Current distribution effects in a 2.6 ka 2G-Roebel bar

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1 Current distribution effects in a 2.6 ka 2G-Roebel bar University of Houston 4th.-6th. Dec W. Goldacker, A. Frank, A.Kudymow, R. Heller, S. Terzieva, A. Kling, C. Schmidt Forschungszentrum Karlsruhe, Institut f. Techn. Physik, Hermannvon-Helmholtz-Platz 1, Eggenstein-LeopoldshafenPhone: , Fax: , wilfried.goldacker@itp.fzk.de Motivation Introduction to new Cable-design Current measurements Interstrand connection Self field effects Conclusions 1 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

2 Reduced size and better economy with high current HTS cables 2 examples with very high operation current : DEMO should follow ITER: HTS Ic > 30 ka Igazu Falls Brazil Water Power Dam Itaipu Brazil One Generator (Siemens): 737 MVA Current: 23.6 ka (winding up to 100 C) 2 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

3 New 2G - Roebel cable design for increased currents Principle of Roebel design New! 15 strands with 3 CC tapes each ( no additional strand coupling ) 3 fold number of CC* ( 45 CC ) Same transposition length* 19 cm Aspect ratio of cross section smaller (* More is a possible option ) Old cable Aspect ratio 13.3 New cable Aspect ratio 5.2 Increased transport current 3 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

4 Variation of CC currents (data provided by Superpower) 3 batches CC (12 mm width) A,B,C ; average 299 A/cm-w. Strand punching gives approx. 3% current loss No.1 No.6 No.7 No.12 No Strand current is assumed as 5/12 of orig. CC I c = 6.5 ka expected for the cable 4 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

5 DC transport current measurements on three selected strands One representative strand from each CC batch 3 to 4 transposition lengths measured simultaneously Contact resistance also measured Cu shunt 25 mm2 in parallel Loose strand stacking Strands touch each other Voltage contacts at step over section 5 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

6 DC transport current of RACC 4 at 77 K s.f. Cable current defined as average value from 3 strands I c = 2628 Amps. (5 µv/cm criterion) Calcul Amps. for no losses J c eng. = 9.52 kacm -2 (5 µv/cm criterion) One warm-up cycle : already slightly new current distribution Spikes and steps come from movements and change of strand connections 6 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

7 Current distribution along different strands Measurement of different sequential sections with one transposition length each Black graph = whole length Also in different transposition sections along one strand very different behavior of the voltage potentials 7 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

8 Self field effect on strands in single Roebel cable The modelling (Biot-Savart-approach) was made assuming a 7 strand and a 8 strand stack, simplified to 21 and 24 layers of CC The highest field 280 mt is inside the stack volume! For load line considerations mean B perp. and max B perp was discussed More: W.Goldacker accept. for SUST Each strand goes through all self field situations along one transposition length : Current redistribution occurs But better situation in windings expected!! 8 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

9 Load lines for the cable from modelling Current degradation from self field: 60 % Model describes the self field effect quite well (graphs are extrapolated from single tape values to calculated cable expectation value) Local self field maximum 9 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

10 Hall probe scanning (R.Fuger et al. ATI Vienna, EUCAS 2007, Proc.) Voltage taps do not contact exactly to a cable cross section Stack 1 Gap Stack 2 The potential of the contact is not clearly defined! 10 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

11 Features of current clamps of RACC-cable Cu stabilisation is cut at inner side from punching Buffer forms insulating layers in the stacks Space between stacks is filled with solder Copper block Solder Half transposition 100 mm length was soldered Half the strands have good current feed in conditions, but only the CC tapes on top 80% of CC have complex current percolation Insulating! Cu Ag YBCO Buffer 11 W.Goldacker Institut für Technische Physik CCA2008 Houston USA Hastelloy

12 Performance of current feed in contacts Length of current feed-in: 100 mm = half transposition length Two ends have different contact resistance Contact E Strand 7 15 on top (= less good batch) Contact K Strand 2-9 on top (= good batch) Contact over whole transposition length 12 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

13 Interstrand - coupling in Roebel - cable Investigation of interstrand coupling technique on 4 mm wide cable: 2 different impregnations applied (under moderate pressure) Steel dummy 2mm 4 mm cable Reduced AC losses Contact resistance measured 12 mm cable Ag resin Resistance between strands ( for one transposition length ) Unfilled resin About 2 - times higher resistance 13 Sample Conductive resin Unfilled resin All strands 0.95 mω ± mω ±0.7 Neighboured strands 0.74 mω ± mω ±0.8 Opposite strands 1.04 mω ± mω ±0.5 W.Goldacker Institut für Technische Physik CCA2008 Houston USA

14 Conclusions and Outlook Upgrade of RACC-cable current successful! Further transport current upgrade possible! Cable behaviour becomes complex: current redistribution not trivial! Current lead/joint needs improvement due to inherently complex CC architecture Moderate strand coupling is necessary! More detailed analysis of current distribution is actually in work, monitoring all strands >> 10 ka class cables need a new design as cabling high current Roebel strands ( cable idea for DEMO? ) 14 W.Goldacker Institut für Technische Physik CCA2008 Houston USA