Perspectives in power applications of low and mainly high temperature superconductors: energy, transport and industry

Worldwide there is an increasing need for a more sustainable form of electrical power delivery with a growing share of renewable energy generation. In the distribution and transmission network, large-scale and small-scale wind and solar power plants will be introduced, in proportion to the annual economic growth. The transmission and distribution network will be expanded, focusing on the electricity transport, however, there will also be a need for exchanges with neighboring countries. Alternative solutions are needed in order to support the changes of the future grid. High temperature superconductors are an alternative to conventional conductors, due to their high current density and very low AC loss, and therefore deserve more attention. The purpose of this study is to explore ways to integrate high-temperature superconducting cables in a future network and to compare their favorable technical properties with, e.g., the conventional XLPE cable. The development of 2nd generation high temperature superconducting tapes results in a high tape quality, making it very attractive for use in superconducting power transmission cables. At the same time, the network requirements placed on the grid, based on society needs are changing, such as low magnetic field emissions, reducing space requirements, lower losses, minimizing visual intrusion, etc. Our study shows that superconducting cables compared to conventional cables score better on these societal requirements. From our comparison of three practical low and high temperature superconductors we can conclude that Yttrium Barium Copper Oxide is the most suitable superconductor for use in transmission cables. Our techno-economic analysis shows that superconducting cables become already competitive with conventional cable in the AC transmission, such as XLPE cables. Possible future problems concerning the transport capacity in the power grid where high temperature superconducting cables can offer a solution have been identified. For one promising location, we have formulated the requirements for the design of a high-temperature superconducting cable. Next, we propose two types of cable systems (with cold and warm dielectric). For both types we describe the core, the electrical insulation, the screen, the cryostat, the cooling system, etc. Also for the distribution grid a techno-economic investigation is conducted. To assess the feasibility of the application in distribution grids, we have experimentally demonstrated a reduction of AC conductor losses from 1 W/m to 0.1 W/m. We also carried out an experimental investigation to improve the developed cryostat design for a 6 km long cable connection. Despite these substantial technical improvements our economic study showed that the high-temperature superconducting distribution cable is not yet competitive with the present conventional distribution cable systems except for niche locations where additional advantages e.g. magnetic emission, reduced space usage, power density weighs more heavily. Besides the HVAC grid the Netherlands has HVDC interconnections with neighboring countries and there are initiatives for the use of DC high voltage connections to wind farms further out to sea. In our study, we make reference to a suitable location, where the above mentioned attractive features of the superconducting cable are applicable as well. We advised a modified design of a HTS HVDC cable which enables a possible upgrade of the transmission capacity of the HVDC link at such location. The main results from the investigation are that: • Based on our techno-economic analysis HTS cables offer the most competitive solution in transmission grids. Introduction of such cables will reduce HTS tape price, which in turn will stimulate further applications. • Conceptual designs of competitive HTS AC and DC transmission cables are formulated along with that for HTS AC distribution cable. Novel designs allow for much longer length between cooling stations. • Our experimental research has shown that HTS cable core losses may be reduced by a factor 10 (down to 0.11 W/m/phase at 3 kArms, 77 K, 50 Hz). • Dedicated low friction cable cryostat was developed and successfully tested for 47 meters length. Patented multi-layer thermal insulation improves the cable cryostat heat leak from 1 W/m to 0.1 W/m.

Recently, various high temperature superconducting (HTS) power applications have been developed and prepared for field tests and commercial applications. Comparing to conventional power applications, it could offer several advantages such as reduced size and weight, high efficiency, decreased losses, no oil, nonflammable and decrease of CO2 emissions. Besides overload operation is possible with no loss of lifetime. For HTS power applications in low temperature and high voltage environments, partial discharge (PD) measurements in cryogenic dielectric materials of HTS power applications are very important because PD was regarded as primary source for ageing and breakdown of cryogenic materials. One of the diagnostic methods for safety of the power components, the detection of PD taking place inside the apparatus has been widely investigated. The first method, phase resolved partial discharge (PRPD) Analysis was developed in the early 1970s taking the phase information of the applied AC voltage into account. We also proposed a pattern analysis method named chaotic analysis of PD(CAPD) for PDs occurred in liquid nitrogen, considering three normalized parameters obtained from the values between two consecutive PD pulses: amplitude difference (P-t), occurring time difference (T-t) and correlation between T-t and P-t. This pattern analysis method can identify the type of defects by means of PD pattern classification without employing the phase information of the applied voltage signal. For the experimental investigation, three artificial defects have been fabricated considering possible defects formed during the manufacturing process of HTS power applications: turn to turn insulation, floating particle and protrusion. And PD signals originated from these artificial defects are measured and analyzed by means of CAPD. Throughout this work, it seems that the correlation between the consecutive PD pulses, depending on the nature PDs, could be clarified by CAPD.

The traditional distinction between tunnel and highly transmissive barriers does not currently hold for high critical temperature superconducting Josephson junctions, both because of complicated materials issues and the intrinsic properties of high temperature superconductors (HTS). An intermediate regime, typical of both artificial superconductor–barrier–superconductor structures and of grain boundaries, spans several orders of magnitude in the critical current density and specific resistivity. The physics taking place at HTS surfaces and interfaces is rich, primarily because of phenomena associated with d-wave order parameter (OP) symmetry. These phenomena include Andreev bound states, the presence of the second harmonic in the critical current versus phase relation, a doubly degenerate state, time reversal symmetry breaking and the possible presence of an imaginary component of the OP. All these effects are regulated by a series of transport mechanisms, whose rules of interplay and relative activation are unknown. Some transport mechanisms probably have common roots, which are not completely clear and possibly related to the intrinsic nature of high-TC superconductivity. The d-wave OP symmetry gives unique properties to HTS weak links, which do not have any analogy with systems based on other superconductors. Even if the HTS structures are not optimal, compared with low critical temperature superconductor Josephson junctions, the state of the art allows the realization of weak links with unexpectedly high quality quantum properties, which open interesting perspectives for the future. The observation of macroscopic quantum tunnelling and the qubit proposals represent significant achievements in this direction. In this review we attempt to encompass all the above aspects, attached to a solid experimental basis of junction concepts and basic properties, along with a flexible phenomenological background, which collects ideas on the Josephson effect in the presence of d-wave pairing for different types of barriers.

This Ph.D. thesis is focused on the development of novel models for calculation of AC losses, current and magnetic field profiles in high-temperature superconductors (HTS). The thesis is concentrated on the modelling of Bi-2223 conductors at 77 K, which for the moment have the most advanced manufacturing technology and will be primarily used in the first large-scale power applications of high-temperature superconductors. The analysis of AC losses in Bi-2223 conductors is the leading thread in the structure of the thesis. An introduction to high-temperature superconductivity is made with special emphasis on the mechanisms of AC losses in HTS. Presented are some of the most promising power applications of HTS materials together with a discussion on the required improvements in their performance. A model for dissociating the hysteresis, eddy-current and resistive flux-creep loss contributions, based on relatively broad-range frequency measurements on Bi-2223 tapes has been used for analysis of the frequency behaviour of the transport-current loss in self-field. The hysteresis, eddy-current and flux-creep loss components have been separated due to their different frequency dependence. The study of eddy current loss in the silver sheath and matrix has been complimented by numerical simulations using a simple electromagnetic model of HTS tapes. Described is an original method for estimating the performance of Bi-2223 tapes in typical power grid perturbations by experiments and analysis of over-critical current excursions of various waveform, frequency and current amplitude up to 20×Ic. For precise calculation of the AC losses and studying the electromagnetic properties of HTS with smooth current-voltage characteristics and complex geometry, the finite element method (FEM) has been used. The implementation of the power-law model of the E-J characteristic of HTS into the FEM software package Flux2D is presented. The Flux2D implementation has been validated by means of comparison with results from theoretical predictions, electrical measurements, and other numerical methods. The significance of the power index n and the lateral distribution of the critical current density Jc in multifilamentary HTS tapes has been evaluated by FEM simulations. New anisotropic models of Jc(B) and n(B) for textured Bi-2223 materials have been developed. The models are based on experimental data; they are fairly simple and take into account the orientation of the local magnetic field. These models have been used in FEM simulations on monofilamentary and multifilamentary Bi-2223 conductors with different geometry and filament arrangement. In conductors with given Jc(B) dependence, the notion of effective AC critical current has been defined. The AC losses in Bi-2223 multifilamentary flat tapes and wires of round and square geometry in various operating conditions have been calculated and compared. The AC loss analysis has been supported and complimented by current density and magnetic flux distributions in the different conductors. The influence of the shape factor of the geometry and the filament orientation with respect to the local magnetic field has been thoroughly investigated. The optimal geometry and filament arrangement have been determined for each application.

Superconducting devices tend to operate in one of three general regimes. Applications which are based on the zero resistance of the superconducting state, those based on the transition between the superconducting and normal state, and those based on Josephson junctions. When speaking about energy applications the main interest is focused on the first and second regime. Electrical power applications have been a challenge for high-temperature superconductor (HTS) materials since their discovery. Today, however, the only power application that has been commercialized is superconducting magnetic energy storage (SMES) and all those units are based on NbTi technology at 4.2 K. Success of HTS devices in the energy sector depends on developing materials to techno-economic maturity and organizing efficient production, making model tests of key components and proving reliability in endurance tests. In general superconducting power applications offer a number of benefits to the electrical system: increased energy efficiency, reduced equipment size, reduced emissions and flexible transmission and distribution. This paper reviews the current status of superconducting power technology in Europe where some consortia have been established. SCENET Power (The European Network for Power Applications of Superconductivity) is formed by working groups of a small number of laboratories collaborating in pilot projects and CONECTUS is the Consortium of European Companies Determined to use Superconductivity.

Future electrical propulsion systems for electric aircraft need lightweight and compact electrical machines with high power density, high efficiency, and high power-to-weight ratio (PWR). High temperature superconducting (HTS) electrical machine is expected to be a promising technology to meet these challenging demands due to the ultrahigh current density, near-zero DC resistance, light weight, and compact volume of the 2 G HTS REBCO coated conductors. However, in traditional superconducting machine design processes, the unique properties of the HTS machine windings are often oversimplified. These oversimplifications make it challenging to accurately estimate the critical current and loss of the HTS machine windings under the complex background magnetic fields inside the electrical machines. This paper proposes a magnetic vector potential boundary (MVPB) method to design and optimize a 3 MW fully HTS synchronous electrical machine (FHTS-SM). The results show that the MVPB method can consider the superconducting properties in the design and optimization of the electrical machine, and it has highly accurate results. Meanwhile, the MVPB method significantly reduces the quantity and complexity of computations, and the computation time is reduced by 96%. Through a joint simulation (modelFRONTIER & MATLAB & COMSOL), multi-objective optimization was conducted on the 3 MW FHTS-SM design to obtain the optimal design. Moreover, the HTS armature winding is optimized by the multifilamentary method to reduce AC loss. The results show that the optimal design can increase the PWR (machine only) by approximately 25% and improve the efficiency by about 8% simultaneously. This work provides a powerful tool to effectively and accurately estimate the loss of the HTS machine windings at a stable operation state. Meanwhile, it offers a good understanding of designing high power-to-weight and high efficiency superconducting electrical machines. Additionally, the simulation results can provide a basis for the R&D of the superconducting electrical machine prototype for future electric aircraft.

According to the recent design of an HTS (high temperature superconducting) power application whose capacity is hundreds MVA, the rated current is over thousand amps. Considering the performance of the recent HTS tapes, it is inevitable to use several HTS tapes in parallel for high capacity current. In this paper, we have investigated the influence of current distribution on the AC loss in the multi-stacked superconducting wire. The dependence of AC loss and critical current on the current distribution in multi-stacked wires was discussed. We fabricated multi-stacked wires and measured the current distribution at 77 K. The current distribution was indirectly found from the self-field distribution. The magnetic field was measured by an array of hall sensors located around the HTS superconducting wires. In the case of non-uniform distribution of current, the performance of multi- stacked HTS wires is worse than that of uniform current distribution.

Large AC loss from the second generation (2G) high temperature superconducting (HTS) wires has been one of the major bottlenecks in power applications with HTS materials. Moreover, the large power applications also require the large current capacity from the HTS wires, which makes them produce larger AC losses. In order to reduce the AC loss from the HTS conductors with large current capacity, an HTS compact cable with some striations on the superconducting layers has been proposed. In this paper, we prepared some sample HTS compact conductors with striations, and measured their magnetization loss from the external magnetic field. We also made some slits on the superconducting layer of the HTS wire by laser cutting to reduce the aspect ratio of the superconducting layers. It would make the low eddy current loss and magnetic decoupling. Finally, the magnetization losses of the sample HTS compact conductors were measured and analyzed.

The technology of supplying the electric power by wireless power transfer (WPT) is expected for the next generation power feeding system since it can supply the power to portable devices without any connectors through large air gap. As such a technology based on strongly coupled electromagnetic resonators is possible to deliver the large power and recharge them seamlessly; it has been considered as a noble option to wireless power charging system in the various power applications. Recently, various high temperature superconductor (HTS) wires have now been manufactured for demonstrations of transmission cables, motors, MAGLEV, and other electrical power components. However, since the HTS magnets have a lower index n-value intrinsically, they are required to be charged from external power system through leads or internal power system. Thus the portable area is limited as well as the cryogen system is bulkier. Thus, we proposed a novel design of wireless power charging system for HTS magnet (WPC4SM) based on resonance coupling method. As the novel system makes possible a wireless power charging using copper resonance coupled coils, it enables to portable charging conveniently in the superconducting applications. This paper presented the conceptual design and operating characteristics of WPC4SM using different shapes' copper resonance coil. The proposed system consists of four components; RF generator of 370 kHz, copper resonance coupling coils, impedance matching (IM) subsystem and HTS magnet including rectifier system. This feasible study deduced the design and analysis methods of the novel WPC4SM.

The effect of stress and strain on the electromagnetic properties of superconducting composite conductors is one of the key issues for the practical application of superconductivity. To discuss these subjects thoroughly, the International Workshop on Mechanical–Electromagnetic Properties of Composite Superconductors (MEM) has been held regularly since 2001. The 6th workshop (MEM11) was held in Okinawa, Japan in 2011, which was the centennial of the discovery of superconductivity by Professor Kamerlingh Onnes, as well as the 25th anniversary of the discovery of high temperature superconductors (HTS). Although it was originally planned that MEM11 be held in Mito, the workshop venue was changed because of the serious disaster in the north of Japan on 11 March 2011. Sixty five scientists participated in this specialized workshop. Fifty six papers were presented in the following six sessions: (1) Intrinsic strain effects on low temperature superconductors (LTS) and HTS, (2) The International Thermonuclear Experimental Reactor (ITER), (3) Strain evaluation by quantum beams, (4) Flux pinning properties of HTS, (5) Standardization and the Versailles Project on Advanced Materials and Standards (VAMAS) and (6) High field magnets. Several large topics were presented and discussed at the workshop: the important progress in methods for non-invasive measurements of the local strain exerted on the superconducting components in superconducting wires and cables using quantum beam techniques. This approach provides powerful tools for investigating the effect of strain in composite superconductors; the intrinsic strain effects in LTS and HTS in the reversible strain region. Recently, it has become easier to determine quantitatively the strain dependence of critical current when the local strain is measured directly, and the mechanical and electromagnetic properties of the superconductors used in the ITER tokamak. This was a major topic at the workshop since the superconducting magnets are a critical component in this project. Standardization with respect to ITER was also discussed, as well as other topics related to improving the mechanical–electromagnetic properties of practical HTS, including (RE)BCO, BSCCO and MgB2 wires. From such exciting and excellent presentations, nineteen papers were selected and invited for publication in this special issue.At the end of the workshop, a senior member of the International community thanked our Japanese colleagues for their exceptional hospitality and for holding this important workshop at this difficult time.

HTS in electric power applications, transformers

High Temperature Superconductors (HTS) are widely considered for large power applications used by industrial end-users and electric utilities. The prominent application areas include power transmission cables, electric motors, generators, current limiters, and transformers. The promising design concepts rely on HTS to be a flexible composite conductor, robust enough to handle an industrial environment. Currently, the most advanced manufacturing method for flexible composite conductor is the Bi-2223-OPIT, used by many organizations. Significant advances in HTS technology have been made, with average critical current performance above 115 A at 77 K which is equivalent to an engineering current density of 13.8 kA/cm/sup 2/. During the past 18 months, American Superconductor increased its HTS wire manufacturing capacity from 250 km to 500 km per year to meet the increased demand for development and demonstrations. While this level of quality and quantity enables impressive demonstrations of prototype power applications, it does not fully meet the requirements of commercial economic viability. Therefore, to further decrease wire price to $50/kA-m, American Superconductor is currently siting a new facility dedicated to the manufacturing of Bi-OPIT-2223 wire in quantities of 10000 km per year. Initial applications for this wire are power transmission cables, industrial motors and electrical generators. This paper will report on the performance and reliability testing of Bi-2223 tapes. We will discuss the electrical, tensile, compression, and fatigue testing results of tapes manufactured for specific key projects. Also, we will review mass availability of High Temperature Superconductors and we will report on technological and price/performance limitations to be overcome to increase the applicability of HTS in research and industrial devices and equipment.

High Temperature Superconductors (HTS) are widely considered for large power applications used by industrial end-users and electric utilities. The prominent application areas include power transmission cables, electric motors, generators, current limiters, and transformers. The promising design concepts rely on HTS to be a flexible composite conductor, robust enough to handle an industrial environment. Currently, the most advanced manufacturing method for flexible composite conductor is the Bi-2223-OPIT, used by many organizations. Significant advances in HTS technology have been made, with average critical current performance of 130 A at 77 K which is equivalent to an engineering current density of 15.1 kA/cm2. During the past 18 months, American Superconductor increased its HTS wire manufacturing capacity from 250 km to 500 km per year to meet the increased demand for development and demonstrations. While this level of quality and quantity enables impressive demonstrations of prototype power applications, it does not fully meet the requirements of commercial economic viability. Therefore, to further decrease wire price to the range of $50/kA-m, American Superconductor is currently siting a new facility dedicated to the manufacturing of Bi-OPIT-2223 wire in quantities of 10,000 km per year. The purpose of this paper is to examine the functional, reliable, and economical aspects of today’s HTS materials with an eye towards application in space missions.

Recently, superconducting power applications by using the high-temperature superconducting (HTS) tape have been widely researched due to the development of the HTS tape. In particular, the HTS coils cowound with insulation materials have been dominantly used for the power applications. However, the insulated HTS coil has low thermal stability at overcurrent operation, which causes much likely to be damaged. To relieve this issue, other insulation methods such as insulation-free or partial insulation have been investigated. These methods have good thermal stability; however, they have weaknesses such as magnetic field saturation and charge-discharge delay as well. In particular, charge-discharge delay is a significant obstacle for superconducting magnetic energy storage due to a frequently variable current operation. Therefore, a quasi-insulation (QI) coil that compensates the disadvantages has been proposed. In this paper, two QI coils according to the width of the insulation tape were fabricated since the performance of this winding method is affected by the exposure area to the coolant. In addition, insulation-free coil and partial insulation coil were fabricated and tested as the control group. The terminal coil voltage and center magnetic field were measured to compare the characteristics of each coil. The experimental result shows the QI coil has good thermal stability at overcurrent operation without magnetic field saturation and degradation. In addition, cooling effect is proportional to the exposed area to the coolant and adjustable with a different width of insulation tape.

<p><b>The term 'high-temperature superconductivity' has long been synonymous with copper oxide-based superconductors (cuprates) up until the recent discovery of the iron-based superconductors in 2008. This new family of superconductors exhibits fundamentally interesting properties such as the interplay between magnetism and superconductivity as well as the very recently discovered topological properties of FeSe1−xTex. Furthermore, from an application point of view, iron-based superconductors have the potential to become the new norm for low-temperature, high-field applications such as MRI and nuclear fusion.</b></p> <p>However, some of the post-processing procedures required to obtain high-quality samples, like the annealing process of FeSe1−xTex, are yet to be fully understood.</p> <p>This thesis reports on the effect of annealing on the structure and composition of FeSe1−xTex and how they manifest as changes in the superconducting properties.</p> <p>Overall, air annealing is shown to improve the critical temperature and critical current density of FeSe1−xTex for almost all investigated doping concentrations.</p> <p>These improvements are the result of a decrease in excess iron driven by the formation of thin iron oxide layers on exposed surfaces of the crystal.</p> <p>Further analysis suggests that the reduction in the excess iron concentration is largest in the region right underneath the oxide layers. Consequently, the improvement in the superconducting properties is also found to be largest in these regions. In terms of the annealing atmosphere, even in nitrogen and low-vacuum atmospheres, annealing still leads to the formation of an iron oxide layer and an improvement in the superconducting properties due to the presence of residual oxygen. In rare cases, annealing was found to induce asymmetric magnetic hysteresis loops as a result of weak bulk pinning and strong surface pinning. Whilst asymmetric hysteresis loops have occasionally been reported in the cuprates and polycrystalline iron-based superconductors, this work reports the first observation of such behaviour in FeSe1−xTex single crystals. This work has deepened the understanding of the annealing process on the intrinsic properties of FeSe1−xTexand facilitates the study of additional post-processing procedures that will further improve the properties of this family of superconductors.</p>