Nexans, a worldwide leading expert in the cable industry, has successfully commissioned the world’s first resistive superconducting fault current limiter (SFCL) based on second-generation superconductor tapes. The SFCL, equipped with superconducting elements developed in cooperation with the Karlsruhe Institute of Technology, has been installed on behalf of Vattenfall Europe Generation AG to provide short-circuit protection for the internal medium voltage power supply that feeds coal mills and crushers in the Boxberg lignite power plant in Saxony, Germany.
A first generation SFCL, based on solid superconducting materials, was installed by Nexans at Boxberg in 2009 as part of a long-term test programme. Following the successful completion of this project, Nexans has returned to the plant for live testing of a new SFCL device featuring superconducting tapes. These tapes reduce the already low losses in the conductor material by around 90 percent, thereby lowering operating costs. They also provide an even faster response to a short circuit than the first generation materials.
A key advantage of the SCL is its inherent safety, as it responds to a short circuit without an external trigger signal. Unlike pyrotechnic devices that need to be replaced after triggering, it can resume normal operation as soon as the short circuit fault is cleared and the material is able to return to its superconducting state.
The new SFCL is designed for a nominal current of 560 A at 12,000 V, but can also allow currents of up to 2,700 A to pass briefly without triggering the device. This is an important pre-requisite for operation so that the coal mills can draw a high current on start-up without experiencing any problems.
Coated conductors provide the core elements of the limiter
The new current limiter is based on superconducting tapes made of YBCO (yttrium barium copper oxide) also known as coated conductors. At temperatures lower than -180°C the thin ceramic layer becomes superconducting and can conduct electricity approximately 10,000 times better than copper.
FCLs are under active development. In 2007, there were at least six national and international projects using magnesium diboride wire or YBCO tape, and two using BSCCO-2212 rods. Countries active in FCL development are Germany, the UK, the USA, Korea and China. In 2007, the US Department of Energy spent $29m on three FCL development projects.
Low temperature superconductors cannot be used for commercial FCLs as the AC losses at liquid helium temperatures mean that the cryogenic cooling cost makes the whole device uneconomic.
First applications for FCLs are likely to be in electric-drive ships: naval vessels, submarines and cruise ships. Many more FCLs will eventually be used to help control land-based electricity distribution and transmission systems.
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