Fast Hotspot Detection in SFCLs by Exploiting Strain Response in Optical Fiber Sensing

Superconducting fault current limiters (SFCLs) can be used to limit fault currents in both meshed DC and AC grids by transitioning from superconducting to resistive state, in the presence of high currents. While the device is theoretically a great way to protect grids, the inherent inhomogeneity of critical current along the superconductor length can lead to localized heating, called hotspots, and ultimately destruction of the SFCL device. At EPFL under the European Union project Fastgrid, an extremely efficient Mach-Zehnder interferometer (MZI) based optical fiber sensing technique has been developed and patented that can detect even singular hotspots within 15 ms to protect SFCLs. The MZI response is characterized by a strain sensitive as well as a temperature sensitive contribution. This paper outlines an investigation by means of FEM modeling into the response sensitivity. A 2-D thermal model of the superconducting tape and optical fiber was made to study thermal transfer to the optical fiber from the REBCO tape. The simulation results showed that temperature rise observed in the optical fiber is slower than the MZI response time, proving a strain sensitive response in the experiment measurements. Sound mechanical coupling between the optical fiber and the superconductor tape can enhance strain transfer to the optical fiber and hence reduce hotspot detection time. With this improved performance, the health monitoring for SFCLs can be much more efficient and reliable.