Utilities serving urban regions across the world face a number of different challenges resulting from ever-increasing load growth, the addition of new distributed and often volatile generation resources, and the demand for improved reliability and continuity of service. The need for substantial investment in urban grid renewal programmes is well recognised. Yet there is substantial resistance from planning authorities, and the public they serve, to grant the rights of way to install new overhead lines, underground cables or other high-voltage equipment. Hence, the drive to develop innovative technologies that can increase the capacity and flexibility of power infrastructure.
|The special termination for the HTS cable to be used in the AmpaCity project|
One of the technologies that offers perhaps the greatest promise to meet these challenges is high-capacity, underground HTS (High-Temperature Superconductor) cables. This century, several HTS cable designs have been developed and demonstrated to take advantage of the much higher power density of HTS materials compared with copper wires. Moreover, because they are actively cooled and thermally independent of the surrounding environment, they can fit into more compact installations than conventional infrastructure, without concern for spacing or special backfill materials to ensure dissipation of heat. They also eliminate EMF concerns.
The potential of HTS cables for highly efficient transmission of large amounts of electrical power means that they are often viewed as a possible replacement for conventional high-voltage cables as the backbone of a transmission grid, carrying power over long distances. That could be feasible in perhaps the next 5-10 years. However, it is Nexans’ view that HTS cables are set to make the most immediate impact when used in short lengths (from a few hundred metres up to a few kilometres), to carry large amounts of power through areas where space and access are at a premium.
For example, in built-up urban areas it is becoming increasingly difficult for utilities to obtain the rights of way to install the new overhead lines or underground cables needed to meet increasing demands for power. When operating at the same voltage, HTS cables can carry between 5-10 times as much power as conventional cables. So simply rethreading existing infrastructure with superconductors could address these bottlenecks to significantly reinforce the grid supply for power-hungry city dwellers.
The compact nature of HTS cables could also help when it comes to crossing obstacles such as rivers. So instead of directional drilling, or constructing a dedicated cable tunnel, the HTS cable could be carried by an existing structure, such as a bridge or service tunnel.
What is an HTS?
Superconductors are materials that do not exhibit any ohmic resistance below a certain critical temperature. This phenomenon has been known since 1911 and is observed in what are now called LTS (Low Temperature Superconducting) materials, because they are cooled with liquid helium at -269°C.
In the late 1980s, a new family of superconductors, the HTS materials, was discovered. They acquire their superconducting properties at a much higher “critical temperature”, in particular they become superconducting at around -180°C, a temperature range that can be achieved using liquid nitrogen, a cheap, abundant and environmentally friendly cooling liquid. All HTS materials are copper oxide-layer based ceramics.
The concept of rethreading existing high-voltage infrastructure was demonstrated successfully in the world’s first HTS power transmission cable system in a commercial power grid, energised in April 2008 at the Holbrook substation on Long Island, New York, US.
The Long Island Power Authority (LIPA) project is a Superconductivity Partnership Initiative (SPI) between the US Department of Energy (DOE) and industry to provide a real-life demonstration of the application of HTS cable within an electric utility’s operational transmission system. The DOE, which funded around half of the project cost, sees HTS cables as a core component of a modern electricity superhighway, one that is free of bottlenecks and can readily transmit power to customers from remote generating sites, such as wind farms.
The 600-metre cable system – at the time the longest superconductor cable in the world – includes three phases connected to the LIPA grid through six outdoor terminations (three at each end). It was designed, manufactured and installed by Nexans, utilising HTS wires produced by American Superconductor, the prime contractor for the LIPA HTS cable project. The liquid nitrogen refrigeration system was manufactured and installed by Air Liquide. Three 600-metre long vacuum insulated cryostats from Nexans provide high quality thermal insulations that maintain the cable cores at cryogenic temperature.
LIPA was the third US electric utility to deploy an HTS cable system. The LIPA cable was the first to operate at the transmission voltage of 138 kV – twice the previous highest voltage achieved by any HTS installation. It was designed to carry 2400 A, resulting in 574 MVA of total power-carrying capacity – enough to power 300,000 homes. The cable system is designed to withstand 51 kA rms fault currents for 12 line cycles.
The LIPA programme is now moving into its second phase, which is focused on replacing one of the existing cable phases with a new phase based on second-generation (2G) HTS tapes. These tapes are designed to be significantly cheaper than the first-generation HTS conductors used in the initial project, and will very likely lead to a more cost-effective cable system.
The existing LIPA cable system is just over 600 metres in length, because that is the longest single length of HTS cable that can be spooled easily for delivery to site, and it was decided in the initial phase to avoid the added complication of cable joints.
However, cable joints will be essential when it comes to creating longer HTS cable links of many kilometres. So this second phase will also encompass the development and demonstration of a suitable cable joint for both the construction of longer links, and to enable HTS cables to be repaired.
Moving from HV to MV – a smarter use of HTS
While the success of the LIPA programme is proving the capability of HTS in high-voltage schemes, there could be an even smarter way to use superconductors. That is to carry the same power as a conventional cable system, but reducing the operating voltage from high-voltage (HV) to medium-voltage (MV). The smaller space needed for cables can free up space for distribution companies to develop simpler networks, and reduce the amount of land needed. It also allows for smaller, less obtrusive substations, and studies have shown that in a typical urban network the number of transformers might be reduced by 40 per cent. These factors all start to tip the financial balance in favour of HTS.
The AmpaCity project
In 2013, the MV concept is being put to practical test in the ‘AmpaCity’ project in the Ruhr city of Essen, Germany where Nexans, in partnership with the German utility RWE and the Karlsruhe Institute of Technology (KIT), will install a 1 km underground HTS cable operating at 10 kV – about a tenth of RWE’s usual 110 kV transmission voltage.
The 1 km link between two transformer stations will mark the longest superconductor cable installation in the world, with a three-phase, concentric 10 kV cable designed for an operating current of 2.3 kA and a transmission capacity of 40 MW. As part of this project, KIT will analyse suitable superconducting and insulating materials. This installation will also be the first to combine a superconductor cable with a resistive superconducting fault current limiter (SFCL) for overload protection. The SFCL will be manufactured by Nexans’ specialized superconductor facility in Huerth, Germany.
The HTS cable features three concentric insulated elements manufactured from bismuth strontium calcium copper oxide (BSCCO), cooled to 68K (-205°C) using liquid nitrogen that flows one way through the centre of the cable and back around the outside to be recooled. This particular design was selected as the most material-efficient -and therefore most cost-effective – use of the relatively expensive superconductor material.
The concentric arrangement also eliminates the need for three separate cables (unlike conventional AC transmission systems), and cancels out the cables’ magnetic fields. A major engineering challenge is in the pumping of two to three litres of liquid nitrogen per metre of cable, especially as careful management is required of the heat exchange that can take place between the central and outer concentric cables.
A new dimension in urban networks
The AmpaCity project could herald a whole new dimension in the restructuring of inner city networks. Following the successful completion of the two-year field test, it should be possible to install 10 kV superconducting links in large sections of the backbone of the Essen distribution network to reduce their reliance on HV installations. In the medium term, this would lead to greater efficiency as well as lower operating and maintenance costs while simultaneously reducing land use. The dismantling of numerous 110/10 kV transformer stations would help to free up valuable space in inner-city areas.
Thanks to the distinctive nature and ambitions of AmpaCity, the project is being supported by the energy research department of Germany’s Federal Ministry of Economics and Technology (BMWi).
The AmpaCity project was preceded by a detailed study in which a number of research institutes, under the leadership of KIT, worked alongside Nexans and RWE to analyse the technical feasibility and economic efficiency of a superconductor solution at medium-voltage level. This study revealed that superconductor cables are the only reasonable alternative to HV cables in city networks and that their use would mean that resource- and land-intensive transformer stations could be demolished. For example, the equipment that might be eliminated from a typical urban network under this new MV concept includes:
- 12.1 km of 110 kV cable systems
- 12 x 110 kV cable switchgear
- 5 x 40 MVA, 110/10 kV transformers
- 5 x 110 kV transformer switchgear
- 5 x 10 kV transformer switchgear
The new equipment required would be
- 23.4 km of 10 kV HTS cable system
- 16 x 10 kV cable switchgear
- 3 x 10 kV bus ties
Although conventional copper or aluminium MV cables could also be used in inner-city areas to transmit high power, the cost efficiency of such solutions is cancelled out by the much higher ohmic loss.
|The cable that is in the process of installation for the AmpaCity project|
Furthermore, conventional MV cables were not feasible in Essen, as they require much more routing space: instead of a single 10 kV superconductor cable, five copper cables would need to be laid in parallel – a largely impossible task given the limited space under the city streets.
Development phase completed
A key stage in the AmpaCity project was reached in March 2013 with the handover of the test certificate for the HTS cable system, marking the completion of the 18-month development phase.
The high-voltage laboratory at Nexans’ Hanover plant subjected the prototype of the HTS cable to a lightning impulse voltage test at roughly seven times the nominal voltage, as well as continuous loading at three times the operating voltage.
During the tests, the system connection joints and the specially developed, compact cable terminations were also tested. These terminations will be used to create the transition from the cryogenic superconductor system to the conventional network.
Following the successful tests on the prototype, production of the 10 kV HTS system is now starting.
Cost trends favour HTS
|The AmpaCity project will mark the longest superconductor cable installation in the world
Looking ahead, there is a general trend for metals such as copper and aluminium, the key elements in traditional electrical infrastructure, to become more expensive. This also will very likely help superconductors to become relatively more cost-competitive.
At the moment, the cooling system is also an expensive component, but as this type of HTS system becomes more widespread, a new concept for an urban grid could emerge with three or four cooling systems shared across all the cables, which would be considerably more efficient and cost-effective.
High-temperature superconductor cables are no longer a laboratory phenomenon.
They are ready now for deployment in congested, energy-intensive urban power grids, where they offer an important new alternative to existing cable solutions from both the technical and financial perspectives.
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