Continental Europe has four large electrical grids. These criss-cross the landscape via heavy-duty wires carrying up to 380 000 volts. If two such cables ever touch, such as when a tree falls over the line, this causes a fault current, colloquially known as a short circuit. This will spark. The spark voltage can be enough to damage or destroy even equipment designed for high voltages. In a domestic circuit, a fuse can prevent damage by instantaneously cutting off the power. Yet, at the extreme voltages of national grids, these devices cannot be used. To date, no practical equivalent has existed; if it did, it would be called a fault current limiter. Today, engineers achieve the same result of limiting (switching off) the fault current by various passive and non-ideal means. In 2014, the Nexans company installed an experimental superconducting fault current limiter (SFCL) into the grid near the German city of Essen. Although that SFCL has effectively protected city power lines, the device was too expensive for commercialisation.
Improving the SFCL
The EU-funded FASTGRID project developed a lower-cost improvement on the initial SFCL. Specifically, FASTGRID uses the superconducting material optimally to reduce costs. Superconductivity is the principle whereby certain materials exhibit zero electrical resistance. However, this only works while the material remains at cryogenic temperatures. For an electrical grid system equipped with the FASTGRID SFCL, in the event of a short circuit, the current rises sharply, by a given and known value, then the SFCL cuts in. The temperature of the superconducting element (tape-like wires made from the new superconducting material) then rapidly rises. So the material loses its superconductivity and resists electrical current. This limits the current to a safe level, preventing damage and making it easy for a switchgear (circuit breaker) to cut power quickly. The conductive material then cools and is again superconducting. The power can safely come on again. Thus the superconductive property allows the SFCL device to function like a switch.
Cheaper and practical
“Our advanced superconducting conductor makes the SFCL much more attractive from an economic point of view,” explains Pascal Tixador, project coordinator. “The new conductor is approximately one tenth the cost of the Nexans conductor.” These savings were mainly achieved by optimising the conductor with alternatives. “We succeeded in developing and experimentally validating an advanced superconducting conductor for SFCLs,” adds Tixador. “But these developments took much more time than foreseen at the beginning of the project, and it was not possible to manufacture all the elements we hoped to.” The team manufactured, and successfully tested, two of a planned six superconducting elements. The next step will be to finalise industrialisation of the superconducting conductor. When this can be manufactured more easily, testing can begin in a full-scale industrial environment. The ultimate result will be a fault current limiter (fuse-like but reusable mechanism, usable at high voltages) cheap enough to be installed throughout national grids. Not only will the final device protect the grid from damage, it will also enable a long sought-after system reconfigurability called grid meshing.
FASTGRID, superconductivity, protection system, electricity grid, fault current limiter, circuit breaker