Paris, January 13, 2012 - 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.

"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 SFCL 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 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.

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.