A superconducting fault current limiter is protecting the auxiliary power supply in the Boxberg lignite-fired power plant in the German state of Saxony against damage caused by short circuits and voltage peaks. The developers have now replaced the first-generation system with a fault current limiter based on YBCO superconducting tapes – which represents a world first. Karlsruhe Institute of Technology (KIT) and Nexans SuperConductors have thus improved the intrinsic safety of the grid and also helped to reduce equipment investment costs.

The BINE Information Service has already reported on this project at the end of 2011. The project head, Wilfried Goldacker from KIT, explains: “For a long time, high-temperature superconductors were regarded as difficult to work with and were considered too brittle and, more importantly, too expensive for general industrial applications.” However, their advantages are significant: No components are destroyed in the case of peak currents, such as after short circuits on the grid; instead, the limiter automatically returns to its normal operating state after just a few seconds. Thus, grid outages are significantly shorter than is the case with conventional fault current limiters. The design of these conventional limiters means that components are destroyed – in a manner similar to household fuses – and it is then time-consuming and costly to replace these components.

Module for network expansion

Reliable, compact fault current limiters allow for more stable operation of electricity grids and also simplify the grid structure. Thanks to the protection that these limiters provide against peak currents, decentralised energy sources such as wind turbines and solar energy systems can be integrated more easily into grids. Expensive components in the existing electricity grid are protected effectively, components in future grids can be designed for lower peak currents, and transformers become superfluous in many cases. The end result is that savings can be made on the investment costs necessary for power plants and grids. In addition, the principle of the superconductive fault current limiter based on YBCO can be expanded to high-voltage grids of over 100 kilovolts in the future, thus better protecting these grids against power cuts.

The second generation

The abbreviation YBCO stands for the components of the superconductor: yttrium, barium, copper and oxygen. This material is coated directly in the form of a crystal layer with a thickness of around one micron onto stainless steel tapes, which themselves are a few millimetres wide. This material becomes superconducting at a temperature of below –183 degrees Celsius. However, the material ceases to be superconducting suddenly if the current in the conductor exceeds the design limits. The superconductive fault current limiter is based on this effect. If there is a peak current in the grid, the superconductor loses its conductivity within fractions of a second and the current can then only flow through the stainless steel tape, which has a significantly higher resistance and therefore limits the current. The heat generated is removed by the superconductor’s cooling system. Normal operation in the superconducting state is then restored again a few seconds after the short circuit. YBCO superconductor layers on stainless steel tapes are more stable and easier to operate than first-generation superconductors made from BSCCO ceramics. Moreover, production does not require precious metals such as silver and is also likely to be less expensive.

BINE publications on this topic

The BINE Projektinfo brochure “Superconducting fault current limiters in a power plant” describes the research work in detail. The Info brochure “High temperature superconductors” was published back in 2010. It describes the production of the various materials along with their potential applications.

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