The superconducting way to protect smart city grids

Superconducting cables carry electricity at exceptionally high currents, reliably and with minimal losses. That makes them an attractive option for meeting the fast-growing power needs of smart cities. However, superconducting properties can also be used to stop current dead in its tracks, almost instantaneously. This enables an ingenious device known as a superconductor fault current limiter (SFCL) to provide vital protection against the increasing threat of fault currents.

Increasing fault currents caused by short circuits are an inherent risk in smart cities. They result from factors such as higher loads, more distributed generation, and more complex load flows. The consequence is that the safety rating of transformers and switchgear in distribution networks may be exceeded. Without suitable protection, vital infrastructure can be damaged, causing outages with consequent impact on business and society.

Fortunately, an answer is in hand in the form of the SFCL. It is based on high temperature superconductor (HTS) material that is cooled to its operating temperature of -200°C by liquid nitrogen, a fluid that is both readily available and inexpensive. The concept behind the device is beautiful in its simplicity.

In normal operation, the SFCL allows current to flow easily and with no losses. But should a fault current start to flow the superconductor heats up above its critical temperature and it transitions from a perfect conductor to having a very high resistance. The transition takes place in less than 2 milliseconds (ms) – about 50 times faster than the blink of an eye. The result is that the fault current is limited immediately, protecting equipment such as transformers, switchgears and busbars on the same circuit.

A key advantage of the SFCL, unlike some fault current limiters that rely on explosive devices to break the circuit, is that it does not need to be replaced or reset after use. As soon as the fault current is cleared and the superconductor is cooled back down, the SFCL can return to operation. That makes SFCLs ideal in case of frequent fault currents or for installation in hard to access parts of the network.

The SFCL can be deployed to protect critical parts of the network infrastructure in smart cities where it offers a very cost-efficient way of stabilizing and optimizing grids as power demand increases.