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Electricity grids without electrical resistance
The construction of an inner-city pilot route for superconductive cable has begun. The superconductive cable has hardly any electrical resistance. As a 10,000-volt cable, it can therefore transport just as much electricity as a 110,000-volt high-voltage cable. That saves space and simplifies the distribution networks. If the pilot project is successful, this can transform the electricity grids in urban areas.
Superconducting cables incur almost no losses when transporting electricity. This is possible because the cables have hardly any electrical resistance with low temperatures of around minus 200 °C. It enables them to transport considerable electricity in a compact design. This is a particular advantage for inner-city districts in urban areas. Here, 110,000-volt high-voltage transmission lines in combination with transformer stations have until now ensured that sufficient electricity is available for supplying the loads in the 10,000-volt distribution grids. However, these take up valuable space in expensive city centre locations.
In the City of Essen in the Ruhr district, for example, a 6,500-km-long electricity distribution grid and 36 transformer substations supply the 570,000 inhabitants and companies. If the “AmpaCity” project is successful, the number of transformer stations could be reduced. This would make sites available for other uses – and not just in Essen. “Above all it is about testing how we can replace the enormous space and volume taken up by 110-kV systems in inner cities with substantially leaner medium-voltage grids. Thanks to their high transferable capacity with small cable dimensions, superconductive systems are predestined for this. In our “AmpaCity” project we want to prove that these also function under tough everyday conditions, which will then enable us to organise our future grid planning accordingly,” says Andreas Breuer, Head of New Technologies at RWE.
The longest superconducting cable is being installed in the Ruhr district
The construction of the pilot route is beginning in March 2013. The superconducting cable is replacing a 110,000-volt high-voltage cable for the kilometre between the Dellbrügge and Herkulesstrasse transformer stations. The superconducting cable can transport large capacities with a smaller voltage. In future, this will make 110,000/10,000-volt transformer stations surplus to requirements. In a study conducted by the Karlsruhe Institute of Technology in collaboration with the cable manufacturer Nexans and the energy supplier RWE, it was determined that four out of ten transformer stations in Essen could be removed.
The technology is based on a physical material property: with temperatures lower than minus 180 °C, the superconducting cable transports 100 times more electricity than copper cable. Despite the necessary cooling jacket, it can conduct five times the capacity of a copper cable of the same thickness. The reduced space requirements will free up capacities in the dense supply grid in urban areas.
Part of the pilot project is also concerned with investigating safety elements. “In the case of a grid fault, such as a short circuit or lightning strike, the superconductive fault current limiter prevents unlimited increases in the current and thus avoids cable faults. In contrast to conventional fuses, it recovers and starts functioning again,” explains Wilfried Goldacker, Head of Superconductive Materials at the Institute of Technical Physics (ITEP) at Karlsruhe Institute of Technology (KIT).
So far the technology has not yet been tested on a large scale. A test route more than 600 metres long is currently being operated in New York. The test section in Essen is intended to provide information on its practicability by 2016. “The cable length is already very informative in terms of the production, installation and operation, and can be seen as a component for an entire city centre ring, whereby a city centre ring is almost a multiplier of this project,” says Goldacker in estimating the significance of the results.
Before its construction, the study from the project partners investigated in detail the technical and economic advantages of superconductivity in inner-city grids. “The results of the study show that as an economic alternative to the current 110-kV grid infrastructure, an equally powerful 10-kV grid with superconductors is feasible, whereby a continuous cable structure based on conventional 10-kV technology is specifically not possible in Essen’s city centre due to spatial restrictions. This ought to apply to an equal extent to other large cities,” explains Breuer. The study forecasts that the use of superconductors will bring economic benefits, including in regards to both the investment and operating costs.
Breuer hopes that the project will have an important signal effect because “although superconductors are not yet seen to be an economic alternative to conventional cable, when assessing the overall system our study has shown that these will be more economic than the current technology in the long term and as a mass produced product.” However, the price/performance ratio of the conductor material needs to be improved, and there is also a need for optimisation in regards to the cable manufacture and the cooling technology.
Further links relating to the “AmpaCity” project
The German Federal Ministry of Economics and Technology is contributing 6.3 million euros to the “AmpaCity” project, which is costing 13.5 million euros altogether. The additional costs are being borne by the project partners. The energy supplier RWE, the cable manufacturer Nexans, the Karlsruhe Institute of Technology and Project Management Jülich are all participating in the project.
The BINE Projektinfo brochure “Superconducting fault current limiters in a power plant” (12/2011) provides background information on the technology.