Andrej Kudymow

Status of high transport current ROEBEL assembled coated conductor cables

Assembling coated conductors (CC) into flat ROEBEL bars (RACC cable) was introduced in 2005 by the authors as a practicable method of reaching high transport currents in a low AC loss cable, which is a cable design suited for application in windings. The transport current of 1.02?kA in self-field at 77?K achieved so far, however, is still too low for several applications in electrical machinery such as larger transformers and generators/motors. A new cable concept for further increased currents was presented just recently. The goal of the new design was primarily to demonstrate the possibility of strongly increased transport currents without changing the important cable features for low AC losses. such as, for example, the transposition length of the strands. We present detailed investigations of the properties of this progressed cable design, which has threefold layered strands, an unchanged transposition pitch of 18.8?cm and finally the application of 45 coated conductors in the cable. A 1.1?m long sample (equivalent to six transposition lengths) was prepared from commercial Cu stabilized coated conductors purchased from Superpower. The measured new record DC transport current of the cable was 2628?A at 77?K in self-field (5??V?cm?1 criterion). The use of three slightly different current carrying batches of strand material (? 10%) was a special feature of the cable, which allowed for interesting investigations of current redistribution effects in the cable, by monitoring a representative strand of each batch during the critical current measurement. Although current redistribution effects showed a complex situation, the behaviour of the cable was found to be absolutely stable under all operational conditions, even above the critical current. The high self-field degradation of the critical current reached the order of 60% at 77?K, and could be modelled satisfactory with calculations based on a proven Biot?Savart-law approach, adapted to the specific boundary conditions given in this new cable design.

Transport and magnetization ac losses of ROEBEL assembled coated conductor cables: measurements and calculations

Many superconductor applications require cables with a high current capacity. This is not feasible with single-piece coated conductors because their ac losses are too large. Therefore, it is necessary to develop superconducting cables with a high current capacity and low ac losses. One promising solution is given by ROEBEL cables. We assembled three ROEBEL cables from commercial YBCO coated conductors. The cables have the same width but a different number of strands, which results in different aspect ratios and current capacities. We experimentally studied their ac losses under a transport current or a perpendicular magnetic field. In addition, we performed numerical calculations, which agree with the experiments, especially for the transport case. We found that in the cables there is good current sharing between the strands. We also found that stacking the strands reduces the magnetization losses. For a given critical current, thicker cables have lower magnetization ac losses. In addition, a conducting matrix is not required for a good current sharing between strands.

ENSYSTROB—Resistive Fault Current Limiter Based on Coated Conductors for Medium Voltage Application

A new German government funded project for a resistive fault current limiter has started in September 2009 (ENSYSTROB) and is presented. The consortium includes partners from industry, research centers and utilities. It aims the construction of a 3-phase medium voltage current limiter (12 kV, 800 A) for the protection of the domestic supply in a power plant. A special feature of this application is the presence of large in-rush currents (4100 Ap for 50 ms, 1800 A for 15 s). The superconducting components are bifilar pancake coils consisting of pairs of YBCO-tapes in face to back configuration. Successful limitation experiments on prototype components with prospective currents in the whole range are shown. Also the stability with respect to high voltage could be demonstrated. The AC-losses under normal operation are shown, by simulation and experiment, to be smaller than the heat input of the current leads. The current limiting components are compared with similar elements based on BSCCO 2212 bulk material (Nexans SuperConductors).

Coated Conductor Rutherford Cables (CCRC) for High-Current Applications: Concept and Properties

In the past few years the Roebel technique has been established as a method for cabling of coated conductor tapes to achieve high current carrying capabilities for low ac-loss applications. We have successfully developed Roebel cables consisting of up to 50 tapes and with current carrying capabilities up to 2.6 kA (77 K, self-field). However, for applications like busbars or fusion magnets current carrying capabilities of more than 10 kA are required. Such high current carrying capabilities cannot be reached by a simple scale-up with additional tapes. We present a concept for Coated Conductor Rutherford Cables (CCRCs) for currents exceeding 10 kA using Roebel subcables as strands. The first steps towards a short subsize CCRC demonstrator cable with electrical and mechanical characterization of commercial coated conductor tapes, strands and a first Roebel strand have been performed and will be discussed.

Improvement of Superconducting Properties in ROEBEL Assembled Coated Conductors (RACC)

Assembling coated conductors (CC) into flat ROEBEL bars (RACC-cable) is a practical method to reach high transport currents in a low AC loss cable design which is suitable for application in windings. Electrical machinery as large transformers and generators/motors need a few kA transport current. The aim of the presented work was demonstrating the possibility of a strong increase of the transport current of such RACC-cables. So far 1 kA was achieved We present a changed cable design with 3-fold layered strands, an unchanged transposition pitch of 18.8 cm which finally leads to 45 coated conductors in the cable. A 1.1 m long sample (equivalent to 6 transposition lengths) was prepared. Cu stabilized coated conductors purchased from SuperPower were used formatting the ROEBEL strands and assembling the new cable. The new cable reached a record transport current of 2628 A at 77 K in self field (5 muV/cm criterion). A special feature of the cable was the use of 3 slightly different current carrying (plusmn10%) batches of strand material. Although current sharing and redistribution effects could be observed, the behavior of the cable was found to be absolutely stable under all operation conditions. The self field degradation of the critical currents, being of the order of 60% at 77 K could be modeled satisfactory by means of a Biot-Savart-Law approach.

Dissipated energy as a design parameter of coated conductors for their use in resistive fault current limiters

Coated conductors are suitable for many power applications like motors, magnets and superconducting fault current limiters (SCFCLs). For their use in resistive SCFCLs main requirements are quench stability and resistance development above Tc. Several coated conductors are available with different kinds of stabilization like thickness or material of cap-layer and additional stabilization. The stabilization can vary and has a great influence on the quench stability and quench behaviour of a coated conductor. Thus, for the dimensioning of a superconducting current limiting element there is a need of reliable and universal design parameters. This paper presents experimental quench test results on several coated conductor types with different stabilization and geometry. The test results show that the dissipated energy during a quench is a very useful parameter for the SCFCL design.

Investigation of YBCO Coated Conductor for Application in Resistive Superconducting Fault Current Limiters

These R&D of YBCO coated conductor wire is progressing very fast. The manufacturing lengths for a single wire have reached up to several hundred meters with high quality. Due to its promising cost predictions, YBCO coated conductor (CC) material might considerably increase the economic feasibility of superconducting power devices (cables, transformers, machines, current limiters and energy storage) in near future. The main requirement for YBCO wire in resistive superconducting fault current limiters (SCFCLs) is quench safety for various short-circuit conditions. Up to now, only a few authors report on this subject. This paper presents experimental quench test results with short and medium length samples of YBCO coated conductor wire for different test conditions. The experimental results confirm the feasibility of YBCO coated conductor wire for application in resistive SCFCLs. Short samples showed fast and effective limitation up to an electric field of 2.7 V/cm for a short-circuit time of 100 ms without material degradation. Even relatively inhomogeneous short samples showed a non-destructive quench. The tests clearly demonstrate that a good contact between the cap layer and the substrate is mandatory to avoid hot spots during quench.

Investigation of the effect of striated strands on the AC losses of 2G Roebel cables

The assembly of meander shaped coated conductor tapes by the Roebel technique is a promising way to manufacture high current cables with low ac losses. The application of longitudinal striations to the single strands can be an option to create a filament structure for further possible reduction of the ac losses. Due to the complex Roebel strand geometry, it was important to identify a reliable technique to produce such structures using a picosecond-infrared (IR) laser for the groove etching process. We analyzed the effects of the filament structure on the magnetization ac loss behavior by comparing the losses of a cable with striated strands with those of a reference one with non-striated strands. The ac loss reduction in the Roebel cable with striated strands was confirmed. The measured magnetization loss of the 125 mm striated single strand is five times lower than that of the non-striated one. In the case of the cable sample the loss reduced by a factor of three, but not in the whole interval of amplitudes of the applied magnetic field. We also compared the results with those for a cable with insulated striated strands: they seem to indicate that the coupling currents occur mostly between the filaments, not between the strands.

Recovery Characteristic of Coated Conductors for Superconducting Fault Current Limiters

Superconducting fault current limiters (SFCL) offer the possibility to reduce the level of short circuit currents in electrical power distribution networks. During the current limitation, the temperature of the superconductor increases because of the resistive losses until the short circuit is switched off. After the current limitation, the superconductor has to cool down to become superconducting again. If recovery under load is required, the superconductor has to cool down under regular load conditions. This paper presents experimental recovery under load tests of coated conductors for the use in fault current limiters or combined applications, like fault current limiting transformers. The results show the limits of the recovery time, depending on the residual current for different coated conductors in boiling liquid nitrogen.

Conceptual Design of a 110 kV Resistive Superconducting Fault Current Limiter Using MCP-BSCCO 2212 Bulk Material

Superconducting fault current limiters (SCFCLs) are new and attractive devices to limit short-circuit currents in power systems. In recent years, the technical feasibility of SCFCLs in medium voltage applications was successfully demonstrated in several field tests. In high voltage power systems the application of SCFCLs is very attractive too, because at this voltage level conventional devices to limit short-circuit currents are hardly applicable and system studies showed considerable economical benefits. Therefore, a German project started recently to develop a first 110 kV, 1.8 kA prototype of a resistive SCFCL. A magnetic triggered resistive concept using MCP-BSCCO 2212 bulk material will be used for the demonstrator. This paper reports about the conceptual design of this SCFCL and the project status. Focus is given on the main data of the 110 kV prototype, the SCFCL modules, the general design of the whole system and the most important high voltage design aspects. The calculations and estimations show that the conceptual design presented in this paper seems feasible and that a major technical challenge is to ensure a reliable electrical insulation system.