Soumen Kar 1,2, Xiao-Fen Li 1, Venkat Selvamanickam 1, V. V. Rao 2 1 Department of Mechanical Engineering and Texas Center for Superconductivity University of Houston, Houston, TX 77204, USA 2 Applied Superconductivity Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur, West Bengal , India Current Distribution Mapping in Insulated (Gd,Y)BCO based Stabilizer-free Coated Conductor after AC over-current test for R-SFCL application Abstract ID: 46

GG G GG Climbing fault current G The fault current challenge Circuit Breaker Generator Load

GG Zero resistance Protection against fault currents using superconductors GG Circuit breaker can operate safely GG Instant rise in resistance limits fault current

SFCL – Working Principle Key characteristics of Fault Current Limiters based on superconducting materials Under normal operation a fault current limiter inserts negligible impedance into the network When a fault occurs the limiter‘s impedance rises rapidly, reducing the current flowing through it Fig.- Operation modes of SFCLs

HTS tapes for R-SFCL Resistive type superconducting fault current limiters (R-SFCLs) reduce fault current levels (5-10 times higher than its critical current, I c ) within the first cycle. The first limited peak current of the R-SFCL i.e. HTS tape is much higher than its I c value and termed as over-current. Stabilizer free (SF) (Gd,Y)BCO based coated conductors (CCs) are used for R-SFCL application as they have the required normal state resistance without current sharing in the stabilizer layers and low AC losses as well as sufficient mechanical strength and thermal capacity. Uniformity of I c over long lengths of HTS tapes over long periods of AC operation is also an important criterion for use in an R-SFCL. Heat generation occurs in HTS tape due to non-uniform current flow in the conductor even though it is partially superconductive. Hence, it is necessary to investigate the local degradation of I c of the HTS tape, if any, after its exposure to AC over-current operations. These studies are useful in predicting the reliable and reproducible performance of R-SFCL based on SF (Gd,Y)BCO tapes.

Tape Ic testing after fault limitation  To check critical current (I c ) uniformity over 1m of stabilizer free 12 mm wide 2nd generation (2G) (Gd,Y)BCO-based HTS tape before and after AC over-current operation.  Non-destructive I c measurement of as purchased SF (Gd,Y)BCO based HTS tape using a static hall probe (Tapestar ® ) with moving HTS tape configuration.  AC over-current of 2kA peak applied on 1m long SF (Gd,Y)BCO based HTS tape for 100ms (5 cycles).  After AC over-current exposure, I c measurement over 1m length using Tapestar ®.  Scanning Hall Probe Microscopy (SHPM) of degraded regions to map the two dimensional (2D) current density (J c ) distribution and to identify exact defective zone.

Non-destructive I c measurement over long length using Tapestar ® ManufacturerSuperPower Tape typeSF12100/ 2G CC HTS material(Gd,Y)BCO SubstrateHastelloy ® C276 Over-layerAg Over-layer thickness (μm)2 Width (mm)12 Thickness (mm)0.105 I c 77 K, self field468 T c (K)90 n-value37.5 Schematic of Reel to reel Tapestar setup along with axis Table 1- Specifications of SF 2G (Gd,Y)BCO CC The calibration is done by measuring a tape with known I c and the calibration factor cf is derived from the following equation. Here, N is the number of sensors and B i is the field at sensor i. As the values of B can be calculated by the measurement is performed by a Hall sensor array across the width of the tape. It is possible to calculate cf for a given sensor geometry.

Figure 2- As purchased SF 2G (Gd,Y)BCO CC, I c over 1m length using Tapestar ® I c over the length of HTS tape using Tapestar before and after AC over-current operation Figure 3 – Over-current limitation waveform of SF 2G (Gd,Y)BCO CC tape for a 5 cycle prospective over-current of 2 kA peak 2 1 Figure 4- Over current tested 1m long SF 2G (Gd,Y)BCO CC, I c over length using Tapestar ®, Red circles portion (1&2) taken for SHPM

Figure 5- Results of SHPM after the AC over current test on SF (Gd,Y)BCO CC (a) 3D magnetic field map of section 1, where no defects are observed. 2D magnetic field map along X-Y directions (Inset) (b) 3D magnetic field map of section 2, where one defect is observed and marked by black circle. 2D magnetic field map along X-Y directions with the defect is marked by black circle (Inset) (c) comparison of magnetic field profile across X direction at y=0 (with defect) and y=2 mm ( without defect) locations Magnetic field mapping of SF 2G REBCO CC after AC over-current test

Figure 6 - 2D current density maps of SF (Gd,Y)BCO CC (a) J x of section 1, where no defects are observed (b) J x of section 2, where one defect is observed at y=0 (c) J y of section 1, where no defects are observed (d) J y of section 2, where one defect is observed at y=0 and marked by black circle. ab c d a b Figure 7 - 2D current density maps of SF (Gd,Y)BCO CC (a) the current density J n of section 1 (b) J n of section 2 where one defect is observed atx=-4 y=0 and marked by black circle Current density distribution (J c ) in SF 2G (Gd,Y)BCO CC after AC over-current test

Summary  Measured I c over 1m length of as-purchased SF 2G (Gd,Y)BCO CC using Tapestar and almost uniform I c over 1m length with an average I c of 471 A, maximum I c of 486 A and minimum I c of 437A and standard deviation (STDEV) of 1.98% are observed.  At the time of exposure to 2kA Peak for 100ms which is almost 4 times higher than the sample’s I c, the tape shows current limiting behavior within 2ms and the 1 st limited peak is at 888A Peak.  After AC over-current test, HTS tape shows an average I c of 460 A, maximum I c of 495 A and minimum I c of 391A and standard deviation (STDEV) of 2% in the tapestar measurement.  Two sections where slight I c degradation is observed were cut for further SHPM to check current distribution uniformity and defects, from magnetic field mapping.  With the measured B z (x, y) data, we calculated the distribution of persistent current J x and J y, and the absolute value of J n (|J n | = √(J x 2 + J x 2 )) ) of both the sections 1& 2.  One defect found in the magnetic field map of section 2 corroborates to the low critical current point in the same section. This defect of section 2 at (x, y) = (-4, 0) has 15% lower J c than its surroundings.  Finally, it is observed that a minor overall J c reduction for a single defect does not affect HTS tape performance due to its uniform current distribution over long length. Hence, this (Gd,Y)BCO based stabilizer free coated conductors can be used for R-SFCL application due to their reliable and reproducible performance.

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