The development of superconducting systems for electric power is driven by the promise of improved efficiency, smaller size, and reduced weight as compared to existing technologies and by the possibility of new applications. Superconducting power components can also contribute to improved power quality and increased system reliability. This paper addresses historical developments and technology status of four superconducting power applications: cables, superconducting magnetic energy storage (SMES), fault-current limiters, and transformers. Today, SMES is the only fully functional superconducting system and it has seen only limited use at grid power levels. A few model or demonstration units exist for each of the other three applications. Superconductivity faces several hurdles on the path to widespread use. Perhaps the most important is the need for operating voltages of 100 kV or more. Though progress in this and other areas has been rapid, considerable development is needed before superconducting devices perform reliably in the utility environment. As a result, today, most initial installations are aimed at niche applications and will be installed where space is limited, where power demands are increasing over existing corridors, and/or where initial development costs can be offset by enhanced power grid performance.

Electric power applications of superconductivity

Design, parameters, and application areas of a superconducting fault current limiter (FCL) are analyzed on the basis of the requirements of power systems. The comparison of resistive and inductive designs is carried out. An example of the effective application of FCLs in distribution substations is considered and the gain from the FCL installation is discussed. It is shown that an FCL not only limits a fault current but also increases the dynamic stability of the synchronous operation of electric machines. The calculation procedure of the parameters of an inductive FCL for a specific application case is described.

Superconducting FCL: design and application

Since the discovery of the first high-temperature superconductors (HTSs) in the late 1980s, many materials and families of materials have been discovered that exhibit superconductivity at temperatures well above 20 K. Of these, several families of HTSs have been developed for use in electrical power applications. Demonstration of devices such as motors, generators, transmission lines, transformers, fault-current limiters, and flywheels in which HTSs and bulk HTSs have been used has proceeded to ever larger scales. First-generation wire, made from bismuth-based copper oxides, was used in many demonstrations. The rapid development of second-generation wire, made by depositing thin films of yttrium-based copper oxide on metallic substrates, is expected to further accelerate commercial applications. Bulk HTSs, in which large single-grain crystals are used as basic magnetic components, have also been developed and have potential for electrical power applications.

https://iopscience.iop.org/article/10.1088/0034-4885/66/11/R01/pdf

Applications of high-temperature superconductors in power technology

A full scale three-phase resistive high-Tc superconducting fault current limiter (SCFCL) designed for 10 kV, 10 MVA, has been developed, manufactured, and tested within a publicly funded German project called CURL 10. The device is based on 90 bifilar coils of MCP BSCCO-2212 bulk material. The operating temperature of 66 K is achieved by cooling of liquid nitrogen using two Stirling cryocoolers. Until today, this is the largest HTS fault current limiter world wide. We report on the design features, the composition, and the operation parameters of the SCFCL system. From April 2004 on CURL 10 is installed and tested within the network of the utility RWE at Netphen near the city of Siegen, Germany. The results of the laboratory test and the field test of CURL 10 are given.

System technology and test of CURL 10, a 10 kV, 10 MVA resistive high-Tc superconducting fault current limiter

The fault current limiter is a very attractive device for electric networks. Meander pattern conductors cut from bulk melt textured YBCO were studied for this application. The meanders are put in series and/or in parallel to match the required current and voltage. The YBCO materials are attractive because they show a very effective limitation with a relative low volume (high engineering current density and normal state resistivity). However, they are sensitive to hot spots. To avoid these destructive hot spots the operating temperature is chosen very close to the critical temperature (above 90 K). This temperature range is reached using a pressurised liquid nitrogen bath. Working close to Tc has two major advantages. The first is reduced values of Jc which limit the power dissipation. The Jc can be matched by changing the pressure on the nitrogen bath. The second is that the proximity of the normal state is favourable for homogeneous quenches along the whole meander as it can be experimentally recorded. Results obtained on single meanders and on the whole assembly are reported under steady state operation as well as during current limitation. Forty three meanders in series limited the current to 740 A (11 000 A unlimited value) under 1 kV.

Current limitation with bulk Y-Ba-Cu-O

Superconducting materials offer the unique possibility to limit fault currents by their self-triggering quench to a resistive state above a given threshold current. Their use in power networks would improve the power quality, which is a real need today. YBCO or BSCO bulk materials are good candidates for current limitation. Their behaviours are rather different, in particular the electric field versus current characteristics. BSCO compounds are protected by their homogeneous quench due to the numerous defects (grain boundaries) distributed along the material. An assembling of 60 sintered Bi bars was tested under 1 kV. The current limitation is effective (a 5000 A short-circuit has been limited to 1080 A) but it occurs at 24 times the critical current. The increase of the critical current density of Bi compounds by their texturation reduces the current excursion above the critical value. In YBCO bulk elements, the defects are localized and make YBCO very sensitive to hot spots. Those are unavoidable and the operating conditions should be adapted in order to make them non-destructive. That is the self-protection concept. To fulfil it, we chose to operate near the critical temperature (above 90 K) to reduce the critical current density. The small difference with T c is very favourable for the quench. A fault current limiter based on 43 YBCO meanders working at 90.5 K in a pressurized liquid nitrogen bath is presented. The meanders are cut in 45 mm in diameter single domain pellets elaborated using the top-seeding technique. Supplied under a maximum voltage of 1 kV, the current was limited at 740 A instead of the theoretical value of 6500 A without quench.

Superconducting fault current limiter with bulk materials

Current limitation based on bulk YBaCuO meanders

The superconducting (SC) current limiter is one of the most attractive applications of superconductivity because it is an innovative device without any conventional equivalence. Following the emergence of ultra low AC loss NbTi strands in the eighties, several devices have been designed and built using these superconductors. The discovery of high T/sub c/ materials has created numerous potential applications such as particularly for current limiters. The operation at higher temperatures and the very different nature of the SC oxides compared to low T/sub c/ strands deeply modify the limitation process. The authors have carried out some experiments to study the transition of high T/sub c/ samples fed by voltages with the objective of making a resistive limiter. The quench behaviour of sintered and textured Bi compounds have been studied and compared. Grain boundaries play a very important part in the limitation process. The resistive transition seems to begin at the grain boundaries. The complete resistive transition of the sample has never been recorded. The propagation of the transition from the resistive part to the entire sample is very low. To overdrive the quench, a magnetic field has been applied. In this paper, the results are reported to show the grain boundaries role in the transition.

Current limitation using high T/sub c/ materials

Forty three meanders were mounted and put in series to build a resistive FCL prototype. The nominal current and the temperature were respectively fixed at 100 A and 90.5 K. The voltage and the mean electric field respectively reached 1 kV and 106 V/m during 20 ms. The current was limited to 730 A whereas it would have reached 5.1 kA without limitation. The superconducting parameters were adjusted so as to reduce the temperature (self-protection) reached by the samples. Bulk YBCO proved in our experiment their strong ability to limit the current. However, meanders suffered large electromechanical and thermal stresses during the transition. In this article, we analyse the prototype results.

E Floch

Papers

Current limitation with bulk Y-Ba-Cu-O

Superconducting fault current limiter with bulk materials

Current limitation based on bulk YBaCuO meanders

Current limitation using high T/sub c/ materials

Current limitation with bulk YBaCuO meanders