Special section: Powered up - Superconducting power cables

Materials World magazine
1 Oct 2012

Superconducting power cables can carry 100 times more electricity than copper, but costs can be prohibitive. Tim Probert explores whether this radical technology can ever gain market traction in a conservative industry.     

A superconductor is a material that conducts electric current with virtually no resistance when cooled to very low temperatures. At around -200°C, the superconducting material is transformed into an almost perfect electrical conductor able to transport 100 times more current than copper. These properties make superconductors very attractive to power cable manufacturers, but progress has been slow since 1911, when Dutch physicist Heike Kamerlingh Onnes discovered that the electrical resistance of a solid mercury wire immersed in liquid helium suddenly vanished at a temperature of -269°C, close to absolute zero.   

Research into superconducting power cables only really got going in the 1960s, but because conventional metallic superconductors required cooling with liquid helium, these cable system designs were too complex and cost-prohibitive. The turning point came in 1986 when researchers Johannes Georg Bednorz and Karl Alexander Müller at IBM’s research laboratory in Zurich discovered that the electrical resistance of a material made from lanthanum, barium, copper and oxygen (LaBaCuO) fell abruptly to zero when cooled below a temperature of -238°C.   

Suddenly, the game changed. The discovery won Bednorz and Müller the 1987 Nobel Prize for Physics and, alive to the possibilities of high temperature superconductors (HTS), work quickly began on new materials. A year later a material containing bismuth, strontium, calcium, copper and oxygen (BSCCO) that superconducts at about -168°C was developed. BSCCO was adopted by American Superconductors (AMSC), which was founded in 1987 by the then Massachusetts Institute of Technology’s Professor of the Department of Materials Science and Engineering, Greg Yurek, in response to the breakthrough. By 1995, AMSC had developed a first generation superconducting wire made by packing ceramic powders of BSCCO into silver tubes. The packed powder was extracted and rolled into a flat tape, which was heated to make it suitable for winding cables or coils for transformers, magnets, motors and generators.   

Commercial potential   
To make a superconducting cable, the tape is wrapped around a copper core, surrounding which are various levels of electrical shielding. The cables are then sheathed with thermal insulation allowing liquid nitrogen to pass through, thus cooling the HTS tape. The challenge for AMSC to commercialise these problematic materials into flexible power cables has proven tough, and uptake in the power industry has been extremely slow, mostly due to cost, currently estimated to be, at best, five times that of the conventional copper alternative. This is due to the need to install cooling systems, which flow the liquid nitrogen back and forth through the centre of the bles to be configured in a concentric arrangement, dispensing with the requirement for three separate wires used by conventional AC transmission systems, as well as negating the cables’ magnetic fields. However, the different flows of liquid nitrogen within one cable result in a heat exchange between the inner and outer flow, which presented HTS cable manufacturers with a significant challenge.   

It was not until 2008 that AMSC finally installed HTS technology in a power transmission cable in a commercial power grid. With US$23.5 million (£14.8 million) of funding from the US Department of Energy, the Holbrook Superconductor project involved 600 metres of underground cable containing 160 kilometres of BSCCO wire, installed at a Long Island substation in New York State. The 138kV cable carried more than 2,000A for a power delivery capability of 574MVA, roughly the same capacity as a 345kV overhead line. The liquid nitrogen cooling system, made by Air Liquide, comprises three 600-metrelong vacuum-insulated flexible cryostats to provide thermal insulation, maintaining the cable cores at cryogenic temperature.   

The cable for the Long Island installation was made by French manufacturer Nexans, which is involved in several HTS power cable projects, including AmpaCity in the German city of Essen. By the end of 2013, in conjunction with giant German utility RWE, Nexans will have installed a one-kilometre underground HTS cable, which will be the world’s longest, transmitting power at 10kV. The HTS system will replace two conventional 110kV lines between two sub-stations in Essen’s city centre. By connecting to other medium voltage parts of the grid, RWE can replace up to five parallel conventional 10kV cables or 110kV/10kV substations, which are used to step down power from the long-distance transmission grid.   

From an economic perspective, the lower footprint of HTS power cables is their primary advantage over conventional copper cables, says Mark Stemmle, Project Manager for Superconducting Cable Systems for Nexans. He explains, ‘If you are looking at an installation in the middle of the country with no constraints on conventional solutions, such as overhead power lines, it will be too expensive. But if you are looking at an installation in a city with space constraints then a conventional solution could be more expensive than superconducting cables. A utility needing to upgrade a substation in a downtown location will have to install additional transformers, which require a lot of space and contain a great deal of oil which can be a fire hazard if they fail. So a superconductor solution could be a very interesting alternative.’   

Counting the cost   
The total cost of AmpaCity is £10.7 million. RWE, Nexans and the Karlsruhe Institute for Technology, which conducted research into the project, has put up 57% of the cost, with the remaining 43% funded by the German Federal Ministry of Economics and Technology. For the foreseeable future such projects will require government funding, says Stemmle. ‘The dominating cost factors for HTS power cables are the material and cryogenic cooling systems, which together contribute more than 50% of the total cost,’ he says. ‘But as more projects are installed around the world we expect the HTS material to become cheaper, as will the cooling systems. At present they are relatively niche products.’   

There are around 12 HTS trials currently underway in Europe, China, Japan, Korea, Russia, Mexico and the USA. To date, no utility firms have announced plans for similar trials in the UK, and National Grid sees the technology as insufficiently advanced for longer distance transmission.   

Utilities are very conservative in the UK and it would be a major change for them to adopt superconducting cable technology,’ says Stemmle. ‘Maybe they can still deal with challenges using conventional solutions, but from our discussions with utilities we see that they are facing many challenges. Their equipment is getting older, the number of grid connections has grown substantially due to the increasing volume of renewables, and the fault current levels are increasing. This takes a toll on switchgear. Eventually, grid utilities may have to upgrade complete substations.’   

But while potential in the UK may be limited, at least in the short term, developments continue apace in the field of HTS power cables. AMSC and other manufacturers, including compatriots SuperCable and Japan’s Sumitomo, are working on a second-generation HTS wire using a combination of yttrium, barium, copper and oxygen (YBa2Cu3O, also known as YBCO). YBCO has a lower superconducting temperature than BSCCO, but it is estimated to deliver much higher current densities than BSCCO.   

YBCO wire will be used in what is an exciting planned application for HTS superconducting power cables, the Tres Amigas Superstation project in New Mexico, USA. The Superstation will interconnect the USA’s three currently isolated power grids – the Western, Eastern and Texas Interconnections – creating the USA’s first energy market hub. It will use ultra-efficient, high-capacity direct current superconductor cables, coupled with voltage source converters (VSC), to enable multi-terminal power transmission. As a result, wind, solar, hydro and geothermal power sources that currently do not have access to transmission lines or customers will for the first time, be able to tap into multiple markets.   

The interconnection points, or terminals, will be tied together with several miles of underground superconductor DC transmission cable called PowerPipelines. The use of underground superconductor DC cables will permit Tres Amigas to transmit several gigawatts of power between the three terminals with virtually no losses or heat generation. AMSC will use its second generation Amperium wire for the project. Amperium contains YBCO ceramic thick film deposited on an oxide-buffered Ni-W alloy substrate. The 4mm-wide, 0.1mm-thick film is coated with silver and laminated on both sides with a metal strip stabiliser (copper, brass or stainless steel) using solder that has a 179°C melting point.   

The US$1.5 billion, privately-funded project is being constructed in three phases, with the first phase of about US$485 million dedicated to building up 750 megawatt transfer capability by early 2015. When complete, the superstation will have a capacity of five gigawatts.   

It has taken some time, but 101 years after Heike Kamerlingh Onnes’ discovery, superconductivity is finally beginning to bear fruit in the field of power transmission cables.