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The world’s first fault current limiter based on second-generation superconductors is commissioned

17 January 2012

Nexans 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 the Boxberg lignite power plant in Saxony, Germany.

A first generation SCFL, 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 SCFL 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.
“We now have a second superconductor  material option for manufacturing power systems, and this will provide us with  an even wider range of applications for our fault current limiters to help  customers improve the safety of personnel and equipment while also reducing  infrastructure costs. The upgrading and expansion of power networks to meet  the fast-changing needs of our customers requires intelligent solutions and  new functions,” explains Jean-Maxime Saugrain, Corporate Vice President  Technical at Nexans. “The power plant’s house load is just one of many  potential applications for SCFL technology. For example, in the renewable  energy sector the capability to supply more power from renewable sources is  frequently restricted by the level of the short circuit  currents.”

Fast response to short-circuit currents
The  current limiter works in a similar way to the low voltage safety cut-out in  domestic homes, but operates on the medium/high voltage network. In addition,  after operating, it does not interrupt the electricity flow completely. Under  normal circumstances, its superconducting elements allow the electricity to  flow unhindered and with practically no resistance. If a critical current  level is exceeded, such as during a short circuit, the conductor drops out  from its superconducting state within milliseconds to act as a strong  electrical resistor. Only a precisely defined residual current will then flow.  This enables the device to protect all the downstream components, such as  switchgear, from the damaging overloading of a short circuit.
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 SCFL 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.
The current limiting components based on  second-generation superconducting tapes were developed over the past two years  as part of the ENSYSTROB project. The project partners are Nexans  SuperConductors GmbH, the Karlsruhe Institute of Technology, the Cottbus and  Dortmund Universities of Technology and the energy group Vattenfall. The  German Federal Ministry of Economics and Technology provided the project with  financial backing of about €1.3 million.

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