Christian Schacherer

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).

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.

Power Hardware-in-the-Loop Testing of a YBCO Coated Conductor Fault Current Limiting Module

In recent years, good progress has been made in improving the quality and quantity afforded by the manufacturing process for YBCO coated conductors. As a result, several programs have started to develop electrical power applications like motors, transformers, and fault current limiters (SCFCL) with these conductors. High voltage resistive type SCFCLs may typically be assembled from modules connected in series and parallel to accommodate the required voltage and current levels. The limited length of such a SCFCL module simplifies the configuration, manufacturing, and maintenance. It also allows testing of these modules under laboratory conditions at reduced power levels. In order to test SCFCL modules under conditions they will experience in high voltage electrical networks, advanced test methods such as power hardware-in-the-loop (PHIL) can offer significant advantages. This method allows studying conditions such as voltage stability and severe system perturbations with the actual SCFCL module in the loop. Hence, the quench and recovery behavior of the SCFCL module can be investigated under conditions characteristic of real electrical power networks without elaborate experimental setups. This paper presents results from PHIL experiments with a SCFCL module consisting of an approx. 10 m coated conductor.

Experimental Investigation of Parallel Connected YBCO Coated Conductors for Resistive Fault Current Limiters

Rapid development of the critical current density, homogeneity and the manufacturing process of commercially available YBCO coated conductors (CC) opens up new fields for power applications. The feasibility of superconducting fault current limiters (SCFCL) at medium and high voltage levels or for high current applications would be unimaginable without the concept of modular assembling. This means that CCs have to be connected in series according to voltage condition and in parallel according to specific current requirements to form the specific modules. The limiting behavior and the AC-losses of such a module can be defined by purposeful selecting components with different physical properties. This work presents the experimental results of a module of parallel connected CC with equal and different physical properties like stabilization and critical current (Ic). The tests are performed under different over-currents from minor overloads to fault currents and demonstrate the limiting behavior of a single CC and of the bundled CC in liquid nitrogen bath under ambient pressure.

Investigation of the Stability Behavior of Coated Conductors

Superconducting applications like cables, current leads and high current power lines made with YBCO coated conductors (CC) require a protection of the CC against overheating in case of over currents. One possible protection of the CCs is the application of a highly conductive stabilization layer, such as electroplated Cu. The duration of over currents can vary between microseconds and several seconds depending on the application of the superconductor and the characteristics of the fault current. If liquid Nitrogen (LN2) is used as coolant, the transient heat transfer into the coolant varies from adiabatic behavior at short term over currents up to a nearly static heat transfer into the coolant at long term over currents. The thermal behavior of CC cooled by LN2 during over currents was investigated experimentally. The time to reach the transition temperature was calculated analytically and measured for different currents and different stabilizer conditions on the CC. To allow a comparison of the stability for different CCs, a stability number is introduced.

COMPARISON OF QUENCH BEHAVIOUR OF DIFFERENT COATED CONDUCTORS

This paper presents experimental results of quench studies at short samples of YBCO coated conductors (CC) cooled in boiling liquid nitrogen at 77 K by varying the current. Coated conductors with different cap layers and stabilisations are used for these studies. Samples without any stabilisation, except the original silver layer as well as composite samples with an additional stabilisation of hastelloy or copper are systematically investigated for the use in superconducting fault current limiters (SCFCL).The variation of the sinusoidal current covers a wide range starting from currents below the critical current (Ic) up to currents of more than 10 times Ic. The test results show the excellent performance of homogeneous CC with a low stabilisation for the use in SCFCL in wide range of short circuit current amplitudes as well as the acceptable performance of inhomogeneous CC with a high stabilisation.