YBCO Roebel Cable Research Articles

Dipole Magnets Wound Using YBCO Cables-FEM Stability Issues Modeling

Quench and protection in magnets wound with coated conductors is of great interest for many applications, including high energy particle accelerators. The relatively large slow normal zone propagation and large MQE of coated conductors and the potential of burn out in well insulated coils has led to an interest in non-insulation coils. A stability of block type dipole magnets wound using non-insulated YBCO Roebel and CORC cables has been modeled by FEM. Critical currents of the magnets have been calculated using a self-consistent method based on anisotropic I_c(B) curves of the cables. Advantages and disadvantages of using the Roebel and CORC cables in dipole magnet windings have been discussed. Measured heat disturbances of different size, intensity and duration in a pancake coil wound using a non-insulated YBCO tape have been analyzed from the viewpoint of the magnet quench and stability. The effect of CORC cable magnetization on the field quality of a canted cosine theta dipole was evaluated.

Performance and Quench Characteristics of a Pancake Coil Wound With the 2G YBCO Roebel Cable

The YBCO Roebel cable is a promising option for the high-energy accelerator magnets and tokamak devices with high-current carrying capability and compactness. Referring to the complex current sharing and normal zone propagation, it is difficult to apply the traditional superconductor quench measurement methodology to such a long Roebel cable (2 m) in an adiabatic condition. An alternative method for the quench measurement is to make a Roebel pancake coil cowound with ribbon fiberglass. This paper starts reporting the methodology to wind the coil with impregnation process and the wire installment for the quench measurement. Some initial performance measurements and a first trial of the quench measurement were processed in LN2 at 77 K to show the robust performance of this Roebel cable at all the current contacts while maintaining the cable effectively adiabatic or similar to real magnets.

Inter-strand current sharing and ac loss measurements in superconducting YBCO Roebel cables

A Roebel cable, one twist pitch long, was modified from its as-received state by soldering copper strips between the strands to provide inter-strand connections enabling current sharing. Various DC transport currents (representing different percentages of its critical current) were applied to a single strand of such a modified cable at 77 K in a liquid nitrogen bath. Simultaneous monitoring of I–V curves in different parts of the strand as well as in its interconnections with other strands was made using a number of sensitive Keithley nanovoltmeters in combination with a multi-channel high-speed data acquisition card, all controlled via LabView software. Current sharing onset was observed at about 1.02 of strand Ic. At a strand current of 1.3Ic about 5% of the current was shared through the copper strip interconnections. A finite element method modeling was performed to estimate the inter-strand resistivities required to enable different levels of current sharing. The relative contributions of coupling and hysteretic magnetization (and loss) were compared, and for our cable and tape geometry, and at dB/dt = 1 T s−1, and our inter-strand resistance of 0.77 mΩ, (enabling a current sharing of 5% at 1.3Ic ) the coupling component was 0.32% of the hysteretic component. However, inter-strand contact resistance values of 100–1000 times smaller (close to those of NbTi and Nb3Sn based accelerator cables) would make the coupling components comparable in size to the hysteretic components.

Stability, Inter-Strand Contact Resistance, and AC Losses in YBCO Roebel Cables

A dc transport current was applied to the strands of a Roebel cable at 77 K in liquid nitrogen bath. The inter-strand contact resistance was measured. It was modified either by applying a pressure on the cable at 77 K in liquid nitrogen bath or using different soldering patterns between the strands of the cable. Magnetization ac losses were measured in frequency range 50-200 Hz in applied magnetic field 4-70 mT perpendicular to the broader face of the cable to test the inter-strand contact resistance effect. High stability and very low level of coupling losses were observed in the cables even with the lowest inter-strand resistances.

The Dependence of Transport AC Loss on Temperature and DC Parallel Magnetic Field in an Eight-Strand YBCO Roebel Cable

We measure transport ac losses in an 8/2 (eight 2-mm-wide strands) YBCO Roebel cable at 77 and 65 K without dc parallel magnetic field, and at 77 K under dc parallel magnetic fields of 0-1.5 T. The transport ac loss in the Roebel cable scales with temperature and dc parallel magnetic field as the square of the cable's critical current.

Development of a 1 MVA 3-Phase Superconducting Transformer Using YBCO Roebel Cable

We present design details for the planned construction of a 3-phase 1 MVA 11 kV/415 V transformer using YBCO Roebel cable (YBCO—Yttrium Barium Copper Oxide second generation high temperature superconducting coated conductor). The YBCO Roebel cable is a promising technology for carrying high currents. It simplifies manufacture of the winding while managing AC loss. We present the transformer winding design which follows a simple layout with the low-voltage windings utilizing a 15 strand <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex></formula> 5 mm (15/5) Roebel cable in a single-layer 20 turn solenoid. The target rated current capacity of the cable is 1500 A rms at 77 K. The high-voltage winding will be in the form of a stack of double pancake coils arranged on a composite former outside the low-voltage windings. The coils will be immersed in sub-cooled liquid nitrogen with a target maximum operating temperature of 70 K. The development methodology is described along with the results of experiments and modeling to validate the performance characteristics of the windings.

Transport AC Loss Characteristics of a Five Strand YBCO Roebel Cable With Magnetic Substrate

Transport AC loss in a short length of 5/2 YBCO Roebel cable (five 2 mm wide strands) with weakly magnetic substrate is measured. The AC loss data are compared with those for a single strand. All the strands composing the cable and the single strand are cut from the same stock material. Transport AC loss in the 5/2 Roebel cable is larger than that predicted by both the Norris strip and ellipse models over a large range of the cable current. The difference between normalized AC losses in the Roebel cable and the single strand is larger at low <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> than at high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> is the amplitude of transport current. These AC loss characteristics were not observed in transport AC loss results for YBCO Roebel cables composed of coated conductor with non-magnetic substrate. Results from an electromagnetic analysis of two parallel stacks of wire with magnetic substrate are presented to elucidate the results.

The dependence of AC loss characteristics on the spacing between strands in YBCO Roebelcables

Transport AC loss in a short length of 9/2 YBCO Roebel cable, (i.e. with9 × 2 mm width strands), with 0.25 mm spacers between the strands is measured. Thefrequency varies from 59 to 354 Hz. The result is compared with the loss for acable without spacers between the strands. Transport AC loss is decreased by thepresence of the spacers. The AC loss reduction due to the extra spacing is moresignificant when the amplitude of the cable current is small compared to the cableIc.The losses in the cable with spacers normalized by the square of the cable currents plotted againstIt/Ic approximately agree with those in a cable without spacers. Electromagnetic modelling wascarried out for 9/2 and 8/2 cables, modelled as parallel stacks, to assist in understandingthe above experimental results. The 8/2 configuration allows the greater use of symmetryto speed computations. Reasonable agreement between the numerical results and themeasured results was obtained. This supports suggestions made in previous publicationsthat the transport AC loss in a Roebel cable is roughly equivalent to the loss in twoparallel stacks carrying the same current in each tape. The electromagnetic analysis in the8/2 stacks shows the flux lines are more perpendicular to the strand face when the verticalspace between strands is smaller, and this leads to a larger induced electricalfield and larger AC loss. At small current amplitudes, the modelling shows thespacing has a strong effect on the AC loss in the surfaced part of the strands.

Development of YBCO Roebel cables for high current transport and low AC loss applications

We discuss production of lengths of up to 27 m of YBCO Roebel cable. Results for 5/2 (5 strands, 2 mm width), 9/2 and 15/5 cables produced from standard 12 mm commercial YBCO wire are presented. We discuss specifications for the input wire and suggest using a statistical correlation function, using data from magnetic field scanning, that is shown to produce high performance strands. We discuss advances in manufacturing techniques including cable insulation processes. Transport and magnetic AC loss data are presented for 5/2 cable which demonstrates the effectiveness of decreased strand width and the transposition of strands. Both losses are predominantly hysteretic in nature. Finally, the cable DC transport is presented and we discuss the possibilities for high current cables in high field applications.

Transport AC loss characteristics of a nine strand YBCO Roebel cable

Transport AC loss in a short length of 9/2 YBCO Roebel cable (nine 2 mm wide strands) ismeasured. The AC loss data are compared with those in a 5/2 YBCO Roebel cable (five 2 mmwide strands) as well as that in a single strand. All the strands composing the cables andthe single strand are insulated and cut from the same stock material. The validity of themeasurement method was reconfirmed by results at a range of frequencies. At a wide range ofIt/Ic, the normalized AC losses in the Roebel cable were around 6.2–6.7 times of those in the singlestrand. This is less than the nine times predicted for a tight bundle of nine conductors. Thenormalized transport AC losses in the 5/2 Roebel cable are much smaller than those in the 9/2Roebel. This should be due to larger superposition of magnetic field in the 9/2 Roebel. TheIc of the 9/2 and 5/2 Roebel cables is determined by serial connection of the strands. Thiseliminates the effect where differing resistances in the current terminations causeuneven current sharing between strands when the strands are connected in parallel.

Transport AC loss measurement of a five strand YBCO Roebel cable

Transport AC loss in a 5/2 YBCO Roebel cable (five 2 mm wide strands) with a pitchlength of 90 mm is measured over a range of frequencies from 59 to 354 Hz. Five rectangularvoltage loops are arranged from pairs of voltage taps attached at one pitch lengthseparation on each strand. There are significant differences in the apparent transport ACloss measured from the different voltage loops at low frequency due to phase shifts in thecurrent in each strand with respect to the phase of the total cable current. The difference intransport AC losses measured from different voltage loops becomes small with increasingfrequency, because current is more equally distributed in each strand due to the higherreactance at high frequency. At 354 Hz, results measured with different voltageloops agree well with each other. As theoretically expected, the transport AClosses at different frequencies calculated from the mean value of in-phase voltagesmeasured from the five different voltage loops agree well with each other. AtIt/Ic = 0.85, the transport AC loss in the 5/2 Roebel cable normalized by the number ofstrands and square of critical current per strand is around 2.9 times that in a singlestrand compared to the prediction of five times for a close bundle composed offive conductors. This difference may be due to the transposition of the strands.

Progress in the Manufacture of Long Length YBCO Roebel Cables

We have manufactured lengths of up to 10 m of Roebel cable from wide strips of second generation YBCO wire. The wide strips are first screened using magnetic scanning to check for superconductor defects to avoid incorporating poor quality wire into the cable. Manufacturing issues such as the long length registration of the strand crossovers and the maximum yield from wide strip is discussed. Differences in cutting and winding due to different tape substrate materials are shown to be minimal. We report electrical measurements on samples of HTS Roebel cable. Insulation which protects the assembled conductors from environmental degradation is reported.

Narrow strand YBCO Roebel cable for lowered AC loss

We have constructed test lengths of Roebel cable from wide strips of second generation YBCO wire. The strand width is 2mm to allow for lowered AC losses in comparison with standard HTS wires. Up to 10 strands can be cut from the 40mm wide strip and assembled into a 10 strand cable with a transposition length of 90mm. Electrical measurements show good retention of critical current through the cutting and cabling processes. Initial AC loss measurements confirm the reduction expected from full width wire. Results from mechanical modeling are presented which have been used to optimise strand geometry to reduce stress concentrations. Manufacturing capability to produce up to 100m lengths has been demonstrated.