
According to Superconductor Technologies, there are several configurations used to construct a superconducting power cable.
In the concentric configuration, three phases are wound concentrically on a single inner copper wire. This cable design integrates each AC phase into a single cable. Southwire markets this design, under the trade name Triax, but other cable manufacturers, such as Nexans, also use concentric designs.
The Triax design uses about half the superconducting wire as single-phase cable cores. Single-phase cable cores require HTS wires to be used for the inner phase conductor, as well as the outer concentric neutral for each phase. Placing the three phases centrally allows the electric fields, which are 120 degrees apart, to self-cancel. This eliminates the need for using superconducting material in the cable’s concentric neutral conductor and therefore reduces HTS material consumption by half.
A second cable design consists of three separate, single-phase, cables inside a single cryogenic envelope. A third design uses three individual, single-phase cables, each encapsulated in an individual cryogenic envelope.
The pioneer installation of a superconducting transmission line as an integral part of a grid was at the Frisbee substation in Detroit, in 2001, by AMSC. Since then, many installations using HTS wire has appeared around the world, all involving short-distance power transmission.
In a pilot project called AmpaCity, the longest superconductor cable integrated into an existing power grid was officially opened on April 30, 2014, in the German city of Essen, by the RWE AG utility. The cable, manufactured by Nexans, is 1km long and connects two substations in Essen’s city centre. The AmpaCity has been operating reliably since early 2014. If successful in the longer term, it can lead to retrofitting 30km of standard technology transmission lines in Essen.
Professor Joachim Knebel, of the Karlsruhe Institute of Technology, which is one of the organisations participating in the project, says the project is an important milestone in the long-term R&D of superconducting grid components. The fundamental change the electricity grid will be going through as the integration of renewable energies continues, is a motivation and challenge for us to continue contributing with innovative, superconducting solutions to a reliable, stable and efficient grid in the future, too.
An HTS DC transmission line, now in construction in St Petersburg, Russia, will be 2.5km long, surpassing the Essen installation in length. It will connect two substations – the 330kV Centralnya and the 220kV RP-9.
In the Long Island Power Authority’s (LIPA’s) Holbrook Superconductor Project at New York’s Long Island, a substation is fed by a 600m-long tunnel containing about 150km of HTS wire of bismuth strontium calcium copper oxide (BSCCO), manufactured by AMSC, installed underground and cooled with liquid nitrogen. The LIPA cable has been operating in the grid since 2008 with a nominal capacity of 574MW.
The Tres Amigas SuperStation is a planned project to unite North America’s two major power grids to allow faster adoption of renewable energy and improve the reliability of the US grid. The project will use HTS wire supplied by AMSC. The project proposes to tie the East Coast, West Coast and Texas grids together via three 5GW superconducting high-voltage DC power transmission lines, which will permit a controlled flow of energy, while also isolating the independent AC frequencies on each side. The project will provide solar, wind and other forms of renewable energy with suitable transmission infrastructure. The superstation will be located on a 57sq km plot of land near Clovis, New Mexico.
A Japanese project, operated by TEPCO (Tokyo Electric Power Company), will install a 300MVA three-in-one HTS cable at the Asahi substation near Tokyo. The 240m cable with an HTS wire made of DI-BSCCO is cooled with liquid nitrogen. It was supplied by Sumitomo Electric Industries.
Two demonstration sites of the Korea Electric Power Corporation (KEPCO) with high-temperature superconductor cables are in operation at Jeju Island, South Korea. One is a high-voltage direct current (HVDC) 500m, 80kV DC cable, the other is a 1000m, 154kV, AC cable. Both use AMSC’s Amperium HTS wire.
In 2011, KEPCO, LS Cable & System, and AMSC energized a 22.9kV AC cable system at the Incheon substation, near Seoul. The cable operated successfully for two years. At the time of installation, it was the longest distribution voltage superconductor power cable in operation.
The crucial factor in the wider utilisation of superconducting transmission lines is cost. Until HTS becomes economically competitive, MgB2-based superconducting transmission lines will see increased interest. If superconducting lines became economically competitive with standard lines, they could replace vast fractions of the existing medium- and high-voltage grid. The whole grid could eventually be changed into a superconducting grid, rendering high-voltage up and down transformers unnecessary with a direct power plant to city connection at the turbine output voltage of 10kV to 30kV.