Physica C: Superconductivity and its applications 530 (2016) 65–67
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Progress in American Superconductor’s HTS wire and optimization for fault current limiting systemsR Alexis P. Malozemoff∗ American Superconductor Corp., 64 Jackson Rd., Devens, MA 01434-4020, USA
a r t i c l e
i n f o
Article history: Received 16 January 2016 Revised 28 March 2016 Accepted 29 March 2016 Available online 7 April 2016 Keywords: HTS wire Coated conductor Fault current limiter HTS power cables
a b s t r a c t American Superconductor has developed composite coated conductor tape-shaped wires using high temperature superconductor (HTS) on a flexible substrate with laminated metal stabilizer. Such wires enable many applications, each requiring specific optimization. For example, coils for HTS rotating machinery require increased current density J at 25–50 K. A collaboration with Argonne, Brookhaven and Los Alamos National Laboratories and several universities has increased J using an optimized combination of precipitates and ion irradiation defects in the HTS. Major commercial opportunities also exist to enhance electric power grid resiliency by linking substations with distribution-voltage HTS power cables [10]. Such links provide alternative power sources if one substation’s transmission-voltage power is compromised. But they must also limit fault currents which would otherwise be increased by such distribution-level links. This can be done in an HTS cable, exploiting the superconductor-to-resistive transition when current exceeds the wires’ critical J. A key insight is that such transitions are usually nonuniform; so the wire must be designed to prevent localized hot spots from damaging the wire or even generating gas bubbles in the cable causing dielectric breakdown. Analysis shows that local heating can be minimized by increasing the composite tape’s total thickness, decreasing its total resistance in the normal state and decreasing its critical J. This conflicts with other desirable wire characteristics. Optimization of these conflicting requirements is discussed. © 2016 Elsevier B.V. All rights reserved.
1. Introduction This article reviews recent progress at American Superconductor Corp. and its collaborators in developing high temperature superconductor (HTS) wire for electric power applications [1]. American Superconductor’s HTS wire [2], the so-called second generation (2 G) HTS wire, is a coated conductor using an approximately 1 μm thick layer of Dy-doped YBa2 Cu3 O7 (YBCO) superconductor deposited by metal–organic deposition (MOD) on a textured tape-shaped template consisting of a 75 μm thick Ni-5 at %W or Ni-9 at %W RABITSTM substrate coated with 75 nm thick magnetron-sputtered buffer layers of Y2 O3 , YSZ and CeO2 . A silver passivation layer is then deposited on both sides of the stack by magnetron sputtering. The initially 46 mm wide strip is roll-slit to desired width, usually either 4 mm or 1 cm, and encased between two soldered stabilizer tapes, either copper, brass or stainless steel, depending on the application, to finalize conductor.
R 28th International Symposium on Superconductivity, ISS 2015, November 16-18, 2015, Tokyo, Japan. ∗ Corresponding author. Tel.: +1 508 243 9693. E-mail addresses:
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http://dx.doi.org/10.1016/j.physc.2016.03.017 0921-4534/© 2016 Elsevier B.V. All rights reserved.
A challenge in manufacturing such wire efficiently and at low cost is dealing with the diversity of applications, each requiring different conductor specifications. This article will focus on two electric power applications: MW-scale rotating machinery (motors and generators) in the power grid, and fault current limiters including fault current limiting power cables. 2. Rotating machinery Designs for MW-scale HTS rotating machinery typically call for HTS wire with performance of 10 0 0 A/cm-width in magnetic fields of 1–4 T and temperatures from 25 to 50 K, along with high conductivity copper stabilizer [2–4]. While performance far exceeds this goal in short samples [5], obtaining consistent performance over length has been challenging. So it is of interest to demonstrate such performance in wires manufactured with the MOD process, which have generally shown more consistent results over length, but have usually achieved only about half the target level, even though the wire performance has been optimized with a combination of pinning centers including insulating nano-precipitates (typically in the 100 nm dia. range), dislocations and stacking faults [2]. Ion irradiation using a particle accelerator offers a controlled post-process for introducing smaller defects into the Ag-passivated
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A.P. Malozemoff / Physica C: Superconductivity and its applications 530 (2016) 65–67
Fig. 1. (a) Critical current per width of American Superconductor tape versus perpendicular magnetic field, for different dosages (per cm2 ) of 16 MeV gold ions from a tandem Van de Graaff facility [2]. (b) 77 K self-field critical current per width over wire length after reel-to-reel irradiation with 6 × 1011 /cm2 16 MeV Au ions [2]. Pre-irradiation value is from measurements on pieces cut off from the ends of the wire before irradiation.
strip. This approach has been investigated by American Superconductor in collaboration with Argonne, Brookhaven and Los Alamos National Laboratories, Western Michigan University, University of Illinois and the Atominstitut of the Technical University of Vienna. Initial work with 4 MeV proton irradiation demonstrated the required doubling of performance, with the protons creating ∼5 nm dia. defects [6], but the time needed for the required dosage of 2 × 1017 protons/cm2 was too long to allow a practical process for long-length wire. A significant acceleration of the irradiation process to double performance was achieved using 3.5 MeV oxygen ion irradiation at a level of 1 × 1013 oxygen ions/cm2 , enabling an exposure of only 1 s per 0.8 cm2 [7]. And most recently, a major further acceleration was achieved using a dose of 6 × 1011 /cm2 of 16 MeV Au ions using the Brookhaven National Laboratory tandem Van de Graaff accelerator. The ion energy was chosen based on SRIM/TRIM calculations to assure relatively uniform penetration of Au ions through the overlying Ag passivation layer and into the YBCO layer [8]. Critical current measurements on short, non-moving irradiated samples in Fig. 1a, taken at 27 K, show that at the optimal dose of 6 × 1011 Au/cm2 , performance exceeds 10 0 0 A/cmwidth at perpendicular magnetic fields up to 5 T, meeting the target [2]. It should be noted that irradiation at this level depresses the superconductor transition temperature Tc by about 2 K, and so the 77 K performance is reduced; nevertheless enhanced pinning gives dramatic performance enhancement at lower temperatures. To confirm that these static, short sample results can be replicated during roll-to-roll processing of long wire lengths, 80 m of 46 mm wide strip was processed in a roll-to-roll configuration in the same Van de Graaff accelerator at a rate of 10 m/h. The ion beam was rastered across the sample to assure uniformity across strip width. So far, measurements over length have only been taken at 77 K, as shown in Fig. 1b, and as discussed above, performance is depressed at this temperature. A key takeaway from Fig. 1b is the process uniformity over the entire 80 m; however critical current uniformity must still be checked in the 27–40 K range of device operation. Since a 46 mm wide strip can be slit into up to ten 4 mm wide tapes, this corresponds to a processing rate for 4 mm wire of 100 m/h. It is believed further optimization of the process may allow this to be commercially viable. Further details are reported in references [2,6] and [7].
3. Fault current limiting (FCL) devices Fault current limiting cables are a major commercial opportunity [9,10]. In many urban grids, substations fed by high voltage power lines supply distribution level power to specific neighborhoods. Grids could be made more resilient against power interruptions by linking substations at the distribution level so that loss of high voltage supply at one substation could be replaced from another substation. Impeding this solution are a) the requirement for the link between substations to carry very high current, since the voltage is only at a distribution level, and (b) the increase in grid fault current resulting from the new link. The latter issue is critical since many urban grids are already near their fault current limit. FCL HTS cables address these issues. They carry large currents and can limit fault currents. To design 2 G HTS wire for such an application, one must understand the remarkable studies of Kraemer et al. [11] on the response of YBCO wire strips deposited on crystal substrates to voltage steps. Using a high speed CCD camera, they detected wire regions which were driven normal—,i.e. “hot spots”—by onset of bubbling in surrounding liquid nitrogen (lN2 ). The higher the voltage, the farther these hot spots extended along the wire length, while they always extended across the full wire strip width; so total wire resistance R was given by ρ L/A, where ρ was the normal state YBCO resistivity (averaged over temperature), L was total hot spot length and A was YBCO wire strip cross section (thickness times width). The wire current was close to the critical current Ic ,; so the voltage RIc = ρ LIc /A was proportional to L. Thus the effective total resistance of the wires was proportional to voltage, an unusual resistance indeed! The same phenomena apply to 2 G HTS wire except that the normal state resistivity ρ net is now the net parallel resistance of all conducting components of the wire stack: the NiW substrate, the solder, the normal YBCO and the Ag passivation layer. Only at sufficiently high voltage does the entire wire switch, at which point the resistance becomes constant (aside from the initially small changes in resistivity as the wire heats up). Why do the hot spots form? It must be recognized that no wire has a perfectly uniform critical current; it varies along length within a certain range, ideally fairly tightly around the average critical current Ic . As the current rises in response to an increase in voltage, the regions with lowest critical current are driven normal first, then the next highest and so forth until the voltage across
A.P. Malozemoff / Physica C: Superconductivity and its applications 530 (2016) 65–67
the normal regions rises to equal the applied voltage. Coexistence of normal and superconducting regions is made possible by the sharp rise in the V–I curve at the local Ic . As long as critical currents along the wire cluster closely around the average, the above formulas apply to a good approximation. In an FCL device, such a process arises when the load resistance is short-circuited, applying the full system voltage to the FCL device (plus any other series grid components such as transformers) [10]. The rapid appearance of the normal (hot-spot) regions keeps the current near Ic , thus protecting the grid from potentially much higher fault currents. This persists till circuit breakers open, cutting the current; this time is called the fault hold time τ . Hot spot temperature must be limited to avoid wire damage in a standalone FCL device; this temperature can usually be well above room temperature. However, in an FCL cable, the limit is the much lower temperature at which bubbles nucleate in the liquid nitrogen (lN2 ), since bubbles can initiate voltage breakdown. Fortunately, because lN2 in HTS cables is pressurized, e. g. 15–20 bar, its boiling point rises to 111–116 K respectively. For cables operating sub-cooled around 70 K, a safe margin for temperature rise could be set at around 35 K. A conservative first approximation to predict temperature rise is adiabatic hot-spot heating, ignoring dissipation down the wire length or into the lN2 bath. Also, once hot spots are driven normal at the top of the ac cycle, they are assumed to stay normal. So if peak current is Ic , the root-mean-square current over a power cycle (50 or 60 Hz) is Ic /2 and the average power absorbed adiabatically into the wire per volume is just ρ net Jrms 2 = ρ net (Ic /A)2 /2 . Then
T = ρnet (Ic /A )2 τ /2C,
(1)
where C is the average specific heat (per volume, averaged over the temperature range T). Eq. 1 shows that to limit heating, lower net resistivity and critical current are desirable, along with higher thickness (i.e. higher A). Of course, from a cost perspective, higher Ic is desirable to reduce the number of parallel wires required to meet the cable’s current specification. A higher normal state electric field ρ net Ic /A (hence higher ρ net ) is also desirable to reduce the wire length required to meet the fault current specification. Fortunately, the required cable length is usually long enough to avoid increasing ρ net . In fact, often ρ net can be kept sufficiently low to avoid having to decrease Ic too much to meet the temperature limit of Eq. (1). For example, with ρ net = 0.03 μm (case of thick copper stabilizer with NiW substrate), τ = 0.2 s, C = 2 MJ/m3 K, Ic per width = 500 A/cm-width and total thickness = 323 μm, T = 33 K, meeting the safe margin set above.
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In short, hot-spot formation requires conductor design criteria which initially seem counterintuitive, but are essential for reliable and safe operation of fault current limiting devices. American Superconductor has used these concepts in designing wires for fault current limiting cable applications such as the Con Edison Hydra Project in New York [9] and the more recently announced ComEd project in the downtown Loop of Chicago [10]. Choices for ρ net (including stabilizer type and thickness) and Ic are strongly dependent on the specifics of each application. While some short cable data has been reported [9], experimental results on the full cables are not yet available. Acknowledgments The author thanks Marty Rupich and Steve Fleshler for input on the irradiation work and careful reading of the manuscript, and Hans-Peter Kraemer and Wolfgang Schmidt for elucidation of their switching results on YBCO thin films. Irradiation work was supported by ARPA-E and Center for Emergent Superconductivity funded by USDOE. References [1] A.P. Malozemoff, The power grid and the impact of high-temperature superconductor technology: an overview, in: C.M. Rey (Ed.), Superconductors in the Power Grid, Woodhead Publishing, Cambridge UK, 2015, pp. 159–170. [2] M.W. Rupich, S. Sathyamurthy, S. Fleshler, Q. Li, et al., Engineered pinning landscapes for enhanced 2 G coil wire, in: IEEE Transactions on Applied Superconductivity (proceedings of EUCAS 2015, Lyon, France, Sept. 6-10, 2015 to be published. [3] J.W. Bray, High-temperature superconducting motors and generators for power grid applications, in: C.M. Rey (Ed.), Superconductors in the Power Grid, Woodhead Publishing, Cambridge UK, 2015, pp. 325–344. [4] G. Snitchler, B. Gamble, C. King, P. Winn, 10 MW class superconductor wind turbine generators, IEEE Trans. Appl. Supercond. 21 (3) (2011) 1089–1092. [5] V. Selvamanickam, M.H. Gharahcheshmeh, A. Xu, Y. Zhang, E. Galstyan, Critical current density above 15 MA/cm2 at 30 K, 3 T in 2.2 μm thick, heavily doped (Gd,Y)Ba2 Cu3 Ox superconductor tapes, Supercond. Sci. Tech. 28 (2015) 072002. [6] Y. Jia, M. LeRoux, D.J. Miller, J.G. Wen, et al., Doubling the critical current density of high temperature superconducting coated conductors through proton irradiation, Appl. Phys. Lett. 103 (2013) 122601. [7] M. LeRoux, K.J. Kihlstrom, S. Holleis, M.W. Rupich, et al., Rapid doubling of the critical current of YBa2 Cu3 O7-δ coated conductors for viable high-speed industrial processing, Appl. Phys. Lett. (2015). [8] J.F. Ziegler, J.P. Biersack and M.D. Ziegler, SRIM, The stopping and range of ions in matter. [9] J. Maguire, D. Folts, J. Yuan, N. Henderson, et al., Status and progress of a fault current limiting HTS cable to be installed in the Con Edison grid, Adv. Cryogenic Eng. 55A (2009) 445–452. [10] A.P. Malozemoff, J. Yuan, C.M. Rey, HTS AC cables for power grid applications, in: C.M. Rey (Ed.), Superconductors in the Power Grid, Woodhead Publishing, Cambridge UK, 2015, pp. 133–188. [11] H.-P. Kraemer, W. Schmidt, B. Utz, H.-W. Neumueller, Switching behaviour of YBCO thin film conductors in resistive current limiters, IEEE Trans. Appl. Supercond. 13 (2003) 2044–2047.