The current limiter protects the grid that is designed for 12 000 volts and 800 amperes against damagefrom short circuits and voltage peaks. It enhances the intrinsic safety of the grid and may help reduce the investment cost of such power plants.

“For a long time, high-temperature superconductors were considered to be difficult to handle, too brittle, and too expensive for general industrial applications,” explains (left) project manage, Wilfried Goldacker from Karlsruhe Institute of Technology. (left) “The second generation of high-temperature superconductor wires based on yttrium, barium, copper, oxide (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 are destroyed. The limiter automatically returns to the normal state of operation after a few seconds only. Consequently, power failure is much shorter than in case of conventional current limiters, such as household fuses, whose components are destroyed and have to be replaced with high time and cost expenditure.



“Superconducting current limiters have a number of advantages for the stability of medium- and high-voltage grids,” says Mathias Noe, (right) Head of Institute of Technical Physics at Karlsruhe. "Reliable, compact current limiters enhance the operation stability of power grids and allow for simplification of the grid structure. As they are protected against current peaks, decentralised energy generators, such as wind and solar systems, can be integrated much better in grids. Expensive components in the existing grid are protected efficiently, components in future grids can be designed for smaller peak currents, and transformers will no longer be necessary. 

"Investment costs of power plants and grids will be reduced. Moreover, superconducting current limiters on the basis of YBCO can also be applied in high-voltage grids of more than 100 kV for better protection against power failures in the future." 


How YBCO works
The YBCO crystal layer of about 1 micrometer in thickness is grown 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.

Superconductivity collapses abruptly when the conductor current exceeds design limits. This  is used by the current limiter and in grid current peaks,  the superconductor loses its conductivity in fractions of a second,the current flowing through the stainless steel strip only, with a higher resistance  thus limits the current.

The heat arising is removed by the cooling system of the superconductor. A few seconds after that short circuit, it is returned to normal operation in the superconducting state. YBCO superconducting layers on stainless steel strips are more stable and operation-friendly than first-gen superconductors based on bismuth, strontium, calcium, copper oxide (BSCCO) ceramics. And  poduction not needing  any noble metals will presumably be cheaper.



The superconducting current limiter was developed in the past two years under the ENSYS-TROB  project. The project partners are Karlsruhe Institute of Technology, Nexans SuperConductors, TU Dortmund, and BTU Cottbus.  The field test is carried out at the user, the Vattenfall utility company. Funded with about €1.3m by the Federal Ministry of Economics, results of the project are of great relevance, as the functionality of current limitation may be integrated into superconducting transformers and energy cables in the future.