Applied superconductivity group Challenges for HTS Coated Conductors Used as RFCL in MV Grids

1. Applied superconductivity groupChallenges for HTS Coated Conductors Used as RFCL in MV Grids Lausanne Mai 23., IEA ExCo Meeting

2. Outline • Foreword • Lack of interest of youngstudents for Electrical Engineering • AppliedSuperconductivityteachingcancelled in Lausanne? • ECCOFLOW FCL • Tape inhomogeneities: • Price and length of a tape ishighlyrelated to itshomogeneity • Whichbehaviour has this FCL in a real grid ? • How to design a FCL withinhomogenous tape ? • Thermal stability • How to enhance thermal stability ? • Speed-up recooling EPFL Applied Superconductivity Group, Lausanne, May 2013

3. ECCOFLOW project (FP7) • CC FCL demonstrator (1kA, 20 kV, 3p) • Impliedstudies: • Experimentalwork • Basic characterization • Ic, n, R(T) • Quenchmeasurements • NZPV measurements • Fastpulsedmeasurements • AC losses • NumericalModelling • FCL device • FCL in the grid • FEM tape modelling EPFL Applied Superconductivity Group, Lausanne, May 2013

4. ECCOFLOW device • Full device • FCL itself • Component quench test • http://ECCOFLOW.org EPFL Applied Superconductivity Group, Lausanne, May 2013

5. Tested in Milano somemonthsago EPFL Applied Superconductivity Group, Lausanne, May 2013

6. Inhomogeneity of Ic Voltage criterion (1 µV/cm) give us an average of “Jc” on the measured length 2 to 3 km needed for a FCL Jcvaries locally How this LOCAL property affects the GLOBAL limiting performance ? Discretization of the length Measurement gives us lowest Ic EPFL Applied Superconductivity Group, Lausanne, May 2013

7. The model Stabilizer Superconductor • The total length (Ltot) of the HTS tape is modeled with n blocks in series. • Every block is modular and implements thermal and electrical properties of HTS-cc layers (Hastelloy, Silver, Ybco, etc...). Buffer Substrate EPFL Applied Superconductivity Group, Lausanne, May 2013

8. Electrical model calibration • Electrical part – experimental validation Experimentsketch Furtherwork: Fastpulsedmeasurements, kA/µs EPFL Applied Superconductivity Group, Lausanne, May 2013

9. Ic inhomogeneity Relative position of [Im] terms implies a diferent amount of thermal conduction between blocks EPFL Applied Superconductivity Group, Lausanne, May 2013

10. Simulation circuit Circuit and simulations EPFL Applied Superconductivity Group, Lausanne, May 2013

11. Rfault ≈ 0 ->Homogeneousquench Thinner stabilizer has higher Rp.u.L. For a given value of Zs and tCB, it allows to face the temperature criterion minimizing the lengths of the paralleled wires. EPFL Applied Superconductivity Group, Lausanne, May 2013

12. Rfault> 0 -> inhomogeneousquench EPFL Applied Superconductivity Group, Lausanne, May 2013

13. Partial quench Danger atlowstabilization EPFL Applied Superconductivity Group, Lausanne, May 2013

14. Safestabilization RF >> 0 → inhomogeneous (partial) quench SP-NEX/ESSEN-20110812MTA, Ic,av = 380 A thC-276 ≈104.5 µm 3.5 µm < thAg < 4.5 µm EPFL Applied Superconductivity Group, Lausanne, May 2013

15. Inhomogeneous tapes summary • Our approach can consider: • Inhomogenetyof Icas a gaussiandistribution. • thermal conduction within the tape where it is not possible to neglect it. • variation of the stabilization • geometrical manufacturing imperfections of silver stabilizer and hastelloysubstrate • With (IF>>Ic,tot,av) there is homogenous quench: for any thickness of stabilizer it is possible to obtain the desired limitation adjusting the length of the tapes BUT • if IF ≃ Ic,tot,min, due to Icinhomogeneity, the quench tends to be partial. Length of the tape that contribute to the limitation drops with silver stabilizer thickness. A too small limiting zone leads to dangerous temperature values • Our model can be used to calculate the real parameters (single tape length and the degree of stabilization etc…) and to evaluate the safe integration of the RFCL in the grid (e.g. external shunt etc…). EPFL Applied Superconductivity Group, Lausanne, May 2013

16. About FCL thermal stability • When a faultoccurs, a bigamount of energy has to beabsorbed by the conductor, the higher the mass the more stable is the conductor • Whentemperatureincreases, heat exchange isgoverned by the « boilingcurve » over a ΔT of ~30K an more, hc drops to verylow values. log EPFL Applied Superconductivity Group, Lausanne, May 2013

17. Thermal stability computation • Insert a second medium between the HTS-CC and the nitrogen bath. • Add mass • Add thermal insulation • This arrangement is compared to • Adiabatic conditions: the tape does not exchange heat with the external bath (Fig. c). • Heat exchange with cooling bath (Fig. b): the boiling curve at the interface nitrogen-tape is modeled through an appropriate form of Newton’s law of cooling EPFL Applied Superconductivity Group, Lausanne, May 2013

18. Effect of additional medium • The additional medium isvery effective a lowcurrents • Athighercurrents the effectisstillpresent but stronglyreduced. • The effectincreasewith medium thickness up to 300-400 µm • Specificenergy: joule integral, energydissipated in a portion of the conductorwith unit resistivity EPFL Applied Superconductivity Group, Lausanne, May 2013

19. Recooling time • There is an optimal thicknessminimizingrecooling time EPFL Applied Superconductivity Group, Lausanne, May 2013

20. Temperature profile of the medium • Weseethatwecanstaybelow film boiling conditions with a thicknessbetween 350 and 500 µm. EPFL Applied Superconductivity Group, Lausanne, May 2013

21. Thanks ! • SUST/430729/PAP, 27June 2012 • http://stacks.iop.org/0953-2048/25/095005 EPFL Applied Superconductivity Group, Lausanne, May 2013