8-O-2B-5 Applied superconductivity group Insulation effect on thermal stability of Coated Conductors wires in liquid nitrogen B.Dutoit, Ecole Polytechnique Fédérale de Lausanne, Switzerland T. Rubeli, Ecole Polytechnique Fédérale de Zürich, Switzerland I. Martynova, A. Makarevich, A. Molodyk, S. Samoilenkov SuperOx, 20-2 Nauchnyi proezd, Moscow, 117246, Russia ICEC 26-ICMC 2016, Delhi, March 8th 2016

Outline Inhomogeneity => need of stability Experimental setup Comparison between adiabatic and real case Effect of thermal insulation Re-cooling speed Conclusions EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

CC are inhomogeneous Critical current distribution over conductor length A reduction of this inhomogeneity would imply higher fabrication costs Stabilization of the weakest points is mandatory EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

This present a risk for low fault currents Temperature of the weakest point of a RSFCL made of inhomogeneous tape at the end of the limitation phase No problem for a « perfect fault » Problems may occur at « high impedance fault » Solutions: Higher stabilization Higher length, higher cost Optimized cooling Study experimentally the cooling for these specific conditions EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

R(T) calibration To measure heat exchange near 77 K we choosed a normal tape First for this experiment superconductivity was destroyed By heating up the sample over 500°C for 2 hours Precisely measure resistance as a function of temperature The tape itself will be our “thermometer” EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Energy balance E brought in by the fault = E stored in the tape + E exchanged to the bath Measuring V(t), I(t), give us electrical resistance R(t) Knowing R(T) give us temperature T Energy exchanged to the bath is the difference between the 2 firsts terms.   EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Schematic of the experiment Pulse sent over a voltage to current converter Acquire I(t), V(t) @ 500 ks/s, 18bits Sample protection by a voltage threshold EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

4 wires setup SuperOx provides insulated samples with different insulation thicknesses Measured with 0 µm, 10 µm, 15 µm, 25µm, 35 µm, and 65 µm of Kapton® insulation Sample surrounded by LN2 EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Measurement Pulses of various amplitudes and durations 3kHz sine modulation added to pulse signal 3 different cooling regimes present in this example R(t) is Over a few periods (2-5) In phase signal is used Then R(t) is converted to T(t) with calibration data   EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Adiabatic compared to real conditions Resistance measurement converted into temperature values Classical adiabatic calculation lead to very high temperature Real conditions in the experiment shown below show a quasistatic state With continuous modulation, re-cooling can be measured too EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Generated, stored and cooling power At constant current the power increase due to resistance increase Power stored in the tape during heating phase Equilibrium at quasistatic phase Cooling after the pulse EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Quasistatic thermal equilibrium Same tape, same pulse amplitude , same pulse duration Bare tape: unstable, not shown here 10 µm insulation: stable @ ~ 85 K 15 µm : stable @ ~ 88 K 25 µm : stable @ ~ 93 K 35 µm : stable @ ~ 102 K 65 µm : stable @ ~ 110 K EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Thermal insulation effect Temperature plateau as a function of current Bare tape: unstable from 22 A 10 µm insulation: stable 15 µm : stable 25 µm : stable 35 µm : stable 65 µm : stable EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Re-cooling speed Curves fitted with an exp decay law Damping factor τ measured   Bare 0.49 s 0.09 s 0.28 s 0.66 s EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Damping factors All curves start with a low damping factor (between 1 and 2 s-1) corresponding to conduction cooling. All insulated wires show high damping factors; meaning an excellent recooling through nucleate boiling. Higher insulation thickness induces lower cooling speed. Small thicknesses (0- 15 µm) reach film boiling with low damping factor again at relatively low ΔT Wires with 20 and 35 µm are stable in nucleate boiling up to a higher ΔT. EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Conclusions The goal is the best stabilization of inhomogeneous tapes Measured heat exchange of a non superconducting tape to the bath Using a 3 kHz sine modulation added to the pulse signal Very important difference compared to adiabatic conditions Tuned thermal insulation can enhance stabilization Thanks for your attention ! EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Some references T. Rubeli, D. Colangelo, B. Dutoit and M. Vojenčiak, “Heat Transfer Monitoring Between Quenched High-Temperature Superconducting Coated Conductors and Liquid Nitrogen” Progress in Superconductivity and Cryogenic, Vol.17, No.1, pp.10-13, March, 2015, 10.9714 Lee S, Petrykin V, Molodyk A, Samoilenkov S, Kaul A, Vavilov A, Vysotsky V and Fetisov S, “Development and production of second generation high T-c superconducting tapes at SuperOx and first tests of model cables” Supercond. Sci. Tech., 2014, 27 044022 Samoilenkov S, Molodyk A, Lee S, Petrykin V, Kalitka V, Martynova I, Makarevich A, Markelov A, Moyzykh M, Blednov A, “Customised 2G HTS wire for applications”, submitted to Superconductor Science and Technolog M. Noe et al., “Conceptual Design of a 24 kV, 1 kA Resistive Superconducting Fault Current Limiter”, IEEE Trans. Appl. Supercond., vol. 22, no. 3, June 2012, 5600304. D. Colangelo and B. Dutoit, “Inhomogeneity Effects in HTS Coated Conductors Used as Resistive FCLs in Medium Voltage Grids”, Supercond. Sci. Tech., vol. 25, 2012, 095005. V. K. Dhir. “Boiling heat transfer” Annu. Rev. Fluid Mech., 30:365-401 (1998). SuperPower Inc., http://www.superpower-inc.com/ A. Berger, M. Noe and A. Kudymow, “Recovery characteristic of coated conductors for superconducting fault current limiters”, IEEE Trans. Appl. Supercond., vol. 21, no.3, 1315-1318, June 2011, 0501605. S. Hellmann and M. Noe, “Influence of different surface treatments on the heat flux from solids to liquid nitrogen”, IEEE Trans. Appl. Supercond., vol. 24, no. 3, June 2014, 0501605. EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Increasing pulse amplitude EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016

Boiling curve EPFL Applied Superconductivity Group, ICEC26 – ICMC 2016, Delhi, March 8th 2016