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Exploitable results
The aim of this project is to realise a demonstrator, based on High-Tc Superconductors (HTS), which combines two devices, cables and fault current limiters, for which the industrial need has been clearly demonstrated. Such a link can find its application as a generator out-line or in the medium voltage power distribution system. Two alternative superconductors, Bi-2212 melt-cast bulk material and Y-123 coated tapes, have been considered as possible candidates for the power link.
As final application of the SUPERPOLI an integration in the power grid was considered as (i) a generator output and (ii) high density distribution network. A numerical simulation of the interaction of the power link with the grid was performed, and, as a result, a general specification for the HTS conductors was defined. In order to minimise the ac loss of the conductor a tubular co-axial structure of the link has been suggested. Stability and quench behaviour of SUPERPOLI functional model has been studied. The safety aspects of the power link are considered, particularly, with respect to hot-spots which appear in the HTS material. A functional model link (single phase, 2 meter long, normal operation 20 kV, 2 kA, fault conditions: 200 V, 5 kA) has been developed. The model link allows operation both with active elements based on Bi-2212 and on Y-123.
The essential part of the work has been devoted to the development of well-specified superconductors that are suited for the application in the power link. The development of Bi-2212 conductor included a study towards an increase of critical current density (by 30 %) and a homogenisation of its distribution. A melt cast tubular material with reduced current path has been developed. The non-superconducting cross-sectional area in the tubes is based on secondary phases which formed during the controlled solidification process at the inner side of the tube. As a result, 650mm long tubes with 3 mm wall thickness, where the HTS layer does not exceed 2/3 of the wall thickness were developed. Development of Y-123 coated conductors included particular development of the conductor architecture and deposition techniques of particular layers. The finally developed Y-123 conductor architecture in based on a 0.1mm-thick stainless steel tape coated with a bi-axially textured yttria-stabilized ZrO2 buffer layer, the Y-123 film (~1 µm thick) and, finally, a metallic shunt/protection layer. A High-Rate Pulsed-Laser-Deposition (HR-PLD) was developed for the Y-123 film processing; a method which allows an increase of the deposition rate by a factor of > 40. The critical current densities achieved finally in these tapes are >1.3 MA/cm2, the critical current in these tapes corresponds to >165 A per cm width of the tape. Electrical contacts and shunting layers with well-specified parameters were developed for coated conductors. Y-123 tubular modules, with Ic > 2000A, enabling temperature compensation and double side cooling, were developed for the final power tests. For both Bi-2212 and Y-123 superconductors, 3 tubular modules with Æ50mm (Bi-2212), Æ55mm (Y-123), 650mm-long, with improved contacts have been delivered for the power tests.
An assessment of the superconductors has been performed with respect to the current limiting capacity, power losses, mechanical strains and current limitation capability. Measurements of E(J,B,T) characteristics at high electric fields were done. Data sheets of material properties including stability under mechanical strain are provided for both superconductors. Means for conductor protection against shocks, thermal shrinkage and curvature are defined.
The final power tests were performed at the industrial site in a 600 MVA Power Laboratory for the Bi-2212 tubes and Y-123 modules (3 pieces of each connected in series). The performance of these materials was rather different. The Bi-2212 tubes yielded a high critical current of 17kAampl., a quenching time of 20ms and a maximal voltage drop of 25V. The Y-123 modules exhibited a critical current of 2.4-3.3kAampl, a quenching time of 0.1-0.5ms, and, under fault current (unlimited current of 50kAampl), a voltage drop up to 150V with corresponding peak power loads up to 450kW. These later parameters, indicating a high performance of Y-123 modules, were found to be sufficient to provide a reliable protection for both a grid and generator by a scaled-up power link.
Techno-economical evaluation of the SUPERconducting POwer LInk combining low loss power transmission (1 GVA) with fault current limitation, to be used typically between the generator and the step-up transformer in a power plant, exhibits significant economical benefits with a Y-123 link. A lower power application (bus bar coupling in the distribution grid) has also been studied, on the basis of a comparison with existing Duplex reactors. In lower power applications, the advantages in terms of dynamic performances of the network are considered. A large range of technical and technological results confirms the high potential of SUPERPOLI for further industrial application.
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