Current Limiter Aids Grid Safety

Monday, January 16, 2012 @ 02:01 PM gHale

For the first time, a superconducting current limiter is now working at a power plant that could help enhance intrinsic safety of the grid.

At the Boxberg power plant of Vattenfall, Germany, the current limiter, based on YBCO strip conductors, protects the grid against damage due to short circuits and voltage peaks. The new technology, co-developed by Karlsruhe Institute of Technology and made by Nexans SuperConductors, enhances the intrinsic safety of the grid and may help reduce the investment costs of plants.

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“For a long time, high-temperature superconductors were considered to be difficult to handle, too brittle, and too expensive for general industrial applications,” said project manager Wilfried Goldacker from Karlsruhe Institute of Technology. “The second generation of high-temperature superconductor wires based on YBCO ceramics is much more robust. Properties have been improved.”

Superconducting current limiters work reversibly. In case of current peaks after short circuits in the grid, no components end up destroyed. The limiter automatically returns to the normal state of operation after a few seconds. Consequently, the power failure is much shorter than in case of conventional current limiters, such as household fuses, whose components usually end up ruined and you have to replace them.

“Superconducting current limiters have a number of advantages for the stability of medium- and high-voltage grids,” said Mathias Noe, head of the Institute of Technical Physics of Karlsruhe Institute of Technology.

Reliable, compact current limiters enhance the operation stability of power grids and allow for a simplification of the grid structure. They end up protected against current peaks. In addition, decentralized energy generators, such as wind and solar systems, can integrate quite a bit easier into rids. Expensive components in the existing grid enjoy greater protection. In the future, components can undergo a design for smaller peak currents, and transformers will no longer be necessary. Investment costs of power plants and grids will be lower. Superconducting current limiters on the basis of YBCO can also apply to high-voltage grids of more than 100 kilovolts for better protection against power failures in the future.

YBCO stands for the constituents of the superconductor: Yttrium, barium, copper, and oxygen. An YBCO crystal layer of about 1 micrometer in thickness grows directly on a stainless steel strip of a few millimeters in width that gives the ceramics the necessary stability.

Below a temperature of 90° Kelvin or minus 183° Celsius, the material becomes superconductive. However, superconductivity collapses abruptly when the current in the conductor exceeds the design limits. This effect sees use by the current limiter. In case of current peaks in the grid, the superconductor loses its conductivity within fractions of a second and the current will flow through the stainless steel strip only, which has a much higher resistance and, thus, limits the current. The heat ends up removed by the cooling system of the superconductor. A few seconds after the short circuit, it returns to normal operation in the superconducting state. YBCO superconducting layers on stainless steel strips are more stable and operation-friendly than first-generation superconductors based on BSCCO ceramics. Moreover, their production does not require any noble metals, such as silver, and cost much less.

A field test is underway at the Vattenfall utility company.

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