Superconducting fault current limiters can contribute significantly to increasing the safety, availability and reliability of electrical systems in power stations. According to experts, they also have an important role to play in expanding the power grid. In 2009 at the Boxberg power station, a superconducting fault current limiter was used for the first time to protect its own power supply. Soon a next-generation superconductor system with optimized characteristics will be tested at the same site.

Short circuits in power plants or electricity grids are expensive. First of all, high currents can damage system components and cause downtime. Then there are the additional costs involved in building the plant because every component needs to be able to withstand the maximum possible load in the event of a short circuit. It is particularly annoying when functioning components in existing systems are suddenly under-dimensioned due to an increased short-circuit power, and therefore have to be replaced. As power grids are upgraded for higher capacities, this is a situation that is likely to occur more frequently. Superconducting fault current limiters (SFCL) represent a new tool for containing these additional costs. They open up entirely new possibilities for designing power plants and grids.

These fault current limiters work on a simple principle: the core of the device is a superconducting material which completely loses its electrical resistance to direct current below a material-specific temperature. With alternating current there is an extremely low residual resistance. Installed in a current path, the superconductor normally does not affect the current flow at all. However, this is only the case up to a certain current density in the superconductor. It can be clearly seen on the currentvoltage curve (see fig. 1). If the current exceeds a certain threshold, a “quench” occurs. Superconductivity ceases and instantaneously – within a matter of milliseconds – an electrical resistance occurs. As a result, the short-circuit current is automatically limited (see fig. 2). Since the current limitation effect is not only very fast but also very strong, the limited current may be lower than desired, e.g. to maintain particular operating states. However, the design of the SFCL can be adapted for the specific requirements. A parallel resistor to the superconductor – a shunt – allows the current characteristics to be controlled as desired in the event of a short circuit. The fault current limiter operates completely independently and is intrinsically safe. After just a brief cooling phase, it reactivates automatically without requiring any further maintenance.

These fault current limiters utilise high temperature superconductors which have a transition temperature above 77 kelvins. Consequently they can be cooled simply using liquid nitrogen, which is an inexpensive industrial product. It can either be replenished or reliquefied directly in the system via a suitable cooling system.